# Commercial diving/Types of Environmental Hazards

Relevance: Scuba diving, Surface supplied diving, Surface oriented wet bell diving.
Required outcomes:
* Describe the potential consequences of, and limitations on, diving in currents in rivers and sea
* Describe the effect of local weather and sea state on diving safety and give the acceptable limitations for diving in swell and surf zones
* List the hazards and potential consequences of diving in shipping, including marine traffic, pinch points, confined environments, propellers, thrusters and anchor points
* Explain the hazards and potential consequences of diving in overhead environments and confined spaces, and the procedures and equipment appropriate for these conditions
* Describe the potential effect of differential pressure whilst diving in (or near} dams, docks, locks, sluices, culverts, and gates
* Discuss the hazards and potential limitations of night diving and appropriate warning lights
* Discuss the hazards and potential consequences of cold water diving and use of dry suits
* Discuss the hazards and potential consequences of diving in very low visibility
* Discuss the hazards and potential consequences of diving in contaminated environments and in fluids of viscosity or density different to those of water and their effect on dive planning and health
* Discuss the hazards and potential consequences diving on offshore structures and installations and from support vessels and platforms
* Discuss the hazards and potential consequences of entrapment and entanglement in diving operations
* Discuss the effects of (weather including wind, sea state, and) surface visibility on safe diving operations including the limitations on diving, tendering and evacuation.
* Discuss the hazards and potential consequences of chemical and biological contamination at the dive site (Proposed additional item)
* Discuss the hazards and potential consequences of marine and aquatic animals (Proposed additional item)

## Underwater hazards

Divers face specific physical and health risks when they go underwater with diving equipment, or use high pressure breathing gas. Some of these factors also affect people who work in raised pressure environments out of water, for example in caissons. This module lists hazards that a diver may be exposed to during a dive, and possible consequences of these hazards, with some details of the proximate causes of the listed consequences. A listing is also given of precautions that may be taken to reduce vulnerability, either by reducing the risk or mitigating the consequences. A hazard that is understood and acknowledged may present a lower risk if appropriate precautions are taken, and the consequences may be less severe if mitigation procedures are planned and in place.
A hazard is any agent or situation that poses a level of threat to life, health, property, or environment. Most hazards remain dormant or potential, with only a theoretical risk of harm, and when a hazard becomes active, and produces undesirable consequences, it is called an incident and may culminate in an emergency or accident. Hazard and vulnerability interact with likelihood of occurrence to create risk, which can be the probability of a specific undesirable consequence of a specific hazard, or the combined probability of undesirable consequences of all the hazards of a specific activity. The presence of a combination of several hazards simultaneously is common in diving, and the effect is generally increased risk to the diver, particularly where the occurrence of an incident due to one hazard triggers other hazards with a resulting cascade of incidents. Many diving fatalities are the result of a cascade of incidents overwhelming the diver, who should be able to manage any single reasonably foreseeable incident. The assessed risk of a dive would generally be considered unacceptable if the diver is not expected to cope with any single reasonably foreseeable incident with a significant probability of occurrence during that dive. Precisely where the line is drawn depends on circumstances. Commercial diving operations tend to be less tolerant of risk than recreational, particularly technical divers, who are less constrained by occupational health and safety legislation.
Decompression sickness and arterial gas embolism in recreational diving are associated with certain demographic, environmental, and dive style factors. A statistical study published in 2005 tested potential risk factors: age, gender, body mass index, smoking, asthma, diabetes, cardiovascular disease, previous decompression illness, years since certification, dives in last year, number of diving days, number of dives in a repetitive series, last dive depth, nitrox use, and drysuit use. No significant associations with decompression sickness or arterial gas embolism were found for asthma, diabetes, cardiovascular disease, smoking, or body mass index. Increased depth, previous DCI, days diving, and being male were associated with higher risk for decompression sickness and arterial gas embolism. Nitrox and drysuit use, greater frequency of diving in the past year, increasing age, and years since certification were associated with lower risk, possibly as indicators of more extensive training and experience. There do not appear to be similar studies on commercial divers.

### General hazards of diving

General hazards of diving include hazards of the aquatic environment, the use of diving equipment and breathing gas at high ambient pressures. These hazards are controlled by using appropriate equipment, training in the relevant skills, and following procedures which are known to reduce risk.
In any liquid environment there is a hazard of asphyxia by drowning. This can be reduced by the use of a robust breathing gas supply, appropriate buoyancy and good buoyancy control, protection of the airways by suitable underwater breathing equipment, adequate fitness and skills to manage normal circumstances and reasonably foreseeable contingencies, monitoring the condition of the diver while in the water, and providing a backup team to assist in an emergency.
When using underwater breathing apparatus, there are associated hazards of equipment malfunction, including delivery of unsuitable gas.
The hazards of exposure to a pressurised environment include pressure changes during descent and ascent, and the hazards associated with breathing gases at high ambient pressure.

### Pre-existing physiological and psychological conditions in the diver
#### Chronic conditions
##### Heart disease

    Consequences: Heart attack. High risk of death as direct consequence, or by drowning as indirect consequence.
    Triggers: Exertion beyond the capacity of the unhealthy heart.
    Screening:
   * Periodical medical examination for diving fitness.
   * Stress ECG when indicated by medical examination.
    Risk reduction:
   * Maintaining good cardio-vascular fitness.
   * Use of Nitrox may decrease risk.

##### Angina pectoris

    Consequences: Severe pain and severely reduced physical strength and endurance, and reduced situational awareness, which increase the risk of further deterioration of the incident.
    Triggers: Exertion beyond the capacity of the unhealthy heart.
    Screening:
   * Periodical medical examination for diving fitness.
   * Stress ECG when indicated by medical examination.
    Risk reduction:
   * Maintaining good cardio-vascular fitness.
   * Use of Nitrox may decrease risk.

##### Patent foramen ovale (PFO)

    Consequence: Possibility of venous gas bubbles shunting into arterial circulation and causing emboli
    Triggers/mechanism of injury: Otherwise low-risk venous gas bubbles formed during decompression may shunt through PFO during anomalous pressure differential episode such as coughing, Valsalva manoeuver, or exertion while holding the breath.
    Screening: Available, but not required for commercial diving medical examination unless indicated by anomalous symptoms. Present in about 25% of adult population to some extent. Not considered a disqualification for diving.
    Risk reduction:
   * Screening for PFO for high risk divers
   * Conservative decompression and ascent
   * Avoidance of exercise which is likely to induce shunting during ascent
   * Surgical correction is possible, but not often indicated.

##### Epilepsy

    Consequence: Loss of consciousness and inability to remain alert and actively control activity. Highly likely to lead to drowning in Scuba divers.
    Triggers: Unpredictable and variable. Flashing lights are one fairly common trigger.
    Mechanism of injury: Epileptic seizure.
    Screening and risk reduction: Divers with a history of epilepsy are generally considered unfit for diving due to the unacceptable risk associated with an underwater seizure.

##### Diabetes

    Consequences: Low blood sugar. Most cases are mild and are not considered medical emergencies, but can escalate rapidly in the underwater environment. Effects can range from feelings of unease, sweating, trembling, and increased appetite in mild cases to more serious issues such as confusion, changes in behavior such as aggressiveness, seizures, unconsciousness, and occasionally permanent brain damage or death in extreme cases. Moderate hypoglycemia may easily be mistaken for drunkenness; rapid breathing and sweating, cold, pale skin are characteristic of hypoglycemia but not definitive.
    Triggers: Overexertion, heat loss, general fatigue, anything leading to high consumption of blood sugar.
    Screening:
    Risk reduction: Diabetic divers must learn to manage their blood sugar effectively.
    Mitigation: Mild to moderate cases are self-treated by eating or drinking something high in sugar. Severe cases can lead to unconsciousness and must be treated with intravenous glucose or injections with glucagon.

##### Asthma

    Consequences: Difficulty in breathing, particularly difficulty in exhaling adequately during ascent, with reduced physical work capacity, can seriously reduce ability to cope with a relatively minor difficulty and precipitate an emergency.
    Mechanism of injury: Constriction of lung passages, increasing work of breathing.
    Triggers: Variable. May include allergies, stress and overexertion.
    Screening: Spirometry test standard at diver medical examinations.
    Risk reduction: Asthmatic divers restricted to light activity and should test lung function before each dive.

##### Trait anxiety

    Consequences: Higher susceptibility to panic under high stress, and associated sub-optimal coping behaviour.
    Mechanism of injury: Panic, inappropriate or inadequate reaction to stressful circumstances
    Triggers: Stress
    Screening:
    Risk reduction:
   * Over-learning of critical skills.
   * Avoidance of high stress dive plans.
   * In-water standby diver

#### Temporary conditions
##### Dehydration

    Consequences:
   * Increased risk of decompression sickness
   * Muscular cramps
    Triggers/causes:
   * Overheating and sweating before dive.
   * Drinking diuretic beverages before diving.
   * Immersion effects of diving.
    Risk reduction:
   * Ensure adequate hydration before diving.
   * Rehydrate during dives if they are several hours long.
   * Rehydrate after dives.
    Mitigation: Treat as indicated by symptoms.

##### Fatigue

    Consequences: Reduced situational awareness, reduced ability to respond appropriately to emergencies
    Triggers: Lack of sleep, excessive exertion prior to dive, excessive exertion during dive, long periods of hard work or concentration, particularly in difficult conditions.
    Risk reduction: Restrict shift duration, limit exertion prior to dive, ensure adequate rest periods, avoid diving in conditions which will require excessive exertion

##### Compromised physical fitness

    Consequences:
   * Reduced ability to respond effectively to emergencies and related cascade effects
   * Muscular cramps
    Triggers: Illness, lifestyle, lack of exercise.
    Risk reduction: Training and exercise, particularly swimming and finning exercise using diving equipment

### Diver behaviour and competence

These are mostly hazards that it is possible to eliminate.

#### Inadequate learning of critical safety skills

    Consequences:Inability to deal with minor incidents, which consequently may develop into major incidents.
    Causes:
   * Inadequate demonstration and assessment of skills by instructor.
   * Ineffective skills taught, due to inappropriate training standard, or misinterpretation of training standard.
   * Insufficient correct repetition of skills during training.
    Elimination:
   * Correct initial training and assessment procedures
   * Quality assurance by training agency
   * Periodic practice and reassessment of skills

#### Inadequate practice of critical safety skills

    Inability to deal with minor incidents, which consequently may develop into major incidents.
    Causes:
   * Insufficient practice of skills during training.
   * Insufficient practice of skills after training.
    Elimination:
   * Clear standards for competence in assessment criteria of training programme.
   * Quality assurance by training agency.
   * Post training practice of vital skills by the diver.
   * Periodical re-assessment of skills by a competent assessor.

#### Overconfidence

    Consequences: Diving in conditions beyond the diver's competence, with high risk of accident due to inability to deal with known environmental hazards.
    Causes:
   * Over-optimistic self-assessment of personal competence by the diver.
   * Insufficient information due to inadequate training.
    Elimination:
   * Objective assessment and accurate feedback during training.
   * Realistic training standards and competence level descriptions.

#### Inadequate strength or fitness for the conditions

    Consequences:
   * Inability to compensate for difficult conditions even though well versed at the required skills.
   * Over-exertion, over-tiredness, stress injuries or exhaustion.
    Triggers:
   * Underestimating severity of conditions.
   * Overestimating fitness and strength.
   * Conditions deteriorate during the dive.
   * Excessive task loading.
   * Use of equipment that requires greater exertion than the diver can produce.
    Risk reduction:
   * Experience and familiarity with local conditions.
   * Use of weather and tide forecasts when planning dives.
   * Maintaining fitness to dive by adequate exercise.
   * Use of equipment and techniques that reduce physical exertion required.
   * Gradual buildup of task-loading to develop appropriate skills and fitness.
   * Training with equipment in benign conditions before using in severe conditions.

#### Peer pressure and organisational pressure

    Consequence: Inability to deal with reasonably predictable incidents in a dive.
    Triggers:
   * Divers may be pressurised into undertaking dives beyond their competence or fitness.
   * Divers may be pressurised into diving with unsuitable buddies, sometimes by supervisors who should know better.
    Risk reduction:
   * Objective and accurate knowledge of the diver's capabilities.
   * Recognising and accepting responsibility for possible consequences of exerting or submitting to peer or organisational pressure.

#### Diving with an incompetent buddy

    Consequence: Injury or death while attempting to deal with a problem caused by the buddy.
    Trigger/Mechanism of injury:
   * The buddy may get into difficulty due to inattention or incompetence, and require a rescue that is hazardous to the rescuer.
   * The buddy may get into difficulty and mishandle the situation or panic, creating an incident that is hazardous to both divers.
    Risk reduction:
   * Diving with a buddy is known to be competent and who can be trusted to behave responsibly.
   * Training to deal with emergencies and rescue.
   * Carrying equipment to be independent of the buddy in most emergencies.
   * In some circumstances it may be safer to dive without a buddy.

#### Overweighting

    Consequence: Difficulty in neutralising and controlling buoyancy, leading to:
   * Uncontrolled descent.
   * Inability to establish neutral buoyancy and maintain required depoth.
   * Inefficient swimming.
   * High gas consumption.
   * Poor trim.
   * Kicking up silt, reducing visibility and increasing difficulty of the task.
   * Difficulty in ascent
   * Inability to control depth accurately for decompression
    Causes: Carrying more weight than needed.
   * Scuba divers do not usually need more weight than is needed to remain slightly negative after using all the gas carried.
   * Surface supplied divers may need to be heavy at the bottom to provide stability to work. The need for diving heavy is often an indication that scuba is not the appropriate mode for the operation.
    Risk reduction: Establish and use the correct amount of weight for the circumstances of the dive, taking into account:
   * Density of water (sea or fresh).
   * Buoyancy of equipment (mainly exposure suit).
   * Buoyancy change of cylinders as gas is used up.
   * Tasks of the dive.
   * Capacity of buoyancy compensator to neutralise buoyancy at depth and provide positive buoyancy at the surface.
   * Use surface supplied equipment or a lifeline if it is necessary to dive heavy.

#### Underweighting

    Consequences:
   * Difficulty in neutralising and controlling buoyancy.
   * Inability to achieve neutral buoyancy, particularly at decompression stops.
    Causes: Not carrying sufficient weight.
   * Scuba divers need to be able to remain neutral at 3m depth at the end of a dive when the gas has been used up.
   * Surface supplied divers need to be able to remain negative throughout the dive. The ability to achieve neutral buoyancy is not always necessary, but should be possible by ditching weights.

#### Diving under the influence of drugs or alcohol, or with a hangover

    Consequences:
* Inappropriate or delayed response to contingencies.
   * reduced ability to deal with problems in time, leading to greater risk of a contingency developing into an accident.
   * Increased risk of hypothermia.
   * Increased risk of decompression sickness.
    Cause: Use of drugs that alter mental state or physiological responses to environmental conditions.
    Elimination: Avoid use of substances that are known or suspected to reduce the ability to respond appropriately to contingencies.

#### Use of inappropriate equipment and/or configuration

    Consequence: Muscular cramps
    Trigger: Use of fins that are too large or stiff for the diver
    Risk reduction:
   * Exercise to develop skills and fitness appropriate to the fins chosen
   * Use softer or smaller bladed fins (this may compromise speed and/or maneuverability)
    Consequence: Lower back pain
    trigger: Use of heavy weight belts for scuba diving:
    Risk reduction:
   * Use of integrated weight systems, which support the weights directly by the buoyancy compensator
   * Different distribution of weights - some weight transferred to the harness, BCD, cylinder or backplate
   * Avoiding excessive weighting

### Diving in currents in rivers and sea
#### Currents

Currents may be produced by wind, by river flow or by major ocean currents. Tidal current will be superimposed on these currents. It may increase or decrease the speed and may produce turbulence. Currents also affect swell and the breaking of waves.
The speed of a wind generated current is usually up to 3% of the wind speed. A Force 7 wind (28 to 33 kt), therefore, will generate a surface current of about 1 knot. A Force 5 wind (17 to 21 kt) will generate a surface current of about 0.6 knots. Because of the earth’s rotation and Coriolis force, the direction of a wind generated current is about 45° to the wind direction. As depth increases, the current decreases and the direction turns further away from the wind. At 10 m depth it is nearly 90° to the wind direction and the speed is usually negligible. River currents are often associated with poor visibility caused by sediment carried by the river, but ocean currents can bring in clear or turbid water. The major ocean currents are normally slow moving and are usually do not greatly affect the diver. The force exerted on a diver and his equipment by a current is proportional to the square of the water velocity. If the velocity doubles, the force increases four times. The diver’s umbilical is subject to considerable drag.
Other considerations for diving in currents are:
* The ability of the surface crew to recover the diver safely after the dive. Conditions on the surface and at the worksite must be taken into account. Surface current can be strong enough to carry a diver underneath a vessel and hinder recovery, although the current at working depth may be no problem.
* The ability of the standby diver to reach the diver in an emergency
* The physical strength and endurance of the diver.
* The type of equipment being used.
* The type of work being carried out
* Whether the work is to be done in mid-water or on the bottom
* Whether both hands are required to carry out the task
* Changes in strength and direction of the current
* The possibility of using an underwater tender, swim lines, etc.
If the diver is working in the lee of a structure he may not be aware of an increase in current strength.
AODC 047 suggests the following restrictions on working in currents, but notes that conditions vary considerably and the restrictions should be applied flexibly, taking into account diver feedback and operational requirements.
| Current in knots | 0.0 to 0.8 | 0.8 to 1.0 | 1.0 to 1.2 | 1.2 to 1.5 | 1.5 and beyond |
| Surface Supply in Mid Water | Normal work | Observation | See Note 1 | See Note 2 | See Note 2 |
| Surface Supply on Bottom | Normal work | Light work | Observation | See Note 1 | See Note 2 |
| Note 1: Diving by means of this method in these currents should not be a routine operation. The Diving Supervisor should consult with the divers involved and any other person he judges necessary about the best way to conduct such an operation. ||||||
| Note 2: Diving by means of this method in these currents should not be considered unless the operation has been pre-planned taking into account the presence of high current from the early stages of the project. Special solutions involving equipment, techniques and procedures should have been evolved to overcome or protect the diver from the effects of current and provide contingencies for foreseeable emergencies. ||||||

#### Tides

Tides are caused by the combined gravitational effects of the sun and the moon. When sun and moon are aligned, at full moon and new moon, the effect is at its maximum and the tidal range is at its greatest. These are known as spring tides. When the sun and moon are at ninety degrees to each other, at first and last quarters, the effect is at its minimum and the tidal range is at its lowest. These are known as neap tides.
The tidal range is the difference in height between low tide and high tide. It depends on the phase of the moon, the bottom profile and the shape of the coastline, and resonance of the local oceanic water mass to the periodic gravitational influence of the sun and moon. Areas where there is strong resonance will have a relatively large tidal range.
The times and nominal heights of tides are given in tide tables. They can be affected considerably by strong winds. On average, the tide rises for 6 hours and 12 minutes. This is called the rising, or flood tide. At the top of the flood, the level remains approximately constant for a short period. This is the high water slack. The falling or ebb tide then runs for about 6 hours 12 minutes until low water slack. Then the cycle begins again. The change in water depth caused by the tide will clearly affect dive times and duration. In some cases, there may also be legal implications. Regulations impose a maximum depth for air diving, of 50 m on surface supply. Working depth at a site may be less than 50 m at low tide, more than 50 m at high tide. A similar effect can influence diving on scuba In South African inshore waters the tide range is relatively small and can often be ignored.
Tidal streams are the currents associated with flood and ebb tides and change direction accordingly. Currents may run in different directions at different depths. During tidal diving, the identification of slack water is essential and tide tables are not reliable because of local variations. Tide meters should be used to measure current.
The greatest rate of rise or fall, and consequently the fastest tidal stream, occurs half way through the tide in open water. The bigger the range, the faster is the tidal stream for a given site. Tidal streams can also increase in speed round headlands and in narrow channels. In some areas they can be as fast as 10 knots, and can develop overfalls and whirlpools.
Tidal streams are generally strongest where a large volume of water is connected to the open sea by a relatively small opening. This is common at river mouths, lagoons and estuaries.
The Rule of twelfths is used to estimate the amount of rise or fall in each of the 6 hours:
    Hour 1 - 1/12 of range
    Hour 2 - 2/12 of range
    Hour 3 - 3/12 of range
    Hour 4 - 3/12 of range
    Hour 5 - 2/12 of range
    Hour 6 - 1/12 of range
Accurate forecasts of Southern African tides are available on-line from and SAN Hydrographers office.

## The effect of local weather and sea state on diving safety
### Waves

There are many different types of waves found in the ocean and other bodies of water including wind waves, shallow water waves, swells, breaking waves, surf, seismic sea waves (tsunamis), rogue waves, capillary waves, and gravity waves. The type of wave depends upon the mechanism of its formation and its history. For example, waves may start as small capillary waves, build up to wind waves, breaking if they get too steep, become swell as they move away from the wind, become shallow water waves as the depth of water decreases, and break as surf on the shore. Tsunamis or seismic waves are caused by movements of the earth’s crust or equivalent large disturbances. Rogue waves are short term superpositions (temporary combinations) of large waves which result in unstable and unusually large and destructive waves.
Waves a the surface of the sea are primarily gravity waves, as the force which tends to restore the surface to its undisturbed level is the force of gravity on the particles of the water. There is also a minor influence of surface tension, which is negligible in waves larger than ripples, and viscosity effects, which cause the loss of energy and the gradual dissipation of the wave over long periods

#### The terminology of waves
##### Wave crest

    the highest point of a wave.

##### Wave trough

    the lowest point of a wave.

##### Wavelength (L)

    the distance measured from any one point on a wave to the equivalent point on adjacent wave It is usually easiest to estimate the distance between crests.

##### Wave height (H)

    the vertical distance from the crest to the trough of a wave.

##### Amplitude (a)

    the distance a wave moves the water above or below sea level. (half of the wave height)

##### Period (T)

    the time it takes for one wave to pass a specified point. Relatively easy to measure and useful to know.

##### Frequency (f)

    the number of waves passing a specified point in a given time unit.

##### Celerity (c) or Phase velocity

    the velocity at which a wave travels. (the water goes round in circles, but the wave shape moves)

##### Depth (d)

    the depth of water to the still water level

#### Creation of waves by wind

How wind causes water to form waves is easy to understand although many intricate details still lack a satisfactory theory. On a perfectly calm sea, the wind has practically no grip. As it moves over the water surface, the friction between the air and water transfers energy to the water and makes it move. As the water moves, it forms eddies and small ripples. These ripples do not travel exactly in the direction of the wind but as two sets of parallel ripples, at angles 70-80º to the wind direction. The ripples make the water's surface rough, giving the wind a better grip by causing local pressure variations which increase the rate of energy transfer. The ripples grow to wavelets and start to travel in the direction of the wind. At wind speeds of 4-6 knots (7-11km/hr), these double wave fronts travel at about 30º from the wind. The surface still looks glassy overall but as the wind speed increases, the wavelets become high enough to interact with the air flow and the surface starts to look rough. The wind is more turbulent above the surface and transfers more energy to the waves. Strong winds are more turbulent and make waves more easily.
The rougher the water becomes, the easier it is for the wind to transfer its energy. The waves become steep and choppy. Further away from the shore, the water's surface is not only stirred by the wind but also by waves arriving with the wind. These waves influence the motion of the water particles such that opposing movements gradually cancel out, damped by friction, whereas synchronised movements are enhanced. The waves start to become more rounded and harmonious. Depending on duration and distance (fetch), the waves may develop into a fully developed sea.
Anyone familiar with the sea, knows that wind waves never assume a uniform, harmonious shape. Even when the wind has blown strictly from one direction only, the resulting water movement is made up of various waves, each with a different speed and height. Although some waves are small, most waves have a certain height rannge and sometimes a wave occurs which is much higher.
The one characteristic of the wave which does not change is its period , as it is necessary for the same number of waves to enter a zone of water as will leave it unless waves are created or destroyed in the zone, and so the period at the entry point must be the same as at the exit point. This means that if the speed changes, the length must also change – as the waves are slowed they get shorter.
The celerity (speed) of a wave in deep water is independent of depth, but this changes when the base of the wave interacts with the seabed. A common expression for this is that the wave feels the bottom. When the wave feels the bottom, the particles at the bottom are constrained to move parallel to it, and this causes surge.

### Wave interactions with Shore and Seabed

The depth at which a wave feels the bottom depends on wave length, and scales geometrically. Twice the wave length will feel the bottom at twice the depth. This interaction slows the wave by friction and dissipates energy.
The celerity (speed) of a wave is dependent on depth when the depth is shallower than a limit which depends on period or wave length. This effect occurs when the base of the wave interacts with the seabed.

#### Refraction

Refraction of waves occurs when they move from deep to shallower water where the depth varies along the wave front, and the wave base interacts with the sea bed. The parts of swells passing through shallower water are slowed more by friction with the bottom than those parts in deeper water. This results in the stretching and curving of the wave front, and the result is a change in direction which tends towards the shallower area, and a loss of energy as the refracted wave front is stretched. Long period waves are refracted more than short period waves as they pass into shallower water. So the short period waves tend to continue in their original direction, and when they eventually reach shallow enough water to refract, their lower energy causes greater dissipation as they refract.

#### Diffraction

Diffraction is the mechanism by which wave energy gets around behind obstructions where the water is deep relative to the wave length. When a wave front passes a steep obstruction, part of it is blocked, and the rest goes past. However the end of the wave front does not remain bluntly truncated beyond the obstruction, as this is unstable, and it spreads out behind the obstruction. The energy available for this must come from the remaining wave front, so as it spreads out it gets lower. If the depth variation does not greatly influence the speed of the diffracted wave front, this will assume a curve centred on the edge where the wave front reached open water after passing the obstruction. Where the wave front is interrupted by an island, it will diffract on both sides of the island, and there will be interactions between the waves from the two sides which will produce interference effects.

#### Reflection

When a wave front hits a vertical or near vertical rigid surface, like a cliff or a harbour wall, the particles bounce off the wall in a way slightly similar to a ball bouncing off a wall. Energy is conserved, Momentum parallel to the reflecting surface is retained except for friction losses, and momentum perpendicular to the reflecting surface is reversed. This results in the wave reflecting at an angle equal to the incident angle to the other side of the normal to the reflecting surface. Gravity keeps the average surface height constant, so any slope from the vertical will tend to cause dissipation of energy if the water can ride up it during the impact. Reflected waves can produce a very lumpy sea state from an otherwise smooth even swell. This is one of the reasons for the ragged structure of breakwaters at harbours. The irregular lumps of rock or concrete prevent regular reflection and dissipate a lot of energy. This will also happen on an irregular rocky shore which is too steep to cause surf.

#### Surf and Surge

The waves that reach a dive site have two direct influences on the diver. These are the surf and surge produced by the waves as they interact with the seabed and shoreline. Deep offshore sites obviously will not be influenced by surf, but there may be breaks over shoal areas or rocks which break the surface in the vicinity of the site and this can affect visibility and safety.
Surf and its effects are directly visible from the surface and will not be discussed in great detail. The height of a breaking wave is dependent on both the swell height and period, and the bottom profile. Shoal areas can often be identified by the effects on waves as they pass over the shallower bottom.
Surge is the deeper motion of water due to the wave oscillation, and it is significantly influenced by the bottom topography. As long period waves have a deeper reaching oscillation, they will produce surge at greater depths, while short period waves will produce relatively shallow surge. Surge strength is also proportional to swell height, and big swells will produce proportionately more powerful surge than smaller swells of the same period. Surge can also be amplified by ridges and gullies on the bottom, and strongly turbulent areas are sometimes found at the back of obstacles to the flow. Surge period will match the period of the wave that causes it.
The indirect effects of waves on the diver include changes in visibility and wave induced currents. Again these are dependant on wave direction, period and water depth. Water movement will disturb bottom sediments and detritus to an extent proportional to the speed of the water over the bottom, and strong surf will aerate the water and may break up kelp and other seaweed to release biological material which forms foam on the surface.

#### Effects of waves on diving operations

In general, wave movement can be felt by the diver down to a depth equal to about half the wavelength. For a typical wavelength of 20 m, the surge will be felt down to about 10 m depth. A diver close to the surface may be badly affected by even a moderate swell. In all sea conditions, however, the primary consideration is not whether the diver can work, but whether he can safely exit from the water.
In addition to the risk of being flung against obstruction, the shallow diver may be subject to considerable variations in pressure as the crests and troughs of waves pass overhead. This may affect his decompression if he is carrying out shallow stops in the water and in extreme cases may cause aural or pulmonary barotrauma. Safe maximum sea conditions are hard to define. Many factors such as wave type and wave period, the behaviour of the vessel and bottom topography must be considered. DP vessels may vary considerably in their sea keeping capabilities. Shore line conditions may vary considerably in the amount of water movement due to wave action

### Diving in the proximity of shipping

When diving operations are conducted from or near to an operational vessel this can put diving personnel at additional risk due to hazards specific to the vessels concerned, such as intakes (sea chests), rotating shafts, propellers and other types of thrusters.
Where practicable, effective measures must be taken to prevent diver access to dangerous parts of ships’ machinery. When this is not practicable, effective measures must be taken to stop the movement of dangerous parts of machinery before the diver enters a danger zone, and ensure that they stay motionless until all the diver is confirmed to be clear of the danger zone.
There are times when divers need to operate in or around dangerous components, particularly when the work to be done is on the dangerous component. The operating controls for dangerous underwater components will normally be remote from the hazard zone, so the operators will not be able to see that there are divers nearby and exposed to the hazard. In such cases it is possible for machinery to be activated inappropriately with devastating consequences for any diver in the danger zone.
Before starting the dive operation the possibilities and risks of reconnection should be identified as part of a joint risk assessment, prepared in consultation with the vessel operators, that should then establish how secure isolation can be achieved. In some cases this can be done by simply removing fuses. For other equipment, it may be possible to physically lock an isolating switch or valve in the appropriate position.
Where it is possible to do so safely, it is better to disconnect the power supply, not just the item that controls the power supply.
All relevant sources of energy must be shut off where required, This may include electrical, mechanical, pneumatic, and hydraulic sources. Any residual stored energy may have to be released, such as discharge of capacitors or accumulators, release of hydraulic or pneumatic pressure, or venting. Mechanical blocking with a chock or wedge may also be necessary. Only the supervisor can confirm that the site is secured for diving operations to start. The supervisor must be satisfied that all machinery and equipment that can affect the safety of the diver or the support team has been securely and reliably isolated so that unauthorised start-up or unsafe movement cannot occur.
All isolations must be signed off by the responsible person, and if there is any doubt, the supervisor should personally inspect the isolations, and if appropriate, request the operators to physically test the effectiveness by attempting to activate the machinery with the isolation mechanism in place.
* Wherever a risk of serious or fatal injury from remote start-up of machinery exists, a method of secure isolation must be used. Lock-out procedures are robust and should be used when reasonably practicable
* A permit-to-work system should be used which specifies all the relevant isolations.
* The Vessel Master and Chief Engineer, or if they are not present, their official deputies, and the diving supervisor must be signatories to the permit-to-work.
* All bridge and engine room staff should be briefed on the proposed diving work, and the vessel's public address system should be used to announce the start and completion of diving operations.
* Warning tags indicating that divers are working must be placed at all control points where machinery hazardous to a diver could be energised, and where possible machinery should be locked off. Ideally the diving supervisor should lock out the machinery and keep the keys under his control
* If lock-out is not possible, the diving team should post guards at each potential energisation position for the duration of the dive to prevent inadvertent or inappropriate start-up of dangerous machinery by other persons.
* A checklist should be used to ensure that all required steps are taken and checked by the responsible person.

### Diving in overhead environments and confined spaces

An underwater confined space or overhead environment is one where direct vertical access to the open water surface is obstructed. Underwater confined spaces have characteristics of other confined spaces such as limited number or size of openings for entry and exit, or the diver may have insufficient space to turn around freely.
Typical underwater confined space environments include:
* Sewers, culverts, manholes and shafts
* Flooded caves, mines and tunnels
* Covered reservoirs
* Some underwater dredging and construction sites
* Ice diving
* Dam, canal lock and drydock inlet and outlet ducting, hydro-electric power penstocks etc
* The flooded interior compartments of ships, shipwrecks and marine structures
* The space under a ship, particularly in dry dock
Some hazards associated with underwater confined space diving:
* The primary hazard is the existence of a physical ceiling which restricts direct access to the surface, making the diver completely dependent on properly functioning breathing equipment.
* There may be a flow through the space and a pressure difference across the openings.
* There may be gratings and valves which control flow
* There is a possibility of disorientation, particularly when the space is branched and there are directional changes
* Narrow spaces may make it difficult or impossible to turn around.
* Ropes, lines and umbilicals may be trapped in restrictions and line traps, or damaged by sharp edges
* There are usually hard objects against which the diver's head may be impacted
The use of surface supplied breathing gas reduces the risk of running out of gas and getting lost, but increases the hazard of entanglement and entrapment, and reduces mobility. Use of a helmet reduces risk of impact injury to the head, but reduces field of vision and mobility.

### Diving with differential pressure hazards

All divers should have a basic understanding of the principles and mechanism of injury associated with differential pressure hazards, as this can improve safety by allowing working divers to make informed contributions to the risk assessment process and exercise behavioural risk control. As a general rule intakes are usually more hazardous than discharges, as the pressure difference at an intake will tend to suck the diver into or against the opening, whereas a discharge will blow the diver away from the opening, but this may result in impact with the surroundings, or rapid uncontrolled changes in depth.
Differential pressure, also known as "Delta-P", or suction, occurs where water can move from a region of higher pressure to one of lower pressure. The pressure difference may be the result hydrostatic pressure difference, or a process driven by powered machinery such as pumps or thrusters. When there is no water flow there is no risk, but once flow starts, considerable force can be generated on any object or person in the flow, and if the person blocks the flow the force will reach its maximum.
A pressure difference could occur, as a result of structural failure, the opening of a valve or sluice gate, a diver cutting into a void, or a pump starting.
When the flow is localised about a boundary between the high pressure and the low pressure areas, such as at an opening in a barrier between the two areas or the intake point of a pump, the flow rate can increase considerably over a short distance, and a diver encroaching on the flow from the high pressure (upstream) side may be caught up in the flow with very little warning and be trapped trapped by the pressure difference or injured by impact with the surroundings. Serious or fatal injuries frequently occur in these circumstances.
The force generated depends on the pressure difference and the area on which it is exerted. A large pressure difference can exert a formidable force over even a small area, but a small pressure difference will also exert a large force if the area is large enough. Divers have been killed in depths as shallow as 3 metres.
Four types of differential pressure hazard can be distinguished:
* When water levels between adjoining regions separated by a boundary vary, as at dams and canal or tidal locks;
* Hollow structure containing gas filled voids at lower pressure than ambient water, such as submarine pipelines, other underwater structures with hollow components and around ships;
* When water is drawn through intakes by machinery, such as at cooling water intakes for power stations or at sea chests on ships.
* When water is drawn towards activated propulsors or other types of thrusters on ships.
Incidents involving open propulsors occur and are usually fatal, but the mechanism of injury is significantly different from most other Delta-P incidents and does not involve being trapped or injured directly by the pressure difference or flow, but by traumatic mechanical injury by impact with the propulsor. This type of injury is often caused by activation of a propulsor while a diver is working on or very close to it and there may not be a significant pressure difference involved.
Significant differential pressure hazards are associated with the following diving environments:
* Dynamically positioned vessels (thrusters, propellers and sea chests)
* Pipeline connections (low pressure in the pipe)
* Pipeline stabilisation
* Single point mooring systems
* Pipeline inspection (low pressure in the pipe)
* Ship work (thrusters, propellers and sea chests, cutting into voids)
* Docks and harbours
* Weirs and locks (sluice gates)
* Spillways, reservoirs and dams (sluice gates, outfalls, penstocks)
* Outfalls
* Airlifts and dredges (Mechanical or buoyant pumped flow)
* Unmanned submersibles (Thrusters, propellers)
* Sluices
* Culverts (head differences)
* Gates (?)
* Sewers and stormwater drains (head differences)
Differential pressure hazards can exist in any depth of water, as pumps and ducted thrusters can provide the suction force. Divers usually cannot detect a pressure differential hazard in time to avoid it, and once caught up in the flow it is often impossible to break free even with assistance. It is usually necessary to equalise the pressure difference before a trapped diver can be freed. Attempts by rescuers at the surface to use force to pull a diver free without first equalising the pressure often result in further injuries to the trapped diver, without releasing the diver. Differential pressure incidents are frequently fatal and standby divers who try to free a trapped diver without first equalising the pressure difference are themselves often injured or killed during attempted rescues.

#### Risk assessment

The diving contractor is responsible for ensuring that risk assessment is carried out before the start of any diving operation, and the client is obliged to inform the contractor of all known hazards. The risk assessment must assume that differential pressure hazards may exist whenever:
* water levels between adjoining areas vary;
* water is adjacent to gaseous voids;
* water can be mechanically drawn through intakes; and
* water can be drawn towards activated propulsors or thrusters on ships.
The risk assessment should be made in consultation with competent staff fully familiar with the dive site, such as the client's engineers and reviewed if new information becomes available that indicates the possibility of a previously unknown hazard. Some differential pressure hazards may only arise as a consequence of a structural failure. In such cases the risk assessment should include an assessment of the integrity of the structures at which diving is to take place. Temporary or damaged structures present a higher risk and should not be assumed to be stable without expert assessment.
The area of fast moving water in the vicinity of a hazard which may dut the diver at risk from water flow, suction or turbulence is called the Differential Pressure Danger Zone (DPDZ). There are methods to estimate the size of a DPDZ and the possible strength of the forces, but they are usually either complex or of uncertain accuracy. Large safety margins are generally advisable.

#### Elimination and and control of differential pressure hazards

When suitable and sufficient controls are in place it is possible for divers to work safely in the vicinity of differential pressure hazards. Engineering control measures are preferred as they are inherently more reliable than procedural and behavioural controls. These might include design features that:
* allow pressures to be equalised;
* only require diver intervention from the low pressure side;
* provide redundant valve systems
* prevent diver encroachment into a DPDZ.
Whenever reasonably practicable, hardware controls should be used in preference to ahead of administrative and procedural measures, and in particular, administrative and procedural controls should not be used to justify the absence of reasonably practicable hardware measures. Both types of control measures may be required for acceptable reduction of differential pressure hazards and risks during diving operations.
Examples of failures in control of these hazards show the necessity of:
* assessing the effectiveness of the control measures before the diver enters the water;
* use of robust physical restraints preventing operation of valves and intakes; and
* the physical separation of divers from a DPDZ.
Whenever possible, equalise any pressure differentials before starting diving operations. If it is physically impossible to eliminate the risk from pressure differential situations and there is no way of avoiding the use of a diver to carry out the work, then control the risk by:
* Use of engineering controls to make the differential pressure as small as possible.
* Only diving from the low pressure side.
* Consider using a remotely operated vehicle to do a pre-dive survey.
If diving work on the high pressure side is absolutely unavoidable, conduct a thorough risk assessment and produce detailed procedures for a safe system of work (SSW) in consultation with competent people familiar with the site. Use a permit-to-work system, with lock-out isolation of any plant or machinery necessary to ensure that unsafe re-connection and operation is not possible.
Check from the low pressure side that any valves that must be closed are fully closed and not leaking. Physically test the efficacy of isolations necessary to safeguard the diver. Whenever a closed valve is the main barrier against exposure to an active pressure differential use more than one valve if possible. It may be possible to fit a blank flange to close off a pipe below the valve.
Check that all submarine structures, machinery and seals are fit for purpose and safe to use prior to diving.
Estimate the extent of any DPDZ, and consider if there may be foreseeable circumstances where the size of the zone could exceed the estimated value or suddenly increase during work, such as a partial blockage to an intake grating which would increase flow velocity over the unobstructed part of the opening. Where reasonably practicable, flow velocity should be measured at appropriate locations immediately before diving. Prevent divers from entering a DPDZ by limiting the length of umbilicals, using underwater tending systems, and/or constructing adequate guards or screens. It may be possible to establish an exclusion zone with a suitable safety margin around the danger zone.
If it is unavoidable for divers to enter a latent differential pressure danger zone they must not tamper with seals or other engineering barriers to water flow. Divers and support staff must be provided with all necessary equipment, information and instructions to work safely before starting a diving operation. Only surface supplied diving equipment (SSDE) may be used for diving where significant pressure differential hazards have been identified.

### Night diving

Night diving is underwater diving done during the hours of darkness. The diver can experience a different underwater environment at night, because many marine animals are nocturnal. There are additional hazards when diving in darkness, such as dive light failure. This can result in losing vertical visual references and being unable to control depth or buoyancy, being unable to read instruments such as dive computers and diving cylinder contents gauges, higher risk of entanglement and entrapment, inability to do the planned work, potential separation from the rest of the diving group, boat, or surface team when on un-tethered scuba. Even with a functioning light, these hazards are still present in night diving. Backup lights are recommended, and may be required by CoP or Operations Manual.
Normal requirements for night diving are a dive light, and adequate protection from exposure. Some precautions and skills for night diving include: avoiding shining the light in other divers' eyes, to be aware of and use surface light signals for bearings, and if appropriate to use an illuminated shotline buoy. A surface marker buoy with an attached strobe light or chemical light stick can be used to indicate the position of the divers to the surface team. The use of a strobe light by a diver under water can damage the night vision of other divers and can hinder the diver's own vision.
Diving at night is considered a normal part of occupational diving, though diving during daylight is generally preferred when possible, as the risk is usually lower all other things being equal..
If the diver team surfaces away from the boat or shotline, dive lights can be used to signal the surface team. A surface marker buoy can be illuminated with a dive light at night to increase the visibility of the divers.

### Cold water diving
#### Exposure to cold water during a dive, and cold environment before or after a dive, wind chill.

    Consequence: Hypothermia: Reduced core temperature, shivering, loss of strength, reduced level of conscuousness, loss of consciousness and eventually death.
       Mechanism of injury: Loss of body heat to the water or other surroundings. Water carries heat away far more effectively than air. Evaporative cooling on the surface is also an effective mechanism of heat loss, and can affect divers in wet diving suits while travelling on boats.
       Controls:
      * Diving suits are available that are suited to a wide range of water temperature down to freezing. The appropriate level of insulation for the conditions will reduce heat loss.
      * In extreme conditions and when helium based mixtures are in use as breathing gas, heated suits may be necessary.
      * On the surface, wind chill can be avoided by staying out of the wind, staying dry, and suitable protective clothing.
      * Some parts of the body, particularly the head, are more prone to heat loss and insulation of these areas is correspondingly important.
    Consequence:Nonfreezing Cold Injuries (NFCI).
       Mechanism of injury: Exposure of the extremities in water temperatures below 12 °C (53.6 °F).
          Hand and Foot Temperature Limits to avoid NFCI:
         * Fully Functional 18 °C (64.4 °F) Non Freezing Cold Injury Threshold < Week.
         * 12 °C (54 °F) approximately 3 hours.
         * 8 °C (46.4 °F) for maximum of 30 min.
         * 6 °C (42.8 °F) immediate rewarming required.
       Controls: Protection in order of effectiveness:
      * Heated gloves (hot water suit)
      * Dry gloves attached to drysuit without wrist seal.
      * Dry gloves with wrist seal.
      * Wet suit (neoprene) gloves.
      * Rubberised cloth or leather gloves.
    Consequence:Frostbite
       Mechanism of injury: Exposure of inadequately perfused skin and extremities to temperatures below freezing.
       Control: Prevent excessive heat loss of body parts at risk:
      * Adequate insulation of the diving suit, particularly the gloves and boots.
      * Prevention of wind chill by use of shelters and additional layers of clothing when out of the water.
    Consequence:Muscular cramps
       Mechanism of injury:
      * Inadequate insulation.
      * Reduced perfusion to the legs and feet (occasionally hands) aggravated by excessively tight suit.
       Control: Better insulation and/or suit fit.

### Diving in very low visibility

Experiments on divers working in poor visibility have shown error levels as high as 30% in work involving measurement or inspection. Diver performance is closely related to field dependency. This is a standard psychological measure of the subject’s flexibility in assessing and dealing with a situation. In general, those who performed poorly in the visibility tests had higher field dependency, that is they were less flexible in their approach. In addition to the practical difficulties of bad visibility, some divers may become apprehensive and more likely to react badly in a crisis. Visibility on the surface is also an important consideration. Visibility must be good enough to locate a diver on the surface. It must also be possible to locate and recover any deck crew who may fall into the water. Most installations stop all over-the-side work when visibility is poor. Diving may also be stopped by the master of a vessel if fog or mist significantly increases the risk of collision.

### Diving on offshore structures and installations and from support vessels and platforms
#### Deep diving and helium based breathing gases

There are some hazards which are more common in the offshore environment and in offshore diving operations. There is more diving at extreme depths than in other applications, and the solutions to this bring their own hazards. In order to reduce the risks of compression arthralgia and decompression sickness, saturation divers decompress only once at the end of a tour of duty, but this introduces hazards associated with living under pressure and requiring a long decompression schedule. Helium gas is used in breathing mixtures to reduce work of breathing and nitrogen narcosis, which would make deep diving work difficult or impossible, but the consequences include accelerated heat loss and higher risk of hypothermia, so hot-water suits are used for active warming, but they introduce a risk of heat injuries or hypothermia if something goes wrong with the temperature control system.

#### Exposure to petroleum chemicals

Work on oilfields may result in exposure to crude oil and natural gas components, some of which (such as hydrogen sulphide) can be highly toxic.

#### Hazards of the oil production industry

Much of the diving work involves moving and handling large and heavy objects, and inherently hazardous tools and equipment. These hazards are usually aggravated by the underwater environment.

#### The offshore environment

The inherent problems with offshore evacuation in emergencies like fire or sinking, which are problematic for ordinary crew, are much more difficult to deal with for divers in saturation. The methods of controlling the risks due to these hazards are usually engineering solutions, and are expensive, and often introduce secondary hazards which must also be managed.

#### Diving from dynamically positioned vessels
### Diving near entrapment and entanglement hazards
## The effects of weather, including wind, sea state, and surface visibility, on safe diving operations
### Weather

Weather forecasting terminology
The following are definitions of terms that are used in most English language weather forecasts.

#### Wind
##### Wind direction

    Indicates the direction from which the wind is blowing.

##### Wind becoming cyclonic

    Indicates that there will be considerable change in wind direction across the path of a depression within the forecast area

##### Veering

    The changing of the wind in a clockwise direction e.g. SW to W.

##### Backing

    The changing of the wind in an anti-clockwise direction e.g. SE to NE

#### Visibility
##### Fog

    Visibility less than 1000m

##### Poor

    Visibility between 1000m. and 2 n.miles

##### Moderate

    Visibility between 2 and 5 n.miles

##### Good

    Visibility more than 5 n.miles

#### Gale warnings
##### Gale

    Winds of at least Beaufort force 8 (34-40 knots) or gusts reaching 43-51 knots

##### Severe gale

    Winds of force 9 (41-47 knots) or gusts reaching 52-60 knots

##### Violent storm

    Winds of force 11 (56-63 knots) or gusts of 69 knots or more

##### Hurricane force

    Winds of force 12 (64 knots or more). Note the term is Hurricane Force: the term hurricane on its own is only used to imply a true tropical cyclone in the Caribbean Sea.

##### Imminent

    Expected within 6 hours of time of issue

##### Soon

    Expected within 6 to 12 hours of time of issue

##### Later

    Expected more than 12 hours from time of issue

#### Movement of pressure systems
##### Slowly

    Moving at less than 15 knots

##### Steadily

    Moving at 15 to 25 knots

##### Rather Quickly

    Moving at 25 to 35 knots

##### Rapidly

    Moving at 35 to 45 knots

##### Very Rapidly

    Moving at more than 45 knots

#### Pressure tendency in station reports
##### Rising (or falling) slowly

    Pressure change of 0.1 to 1.5 mbar in the preceding 3 hours

##### Rising (or falling)

    Pressure change of 1.6 to 3.5 mbar in the preceding 3 hours

##### Rising (or falling) quickly

    Pressure change of 3.6 to 6.0 mbar in the preceding 3 hours

##### Rising (or falling) very rapidly

    Pressure change of more than 6.0 mbar in the preceding 3 hours

##### Now rising (or falling)

    Pressure has been falling (rising) or steady in the preceding 3 hours, but at the time of observation was definitely rising (or falling)

### Weather systems

All weather systems are driven by the heat received from the sun. The sun heats the earth's land and sea, which in turn heat the atmosphere. This heated air has a lower density and produces an unstable system, with hot air close to the surface continually rising into the atmosphere. Air pressure near the equator where the air is hotter and less dense is relatively low, and is higher at the poles where it is cold and dry. This pressure difference sets up an overall air flow from the poles to the equator, where the air rises and returns toward the poles at high altitude. This overall flow is complicated by a large number of factors, including the rotation of the earth, ocean currents, the periodic El Niño event, uneven heating of the continental land masses, the greenhouse effect, catastrophic events like large volcanic eruptions and long term effects like changes in solar activity and changes in the tilt of the earth. These effects are generally more pronounced in the Northern hemisphere due to the larger continental area At present, the general circulation is driven by alternating bands of high and low pressure which give the familiar pattern of prevailing winds:
* Easterlies in the polar regions.
* Westerlies in the temperate region
* Easterlies in the tropical regions.
Because of their importance in world trade, the tropical easterlies which carried European sailing ships across the Atlantic, are still referred to as the trade winds. There are also two calm zones, the Doldrums at the equator and the warm sunny high pressure areas on the edges of the tropics.

#### South Atlantic

Superimposed on this pattern are the regional and seasonal weather systems: depressions, anticyclones, monsoons, hurricanes, tornadoes and all the winds and fogs that are governed by local conditions. Winds blow into low pressure systems and out of high pressure systems, but are deflected by the rotation of the earth (Coriolis effect). In the southern hemisphere, if you stand with your back to the wind, the centre of low pressure is to your right, in the northern hemisphere it is to your left. Most of the bad weather in temperate regions is caused by lows or depressions, which are compact and mobile low pressure systems. A typical temperate zone depression has a diameter of about 1600 km (1000 miles) Associated with most depressions are warm and cold fronts. A warm front is the leading edge of a mass of relatively warm air, a cold front is the leading edge of a mass of relatively cold air. The temperature difference between the warm and cold air masses may only be a few degrees. An approaching depression, with warm and cold fronts, shows a well defined sequence of weather:
* High cirrus clouds are driven ahead of the storm by high altitude winds. The sky remains clear and visibility is often exceptionally good. Pressure begins to fall slowly. There may also be the onset of a long swell, originating from the storm.
* The clouds thicken and become lower. Initially, the cloud layer is translucent and there is often a halo around the sun or moon.
* The wind freshens and backs. There is a slow temperature rise, which may only be noticeable with a thermometer. Wave height increases.
* The clouds become low and dense and steady drizzle, rain or snow starts to fall.
* As the warm front passes, the wind slackens and veers. The rain or snow decreases or stops and
* the clouds become higher and thinner. Visibility is generally poor.
* As the cold front approaches, the wind backs and becomes squally. Clouds thicken and become heavy and towering cumulo-nimbus.
* The passage of the cold front is characterised by squally, unstable conditions, cumulo-nimbus clouds, heavy rain, sleet or snow showers and sometimes thunder.
The rate at which these changes occur depends on the speed at which the depression is moving. If the centre of the depression passes directly over the worksite, pressure will fall and then rise again and there will a be period of calm as the centre passes over. The pressure is unlikely to fall lower than about 980 mb in the centre. Wind speeds around a depression are typically 40-50 knots. Tropical Cyclones are small areas of low pressure with very high wind speeds that form only over warm seas, where there is a layer of warm moist air close to the surface. The typical Cyclone has a diameter of only 160 km (100 miles), and a pressure of 950 mb in the centre. The centre of the Cyclone is calm and wind speeds around the cyclone are 100 knots or more. High pressure systems are stable and slow moving with clear skies and low wind speeds. In summer they are typified by bright sunshine and calm seas. In winter the temperature is low because of the absence of insulating cloud cover and there is often fog or poor visibility. Pressure may be up to 1010 mb.
Wind strength and the Beaufort scale :
| Beaufort Scale | Wind speed (knots) | Description |
| 0 | 0 | Calm, sea like a mirror. |
| 1 | 1 - 3 | Light air, ripples only |
| 2 | 4 - 6 | Light breeze, small wavelets (0.2m), crests have a glassy appearance. |
| 3 | 7 - 10 | Gentle breeze, large wavelets (0.6m), crests begin to break |
| 4 | 11 - 16 | Moderate breeze, small waves (1m), some white horses |
| 5 | 17 - 21 | Fresh breeze, moderate waves (1.8m), many white horses. |
| 6 | 22 - 27 | Strong breeze, large waves (3m), probably some spray. |
| 7 | 28 - 33 | Near gale, mounting sea (4m) with foam blown in streaks downwind |
| 8 | 34 - 40 | Gale, moderately high waves (5.5m), crests break into spindrift |
| 9 | 41 - 47 | Strong gale, high waves (7m), dense foam, visibility affected. |
| 10 | 48 - 55 | Storm, very high waves (9m), heavy sea roll, visibility impaired. Surface generally white. |
| 11 | 56 - 63 | Violent storm, exceptionally high waves (11m), visibility poor. |
| 12 | 64 and over | Hurricane force, 14m waves, air filled with foam and spray, visibility bad |
| Wave heights given are those that might be expected in the open sea and there is usually a time lag between the increase in wind speed and the increase in wave height. In enclosed water, the waves will normally be lower, but steeper. |||

### Local weather
#### Thunderstorms

Thunderstorms may occur in the cold sector of a depression or may develop locally during warm weather. The development occurs when a mass of air is heated from below. Powerful convection currents are established and the air mass becomes turbulent. A pre-thunder sky is characterised by dense cirrus cloud, associated with banks of attocumulus and sometimes cumulus. Thunder clouds have a huge vertical development with a rapid development and expansion at higher altitudes. The top of the cloud is often blown out into a characteristic anvil shape by high altitude winds. Below the thunder clouds are squalls, violent gusts of wind and heavy falls of rain or hail. The thunder clouds typically collapse quickly, to be replaced by others. A thunder sky shows a confused mass of dense clouds, building, collapsing and re-forming.

#### Land and sea breezes

Close to land, winds may be influenced by the difference in heating between land and sea. During the day, the land heats more rapidly than the sea, the air heats and rises and cool air is drawn in from the sea. At night, the land cools but the sea retains its heat. Air sinks over the land and is flows out to sea. Onshore winds during the day and offshore winds during the night are typical in otherwise clear, stable conditions.

#### Berg winds

A berg wind is a hot, dry wind that blows from the interior of South Africa to the coast and usually is blustery. It can be mild at times blowing at about 10km/h, but sometimes it can be really strong and may gust up to 100km/h causing some structural damage to buildings and uprooting trees. Berg winds usually occur when a strong high pressure exists south or south-east of Southern Africa and a high pressure is also situated over the continent. These conditions usually occur in winter, so berg winds are more common in autumn, winter and spring. Since air in a high pressure warms up as it descends the off-shore winds will be warm to hot and the temperature will commonly rise about 10 degrees Celsius from the interior of SA to the coast. At the same time a coastal low will develop along the coast. Off-shore flow ahead of the coastal low is usually easterly to north-easterly in direction along the west coast and north-westerly along the east coast. Humidity is usually very low. Behind the coastal low the wind is on-shore and usually north-westerly to south-westerly in direction. It is cool and moist and often associated with fog. The coastal low moves down the west coast and around the Cape Point and then up the east coast of South Africa until it fills up near Maputo in Mozambique. The berg winds follow the same pattern. Berg winds are usually followed by a cold front in winter

#### Fog

Fog may form when warm air blows over a cold surface. This can occur along a coast or where warm and cold ocean currents meet. Fog may also form when air cools below dew point at night.

#### Surface visibility
### Diving in contaminated water
### Animal hazards

(Including dangerous marine and aquatic animals)