Saffir-Simpson Hurricane Wind Scale Saffir-Simpson Hurricane Wind Scale Climatology | Names | Wind Scale | Extremes | Models | Breakpoints The Saffir-Simpson Hurricane Wind Scale is a 1 to 5 rating based on a hurricane's sustained wind speed. This scale estimates potential property damage. Hurricanes reaching Category 3 and higher are considered major hurricanes because of their potential for significant loss of life and damage. Category 1 and 2 storms are still dangerous, however, and require preventative measures. In the western North Pacific, the term "super typhoon" is used for tropical cyclones with sustained winds exceeding 150 mph. Category Types of Damage Due to Hurricane Winds 1 64-82 kt 119-153 km/h Very dangerous winds will produce some damage: Well-constructed frame homes could have damage to roof, shingles, vinyl siding and gutters. Large branches of trees will snap and shallowly rooted trees may be toppled. Extensive damage to power lines and poles likely will result in power outages that could last a few to several days.

2 83-95 kt 154-177 km/h Extremely dangerous winds will cause extensive damage: Well-constructed frame homes could sustain major roof and siding damage. Many shallowly rooted trees will be snapped or uprooted and block numerous roads. Near-total power loss is expected with outages that could last from several days to weeks. 3 How to Measure Hurricane Strength: Saffir-Simpson Scale | The Old Farmer's Almanac Average: 3.2 (92 votes) The Saffir-Simpson Hurricane Scale is a 1-5 rating based on the hurricane's present intensity. This hurricane scale is used to give an estimate of the potential property damage and flooding expected along the coast from a hurricane landfall. Wind speed is the determining factor in the scale, as storm surge values are highly dependent on the slope of the continental shelf in the landfall region. Wind speeds are measured using a 1-minute average. Note:The Saffir-Simpson Hurricane Wind Scale (SSHWS) was updated in 2012 to accomodate number rounding issues.

Category Greenfieldgeography - Measuring Disasters Measuring Disasters Export (PDF) Measuring Disasters Most hazard events are measured on some kind of scale to give them a quantifiable outcome. Some scales measure only one variable and are extremely objective e.g. the Richter Scale which measures the amount of seismic energy released in an earthquake, while others measure several variables e.g. the Saffir-Simpson scale which looks at both wind speed and height of storm surge and others have a more subjective nature e.g. the volcanic explosivity index which includes visual observations. Below is a summary of some of the most common scales for measuring disasters: The Richter Magnitude Scale The Richter scale was developed by Charles Richter in partnership with Beno Gutenberg in 1935. Both worked at the California Institute for Technology and designed to measure differences in local earthquakes. The Richter Scale is base 10 logarithmic scale e.g. a 5.0 magnitude earthquake is ten times larger than a 4.0 magnitude earthquake. The magnitude of an earthquake is calculated the amplitude of waves. This is measured on a seismograph. The largest earthquake ever recorded was a 9.5 in 1960 in Chile.

The devastating earthquake that caused the 2004 Indian Ocean tsunami was a 9.2 and the recent earthquake (2011) that caused the Japanese tsunami was a 9.0. The USGS records the magnitude and location of most of the world's earthquakes. The Mercalli Intensity Scale The Mercalli Scale was based on the Rossi-Forel Scale. It was developed by the vulcanologist Giuseppie Mercalli who updated the Rossi-Forel Scale in 1884 and 1906. It differs from the magnitude of the Richter Scale because it measure the intensity of an earthquake in terms of its impacts and recognises that magnitude is not the only factor in determining this. It also considers other factors like depth of earthquake, terrain and population density. Because the Mercalli Scale looks at a numbers of variables a weaker earthquake on the Richter Scale might have a higher Mercalli Scale than a stronger earthquake on the Richter scale.

For example: Japan Tohoku earthquake of 2011 (deaths = 18,500 - including deaths from subsequent tsunami) Richter Scale = 9.0 Doctors per 1000 of population: 2.3 Healthcare expenditure per capita: $6,096.2 Infant mortality: 6.81 Aid Government initially declined aid. First aid flights arrived on 7th May but rescuers found it hard to obtain visas. UK biggest single donor with about $33.5 million. The UN requested a total of $841 million in aid to help victims, but only about $190 million was ever collected from donor countries. US also delayed asking for international help. Officially they asked the EU for help one week after the disaster. Most relief is in the form of assistance e.g. tents, food, blankets and not money. Cuba did offer to send over 1,500 doctors, but this offer was declined. In total it is estimated $854 million in aid was offered, but up to half was never taken. N.B. All data comes from , or .

For the US I have used the nearest available data to 2005 and for Myanmar the nearest available data to 2008. In terms of magnitude Hurricane Katrina (category 5) was actually slightly stronger than Cyclone Nargis (category 4). However, in terms of deaths and damage Cyclone Nargis was actually far more devastating. The information in the table is statistical evidence that helps support some of the reasons below of why Nargis was more devastating than Katrina: Education: Even though levels of primary enrollment are the same in the US and Myanmar, the overall literacy rates are much higher in the US. It is not only important to be able to read and write so that you can understand the risks posed by hazards and how to minimise the risks, but it is also important that as a population you are educated about hazards by the government. The US local and national government is good at educating its citizens about how to prepare and react to natural disasters. They education people in terms of evacuation plans and also educate rescuers on how to save lives and rescue people. Transport: Car ownership is much high in the US than Myanmar (over 100 times higher). The US also has a much more extensive road network.

In fact in the Irrawaddy Delta where Nargis hit there are virtually no roads and the main form of transport is by boat. The US also has a much bigger air network with established airlines like United, Delta, Continental and American. This means that people in the US were much better able to evacuate the oncoming hurricane and also rescue teams were more easily able to reach victims, thus reducing the overall number of casualties. It also allows water, food, tents, medical supplies to reach affected areas. In times of flooding helicopters are also important - the US who spends more money on its army than any other country in the world has an abundance of helicopters. Communications: Internet and phone usage is extremely low is Myanmar, where they are both over 70% in the US. Good communications are vital in reducing casualties because it allows people to receive warnings and updates of approaching hazards as well as call for help if they have been impacted by hazards. Again the Irrawaddy Delta is very remote and will probably have even lower internet and phone ownership than the rest of Myanmar. Healthcare: The US spends a lot more money on its healthcare than Myanmar and has significantly more doctors per person than Myanmar.

Therefore it is in a much better position to treat victims of natural disasters and in a better position to reduce many of the secondary hazards like disease common with natural disasters like Nargis and Katrina. Level of Development: The US is an MEDC and has the largest economy in the world. Myanmar is considered by most to still to be an LEDC. Because of the US's level of development it has much more money to prepare for natural hazards and a lot of money to recover from them. Countries like Myanmar often become reliant on aid in times of natural disasters - something that they were actually slow to accept. Aid: Both countries were slow to accept aid and the US even declined aid including the offer of over 1500 doctors from Cuba. However, Myanmar's delay in accepting aid and actually declining visas to maid aid workers was much more significant because they were much less able to help themselves. By not accepting aid more victims would have died because of their initial injuries and more people would have suffered from secondary hazards like exposure and disease.

Building Design: Most people living in the Irrawaddy Delta are poor farmers and fishermen who nearly all live in temporary structures made out of things like plastic, wood and reeds which are unable to withstand the strong winds and rain associated with cyclones. On the other hand the US has strict guidelines about building regulations and although many buildings did flood in New Orleans most stood up to the winds associated with Katrina. Defences: Even though many of the levees protecting New Orleans failed and caused significant flooding, there were actually many other levees and coastal defences that held and reduced the impact of the hurricane. In the flat Irrawaddy Delta there were virtually no defences and even many of the regions natural defences like mangroves had been removed making the region more vulnerable and increasing the number of victims. Insurance: Most people and businesses will have insurance in the US. This means that they can afford to live in temporary accommodation if their house has been damaged and afford for totally to be repaired. Businesses can also claim for lost income and repair any damage. This means that people are able to recover much quicker and are less likely to suffer from secondary hazards and future primary hazards.

In the Irrawaddy Delta in Myanmar most people would never of heard of insurance, let alone have it, meaning that they have to rebuild their houses from temporary materials and will lose significant income which may lead to secondary hazards like famine, exposure and disease. So even though the Hurricane Katrina was one of the US's worst ever natural disasters, its impacts were not as great as Cyclone Nargis because of peoples education and the country's transport, communications and healthcare networks as well as the US's higher level of development and more stringent building regulations and defences. Cyclone Nargis - Special Reports - BBC Temporal Changes that have affected the impact of hazards As well as spatial variations (i.e. between countries and regions) in the impact of hazards, there are also factors that have changed overtime (temporal). Some of these factors have increased the impacts of hazards, while others have reduced the impact. Below is a summary of the temporal changes that have changed the impact of hazards: Growing population: The world's growing population (now $7 billion) means that there is a greater risk of drought and famine because of demand on resources, It also means diseases can spread more easily and more people live in hazardous areas e.g.

near volcanoes, by rivers or on the coast. Frequency and magnitude of hazards: The number of hazards are increasing because more people live in hazardous areas, but the frequency of hazards are also increasing because of things like climate change, deforestation and urbanisation. Things like climate change are also increasing the magnitude of hazards like drought, floods and hurricanes. Urbanisation: More and more people are moving from rural areas to urban areas. This can mean people have better access to healthcare and education which can reduce the impacts of hazards, but it can also increase the impact of hazards because people are living in higher population densities which can aid the spread of disease and mean more people are affected by earthquakes and hurricanes that strike urban areas. Building design: Generally building design has steadily improved and become more resistant to earthquakes, hurricanes etc. This means that less people are impacted by some hazards. However, building design can also create false security. People might believe there building is safe, bur in reality building codes have not been followed and it is still vulnerable e.g. buildings in Vam during recent Turkey earthquake. Education: With improvements in education people are now more aware of hazards and know how to prevent them.

For example, they know how to improve personal diet and hygiene to reduce disease and also how to make buildings safe against hurricanes and what to do if a tsunami is approaching Healthcare: Ongoing improvements in healthcare have not only met less people are impacted by diseases, they have also meant more people can survive injuries caused by hazards e.g. broken limbs, infections, etc. Hazard prediction: Organisations are now getting much better at predicting the location and or/track of hazards. Therefore organisations can give countries and communities greater warning to either evacuate and prepare. In years gone by, people may have no idea that a hurricane or flood is approaching. Transport and communication: With improvements in communications people now hear about approaching hazards, but also hear about how to prepare and make themselves safe. They can also alert family or rescue services if they have been affected by7 a hazard. In addition im proved transport means that people can escape hazards more easily and also rescue attempts can be made e.g. helicopters can rescue flood victims. Preparedness: Countries and communities now tend to be a lot more prepared, not only in terms of building design and hazard mapping, but also in terms of evacuation drills, medical training, rescue teams, warning sirens, etc.

Metrics and Measures - SAMATE Metrics and Measures 4.5 Aspect Measured Metrics vs. Measures The terms metric and measure have some overlap. We use measure for more concrete or objective attributes and metric for more abstract, higher-level, or somewhat subjective attributes. For instance, lines of code (LOC) is a measure: it is objective and concrete. (Unfortunately LOC varies wildly for different implementations of the same algorithm [ Schofield 2005 ]. Function points (FP) are much better.) Robustness, quality (as in "high quality"), and effectiveness are important attributes that we have some consistent feel for, but are hard to define objectively. Thus these are metrics. Measures, such as faults/FP, are bases for metrics. Measures help us approximate less tangible metrics. We published these definitions in Paul E. Black, Karen A. Scarfone, and Murugiah P. Souppaya, Cyber Security Metrics and Measures , Wiley Handbook of Science and Technology for Homeland Security, 2008. The above definitions are not incompatible with those in the International Vocabulary of Metrology ... (VIM) 3rd edition, 2012. Section 2.1 is the following definition.

measurement process of experimentally obtaining one or more quantity values that can reasonably be attributed to a quantity In the Sept 1999 version of "Methodology for Evaluation of Collaboration Systems", Section 5 Metrics and Measures lays out the difference quite clearly but assigns exactly opposite meanings: "Metrics ... are directly measurable and can often be collected automatically. ... Measures are derived from interpretations of one or more metrics." Measurement Scales When we talk about metrics and measurements, we usually think about those for mass (gram) and time (second). We can add, compare, and average values. But other scales can be useful. In the taxonomy proposed by Stevens, the most basic "scale" is nominal, which is just classification. VIM calls them "nominal properties" (1.30). Some nominal properties are gender, feelings (happy, serious, angry), language (C, Java, Ada), and ISO two-letter country code. Nominal properties may be hierarchical. For example, Italic, Celtic, and Germanic are all Indo-European languages , and computer instruction sets are RISC or CISC. An ordinal scale is a nominal scale that has a natural order.

For instance, weather reports give hurricane intensity on the Saffir-Simpson hurricane scale a discretization of wind speed. The Mohs scale grades relative hardness from 1 (talc) to 10 (diamond). The scale is not uniform: diamond (hardness 10) is 4 times harder than corundum (hardness 9) and 6 times harder than topaz (hardness 8). An interval scale has equal distance between values. Values on an interval scale cannot be added: it makes no sense to ask what is the sum of 4 July 1776 and 23 December 1805. The number of days between them (difference) is reasonable (10 763 days). A ratio scale is an interval scale with a zero. 20 K (degrees Kelvin) is twice as hot as 10 K. (20°C is not twice as hot as 10°C: Celsius is an interval scale.) Considerations of Stevens' Taxonomy Some scales naturally report in logarithmic terms. The Richter scale was a logarithmic scale for earthquakes, which measured the size of earthquake waves, not an earthquake's total energy.

Relative sound energy is often measured with a logarithmic unit, the bel . Wikipeida's Level of measurement explains what statistics make most sense with what scales. However, transforming data from one scale to another may help. Suppose an uncalibrated thermometer has a non-linear (monotonic) readout. As is, the data is on an ordinal scale, but with experiments, we can transform the data into Kelvin, a ratio scale. Linear scales may not always fit well. Color is better described in three-dimensional models, such as the hue, value, and chroma in the Munsell color model . Note there are many other color models . VIM uses color as an example of a nominal scale. Percentages and probabilities are real numbers bounded by zero and one. Measurement Unit Good metrics depend on good measurement units. As mentioned above, lines of code (LOC) is a seductive measure: it is objective and easily understood. Since LOC varies wildly between different implementations for the same functionality [ Schofield 2005 ], it is doubtful that high level measurements can be based on it.

However, faults often occur at a specific rate per LOC (in addition to higher level errors in design, protocol, etc.) Function points seems to map much better to total intellectual effort. A unit of specification might be "shalls", that is, single statements of requirement. One "shall" yields very roughly 25 lines of code. This is similar to Mosaic, Inc.'s Testable Requirements . The Objects of Measurement There are at least three related, but distinct, entities that might be measured: Algorithms, Programs, and Executions. An algorithm is the abstract thing. For instance, one can analyze a cryptographic protocol (algorithm) to show that it is not vulnerable to cryptographic attack such as replay, man-in-the-middle, chosen plaintext, etc. Similarly, one can check that a quicksort algorithm is correct. Why even bother implementing an algorithm if the algorithm is flawed? Any (faithful) implementation will have the flaws of the algorithm, plus any flaws introduced in the implementation. Algorithms must be instantiated in some form: they must be rendered in some form. We do not (and cannot?) have pure representation. Executable instantiations are programs, source code, implementations, etc. Acceptance and post-development analysis must be at this level.

The algorithm is inferred from the program. It is not clear whether byte code or binaries should be included with programs above or whether they should be a separate category. On one hand, they are below the programming language abstraction and are not subject to compilation (or interpretation). On the other hand, they are still static globs of bits that must be executed according to some semantics by some engine, whether a virtual machine, a processor, or something else. Finally one may check an execution. What did the program actually do? (Dynamic) tests apply to executions, not programs, strictly speaking. Executions include the system, such as library versions, file system permissions, and operator input. It follows that there should be measures or metrics that apply to algorithms, programs, or executions. Design One might argue that the highest level of a system, above algorithms, is actually design. If so, there must be metrics for designs, too. Requirements, Specifications, and Constraints There is another class of entities: requirements, specifications, constraints, etc. These might be distinguished from the above in several ways. They are not (practically) executable. They are higher level or have some orthogonal or cross-cutting view to clearly express certain aspects.

From this it follows that they are often incomplete. For instance, we may specify that a protocol has liveness and safety, but that is far from completely describing what it should do. These might be analyzed for, say, consistency and some completeness, examined with regard to use cases, and so forth. So these also may be included as objects of measurement. Hardware vs. Software Is there a qualitative difference between a VCR and a web server program? Both are designed by people; both run on hardware; both have outputs to and inputs from the outside (including users); both serve certain purposes. But we tend to think of a VCR as hardware, with some software, and a web server as software, running on hardware. Some function may be carried out by hardware, some by software, and some by a combination. A "black box" system is likely to have both, and some functionality may depend on both. For example, security may depend on encryption, which may be done either by a program or a special computer chip. Classifying Metrics and Measures For completeness and comparison, it is convenient to classify metrics and measures. Intrinsic vs. Relative Some properties are intrinsic to target of measurement while other properties are more properly relationships relative to the operational environment.

For instance, the number of lines of code (LOC), the number of function points (FP)in a program or the computational complexity of an algorithm depend only on the thing itself, not the environment in which it is run. (A model of execution is given or assumed for most artifacts, be it C semantics, a Java Virtual Machine, or an instruction set architecture.) In contrast a vulnerability, or exploitable weakness, cannot be determined without knowing (or assuming) an operational context. A buffer overflow weakness is not a vulnerability if it comes from the configuration file which is (assumed to be) only editable by the (trusted) operator. Basic or Primitive vs. Computed or Derived This is similar to the measure/metric dimension. Static vs. Dynamic Lines of code can be counted statically. Number of tests failed can only be counted dynamically - by running the program. Objective vs. Subjective An objective measure typically can be automated and is more repeatable than subjective measures. Objective measures are often easier to model and analyze to establish a theoretical basis.

Aspect Measured New modification of Saffir-Simpson hurricane wind scale New modification of Saffir-Simpson hurricane wind scale in categories Hurricanes , Severe weather The Saffir-Simpson Hurricane Wind Scale (SSHWS) is undergoing a minor modification in 2012 to resolve "awkwardness" associated with conversions among the various units used for wind speed in advisory products. The new modification is to help clarify categorization of wind speed measurements based on miles per hour (mph), kilometers per hour (km/h) and knots. There is an inherent uncertainty when estimating wind speeds for tropical cyclones. Wind speeds are basically rounded because it's to hard to identify exact wind speeds. The change broadens the Category 4 wind speed range by one mile per hour (mph) at each end of the range, yielding a new range of 130-156 mph. This change does not alter the category assignments of any storms in the historical record, nor will it change the category assignments for future storms according to NHC. (The reasoning behind this change and a tabulation of the old and new scales is available here ).

The new categories are shown in this NHC graph below: The Saffir-Simpson Hurricane Wind Scale is a 1 to 5 categorization based on a hurricane's intensity at an indicated time. The classifications can provide some indication of the potential damage and flooding a hurricane will cause upon landfall. It classifies hurricanes that exceed the intensities of tropical depressions and tropical storms — into five categories distinguished by the intensities of their sustained winds. To be classified as a hurricane, a tropical cyclone must have maximum sustained winds of at least 74 mph (33 m/s; 64 kn; 119 km/h) (Category 1). The highest classification in the scale, Category 5, is reserved for storms with winds exceeding 156 mph (70 m/s; 136 kn; 251 km/h). Officially, the Saffir–Simpson Hurricane Scale is used only to describe hurricanes forming in the Atlantic Ocean and northern Pacific Ocean east of the International Date Line. Other areas use different scales to label these storms, which are called "cyclones" or "typhoons", depending on the area.

Most weather agencies use the definition for sustained winds recommended by the World Meteorological Organization (WMO), which specifies measuring winds at a height of 10.1 m (33 ft) for 10 minutes, and then taking the average. By contrast, the U.S. National Weather Service defines sustained winds as average winds over a period of one minute, measured at the same 10.1 m (33 ft) height. Central pressure and storm surge values are approximate and often dependant on other factors, such as the size of the storm and the location. It is not uncommon for a pressure to be significantly higher or lower than expected for a specific category. Generally, large storms with very large radii of maximum winds have the lowest pressures relative to its intensity. After the series of powerful storm systems of the 2005 Atlantic hurricane season , a few newspaper columnists and scientists brought up the suggestion of introducing Category 6, and they have suggested pegging Category 6 to storms with winds greater than 174 or 180 mph (78 or 80 m/s); 150–155 knots (280–287 km/h).

Only a few storms of this category have been recorded, most were West Pacific and titled as super typhoons, most notably Typhoon Tip in 1979 with sustained winds of 190 mph (310 km/h). According to Robert Simpson, there are no reasons for a Category 6 on the Saffir-Simpson Scale because it is designed to measure the potential damage of a hurricane to manmade structures. If the wind speed of the hurricane is above 155 mph (249 km/h), then the damage to a building will be "serious no matter how well it's engineered". (Wikipedia) Featured image: Hurricane Earl along the North Carolina Coast (NASA image courtesy Jeff Schmaltz, MODIS Rapid Response Team at NASA GSFC using data courtesy of the University of South Florida) Share: Hurricanes, Typhoons, Cyclones | UCAR - University Corporation for Atmospheric Research UCAR - University Corporation for Atmospheric Research Background on the science, people, and issues involved in hurricane research Frequently asked questions Hurricane links Why do hurricanes happen? Most hurricanes start life as areas of rough weather and thunderstorms in the tropics.

Many of these disturbances, or tropical waves, produce little more than heavy rain and gusty winds. But if a tropical wave succeeds in spinning into a complete circle of winds rotating around an area of low air pressure at its center, it's given the name tropical depression. When a depression's peak sustained winds reach 39 miles per hour (34 knots), it's called a tropical storm. As a tropical system strengthens, its winds spiral inward, concentrating moisture near the center. This spiraling, a result of Earth's rotation, can't happen near the equator. To benefit from the curving winds produced by the Coriolis effect , a storm needs to be at least 300 miles (500 kilometers) north or south of the equator. When a tropical storm maintains wind speeds of at least 74 miles per hour (65 knots), it's known as a hurricane, at least in North and Central America (see What's the difference between a hurricane and a typhoon or tropical cyclone? ). Each year, hundreds of tropical depressions spin up over warm waters worldwide. But more is needed for a depression to grow into one of Earth's most powerful storms.

Besides warm ocean temperatures, the depression needs lots of warm, moist air to feed on. Winds also matter. Strong differences in wind direction and strength at different heights in the atmosphere, known as wind shear, can pull a tropical system apart, dissipating its energy. So low wind shear is also essential to tropical storm formation. (But note that high wind shear plays a role in tornado formation, including tornadoes that form over land as a hurricane dissipates .) For more about the life cycle of tropical storms, from their birth as tropical depressions, to full-blown hurricanes or cyclones, to their ultimate demise, see Hurricanes & Tropical Cyclone Life Cycles (from UCAR COMET's Hurricane Strike!). back to top What's the difference between a hurricane and a typhoon or tropical cyclone? When a tropical disturbance organizes to the point where its sustained winds top 34 knots (39 miles per hour), it's known as a tropical cyclone . But various parts of the world use a variety of terms once a tropical cyclone packs winds of at least 65 knots (74 mph) in a particular " basin " (an oceanic region where these storms occur). Around North and Central America, they're called hurricanes.

The god of evil for the Carib people was named Hurican, according to the authors of Hurricane Strike! That's the source, with a slight twist in spelling, of the name used in the Atlantic Ocean, Caribbean Sea, Gulf of Mexico, and Northeast Pacific Ocean. In the Northwest Pacific, the same powerful storms are called typhoons. In the Southeastern Indian and Southwest Pacific Oceans they're called severe tropical cyclones. In the North Indian Ocean, they're called severe cyclonic storms, while in the Southwest Indian Ocean, they simply keep the name tropical cyclone. Hurricanes that make Category 3 status on the Saffir-Simpson Hurricane Wind Scale (winds of at least 96 knots or 111 mph) are labeled intense hurricanes. If a typhoon hits 132 knots (150 mph), it becomes a supertyphoon. When a system weakens below hurricane strength, it is typically reclassified as a post-tropical cyclone. This was the case with Hurricane Sandy in 2012 just before landfall.

In order to reduce confusion, the National Weather Service introduced a new policy in 2013 in which hurricane warnings may be continued even after a storm has technically become post-tropical. back to top When is hurricane season? Hurricane season in the Atlantic Basin (including the Caribbean and the Gulf of Mexico) runs from June 1 through November 30, with most activity clustered from August to October. The typhoon and cyclone seasons follow their own patterns. In the Northeast Pacific, the official season runs from May 15 to November 30. In the Northwest Pacific, typhoons are most common from late June through December. The North Indian Ocean sees cyclones from April to December, with peaks in May and November. The Southwest Pacific and South Indian oceans (including the waters bordering Australia) get most of their activity from November to May. If the conditions are right, tropical cyclones can develop outside their official seasons, especially in the Northwest Pacific, where they occur year round. NOAA's Hurricane Research Division has more about seasonal timescales . back to top How big and how strong can hurricanes get? Whatever you call them, these monster storms are the most powerful atmospheric phenomena on Earth.

Hurricanes gather energy from water vapor in the atmosphere stretched for hundreds of thousands of square miles across the warm ocean water of the tropics. The storms themselves can bring heavy rains and gale-force winds blowing 38 to 73 miles per hour (62–117 kilometers per hour) across an area the size of Louisiana. Hurricane-force winds can extend 50 miles (80 km) or more from the storm center. How big? According to NOAA's National Hurricane Center , the average hurricane eye —the still center where pressure is lowest and air temperature aloft is highest—stretches 30 miles (48 km) across, with some growing as large as 120 miles (200 km) wide. The eye typically shrinks as a hurricane intensifies, sometimes narrowing to less than 10 miles (16 km) in width. Eventually, a new eye may form around the old one; hurricanes often weaken during this transition but can intensify again as the new eye contracts. How strong? Hurricane intensity is categorized on different scales around the world.

The strongest storms, equivalent to Category 5 on the Saffir-Simpson scale , have sustained winds that exceed 155 miles per hour (135 knots). In order to grow, hurricanes need plenty of warm, moist air to form showers and thunderstorms, along with winds that don't change direction much with height, which allows the central circulation to develop undisturbed. If these atmospheric conditions are right, then hurricane strength is dictated largely by the presence or absence of deep, warm ocean water (ideally 79°F or warmer [26°C or warmer]). As winds strengthen, more water evaporates, releasing energy stored in the warm seas. Using ocean data, scientists can estimate the maximum potential intensity (MPI), or the peak hurricane strength one might expect at a given location. Scientists are now exploring how best to calculate and predict MPI in computer models. Size does not determine intensity. Some of the most destructive hurricanes to hit the U.S. coastline, including Hurricane Andrew in 1992, extended over a relatively small area. Whatever their size or strength, long-lived hurricanes normally "spin down" after they come ashore. The land surface creates drag and the oceanic heat source disappears.

The high winds gradually weaken and spread over a larger area. Even as they decay, hurricanes can still bring damaging winds more than 100 miles inland. Also, the frictional effects on surface winds can help create the wind shear needed to spawn tornadoes. One hurricane can produce dozens of tornadoes as it moves ashore. But the worst hurricane damage is often the result of a storm surge that causes coastal flooding . Along parts of the Mississippi coast, the surge from Hurricane Katrina was 28 feet or more above mean sea level, putting it among the highest surges ever recorded in the United States. Large waves on top of a storm surge cause even more damage. Surge risk varies depending on the strength and structure of a given hurricane as well as geography and tidal cycles along the coast where it strikes. Although Superstorm Sandy fell just below hurricane strength several hours before its center reached southern New Jersey in October 2012, Sandy was an extremely large system, and it struck near high tide. As a result, the storm pushed huge amounts of water into the New York and New Jersey coastlines, producing a catastrophic surge that set records in several places. Originally, the Saffir-Simpson scale included typical storm surges expected for each category.

In the case of Sandy and several other recent hurricanes, storm surges have been higher than the levels once assigned to each Saffir-Simpson category. The categories have been revised so that they now refer only to wind speed, and the National Hurricane Center is moving toward new ways of depicting the threat from storm surge, including storm surge watches and warnings that will be distinct from hurricane watches and warnings, as described in this 2012 article . Storms that move inland often bring much-needed rain that farmers and water managers count on. Recent studies indicate that across parts of the U.S. Deep South, up to 15% of all warm-season rainfall typically comes from hurricanes and tropical cyclones. But if too much falls at once, the rain can quickly overwhelm stream and river beds, producing serious river flooding . back to top Can we control hurricanes? Even smaller hurricanes pack a mind-boggling amount of power. The heat energy released by a hurricane equals 50 to 200 trillion watts—or about the same amount of energy released by exploding a 10-megaton nuclear bomb every 20 minutes.

We are unlikely to come up with methods to control such overwhelming natural power for the foreseeable future—though that has not kept people from speculating about what it would take . Researchers instead focus on understanding every aspect of hurricane structure and behavior, with the hope their work will lead to better predictions of storm tracks and intensity so warnings can be issued to protect life and property. The societal side of the equation includes communication between forecasters and emergency managers who make decisions about when and where to call for evacuation from threatened areas and then get the word out with help from local public safety offices and the mass media. Researchers at NCAR are collaborating with colleagues elsewhere to address the human side of hurricane forecasts and warnings through the Collaborative Program on the Societal Impacts and Economic Benefits of Weather Information (SIP). Research Updates Studies show 63% of hurricane-related deaths occur inland. NCAR scientists have been using tropical storm winds, census data, and flood maps to help emergency managers identify the most vulnerable populations and plan their response. The resulting 2012 map reveals the surprising extent of risk . An overview in the Bulletin of the American Meteorological Society (August 2012) summarizes recent work by NCAR scientists and collaborators on the communication of hurricane risk.

back to top Hasn't the number of hurricanes been going down lately? On average there are about 70 to 110 named tropical cyclones per year across the world, including about 40 to 60 that reach hurricane strength. This range has held remarkably steady within the last 40 years. Within each basin , the numbers often vary more dramatically than the global average. In part this is due to ocean-atmosphere cycles such as El Niño and La Niña . Because they affect where showers and thunderstorms develop, these cycles can suppress hurricane activity in one basin while enhancing it in another. During El Niño years, the Atlantic tends to be less active than usual, while parts of the central and northeast Pacific are typically busier than usual. While the long-distance effects are not exactly opposite during La Niña events, there is a tendency for more activity in the Atlantic than the Pacific. Even though the total number of tropical cyclones around the world holds fairly steady, some years are more active than others. Scientists often use an index called Accumulated Cyclone Energy, or ACE , to measure the overall intensity of a given year’s cyclone activity.

ACE takes into account a storm’s peak winds at each six hours of its lifetime. After dipping to 30-year lows in the early 2010s, global ACE values have begun rising again ( see graph ). Of all the hurricanes that build over the North Atlantic and Gulf of Mexico each year, only a small fraction make it to the U.S. coastline at hurricane strength. As of early 2013, the nation had not seen a major landfall (Category 3 or stronger) since 1995. However, the total number of hurricanes swirling across the North Atlantic remains unusually high. back to top Are hurricanes striking in new places? In recent years, we’ve seen several hurricanes and tropical storms strike in unfamiliar places. This could be a result of improved monitoring, regional changes in ocean temperatures and upper-air circulation (perhaps linked to global warming in some cases), natural variability, or all of the above. The South Atlantic had been considered free of tropical cyclones—that is, until March 2004, when a mysterious storm later dubbed Hurricane Catarina made landfall in Brazil. In October 2005, Vince became the first tropical storm ever recorded in Spain.

And in the Arabian Sea, Gonu became a Category 5 in June 2007—that region’s strongest tropical cyclone on record. After weakening, Gonu brought unprecedented damage from rain, wind, and flooding to parts of Oman and Iran. Although the Bay of Bengal has seen the world's deadliest tropical cyclones on record, most of these strike India and Bangladesh, while the coast of Myanmar tends to experience only weaker cyclones. The catastrophic Cyclone Nargis , which struck Myanmar in May 2008, appears to be the first major cyclone (Category 3 strength or higher) ever recorded in the populous Irrawaddy Delta. More than 90,000 people died in the storm and its aftermath. The mid-Atlantic and New England can see intense hurricanes, but quite rarely. These are almost always moving north to northeast, often sideswiping the coastline. However, unlike any other hurricane in modern records, Hurricane Sandy moved west-northwest toward New Jersey, making landfall just after becoming post-tropical yet still delivering an unprecedented storm surge to the region. Is global warming affecting hurricanes? Are hurricanes getting worse as the global average temperature rises ? What can we expect in the future?

A growing amount of research is looking into these questions, and not all of the answers are in just yet. Here’s a summary of what research tells us so far. Oceans are warmer; more rainfall expected The bottom line: As summarized in 2007 in the Fourth Assessment Report of the Intergovernmental Panel on Climate Change : There is observational evidence for an increase of intense tropical cyclone activity in the North Atlantic since about 1970, correlated with increases in tropical SSTs [sea-surface temperatures]. There are also suggestions of increased intense tropical cyclone activity in some other regions where concerns over data quality are greater. —IPCC Working Group I, Summary for Policymakers, 2007 Sea-surface temperatures across the tropics have risen along with global temperature over the last 100 years and are expected to warm further in the next century. All else being equal, warmer oceans can support stronger hurricanes. The extra water vapor evaporated by oceans in a warmer climate can be expected to boost rainfall from hurricanes by as much as 8% for every 1°C (1.8°F) of warming, according to Kevin Trenberth of NCAR ( view a PDF of this paper ).

Examining the record Several studies in the last decade have reported enhanced hurricane activity in some regions. For example: Roughly twice as many hurricanes are now reported in the Atlantic compared to a century ago. The rise, identified by Greg Holland (NCAR), occurred over three distinct periods associated with rises in sea-surface temperature (see NCAR news release ). In both the North Atlantic and North Pacific oceans, the duration of tropical cyclones as well as their strongest wind speeds have both increased by about 50% over the past 50 years, according to Kerry Emanuel, Massachusetts Institute of Technology (see MIT news release ). The number of Category 4 and 5 hurricanes worldwide nearly doubled from the early 1970s to the early 2000s, found a team from the Georgia Institute of Technology and NCAR (see NCAR news release ). Changes in observing techniques pose a major challenge when studying past hurricanes. Tropical cyclones have been routinely monitored by aircraft for 60 years or less, and by satellite for only 20 to 40 years, depending on the ocean.

In earlier years, many storms over the open ocean may have gone unobserved, as noted in a 2007 study by Christopher Landsea (NOAA, view a PDF of this paper ). Thus, scientists must take special care in analyzing global hurricane records prior to the 1970s. Studying the future The bottom line: According to the 2012 IPCC report Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation ( PDF ): Average tropical cyclone maximum wind speed is likely to increase, although increases may not occur in all ocean basins. It is likely that the global frequency of tropical cyclones will either decrease or remain essentially unchanged. (Section 3.4.4) Computer modeling Because the inner core of a hurricane only spans a few miles, and the space between data points in a typical global climate model is wider than that, global models cannot yet produce realistic hurricanes. Scientists are using several techniques to get around this roadblock.

For example: A 2008 study (see NOAA news release ), led by Thomas Knutson at NOAA’s Geophysical Fluid Dynamics Laboratory, combined a higher-resolution regional model with IPCC projections for the late 21st century of global-scale atmosphere and ocean conditions. The team's technique indicated an average drop of 18% in the number of Atlantic hurricanes and a 2% rise in average intensity. One limitation to this study is that the model had difficulty producing hurricanes at Category 3 strength or greater. (Christopher Landsea discussed this issue in a 2007 article in EOS , as did Knutson et. al. in an online FAQ .) Using statistics on hurricane development, MIT’s Kerry Emanuel "seeded " a set of IPCC global projections with weak tropical waves, then tracked those that developed, using a more-detailed, hurricane-scale ocean-atmosphere model. His 2008 study (see MIT news release ) showed a tendency toward more and stronger tropical cyclones in the North Atlantic and Northwest Pacific, with fewer but more intense cyclones globally. However, there are substantial differences among the various IPCC projections used, plus a good deal of variability from decade to decade ( view a PDF of this paper ).

Several groups are attempting to produce global models that can directly depict hurricanes without an intervening small-scale model. The first such study, produced on Japan’s Earth Simulator computer and released in 2005, indicated more and stronger hurricanes in the Atlantic by later this century. Globally, it showed a 30% drop in the number of tropical cyclones, but a rise in the number and strength of the most intense hurricanes ( view a PDF of this paper ). Meanwhile, research to boost the resolution of computer models continues. At NCAR, researchers are experimenting with the global Community Climate System Model (see Global model goes local ), the Weather Research and Forecasting model (see Weather forecast goes global ) and idealized models of hurricane structure that reveal details never depicted before, such as fine-scale turbulence in a hurricane eyewall . Multidecadal patterns in the Atlantic Hurricane researcher William Gray (Colorado State University) and others have emphasized the role of a natural 20- to 40-year cycle in ocean temperature, dubbed the Atlantic Multidecadal Oscillation (see NOAA FAQ ), in shaping hurricane activity across the North Atlantic.

If only the AMO were considered, Atlantic hurricane counts would be expected to drop for several decades beginning in the 2010s or 2020s. However, analyses by other researchers, including Kevin Trenberth (see NCAR news release ), have examined the AMO influences in light of global warming changes in ocean temperatures to conclude that the AMO is only a minor factor. Instead, they argue, the recent uptick in Atlantic hurricane activity is more closely related to overall global warming and thus may continue for decades to come. Wind shear and hurricanes Some scientists are exploring how interactions among different parts of the tropics will unfold in a warmer climate. For example, Gabriel Vecchi (NOAA) and Brian Soden (University of Miami) have found that wind shear over the North Atlantic may increase in the coming century, with the western tropical Pacific warming even more than the tropical Atlantic (see American Geophysical Union news release ). This setup would help to inhibit Atlantic hurricanes. However, not all of the IPCC models examined by Vecchi and Soden show this pattern—a sign of the continued challenge in portraying tropical ocean circulation in global models.

Understanding hurricane structure and behavior Flying into the storms One of the best ways to figure out how hurricanes operate is to fly right into them. Each year, skilled pilots steer research aircraft as close as safety allows to storms forming in the North Atlantic and Pacific. The planes are part of NOAA's hurricane hunter fleet and other research operations supported by the U.S. Navy, Air Force, universities, and research labs. Improving forecasts close to shore In 2008 NOAA adopted a technique called VORTRAC to provide detailed 3-D views of an approaching hurricane every six minutes. The three-dimensional views help to determine whether the storm is gathering strength as it nears land (see NCAR news release ). VORTRAC (Vortex Objective Radar Tracking and Circulation) was developed by researchers at NCAR and the Naval Research Laboratory. The technique requires no major new hardware, but instead relies on the existing NOAA network of Doppler radars along the Southeast coast to closely monitor hurricane winds. About 20 of these radars are scattered along the Gulf and Atlantic coastlines from Texas to Maine. Each radar can measure winds blowing toward or away from it, but no single radar could provide a 3-D picture of hurricane winds before now.

NCAR scientist Wen-Chau Lee and his collaborators developed a series of mathematical formulas that combine data from a single radar with general knowledge of Atlantic hurricane structure in order to map the approaching system's winds in three dimensions. The technique also infers the barometric pressure in the eye of the hurricane, a very reliable index of its strength. Forecasters using VORTRAC can update information about a hurricane each time a NOAA Doppler radar scans the storm, which can be as often as about every six minutes. That could enable forecasters to monitor it for the critical 10-15 hours before landfall. Wringing moisture data out of GPS signals At UCAR’s COSMIC program , researchers have shown that computer models do a better job of predicting hurricanes when they’re fed extra data on how much moisture is in the air surrounding a developing storm. The data to help predict which potential hurricanes are most likely to develop come from SuomiNet , a UCAR-managed network of ground-based sensors that assess moisture by measuring tiny changes in GPS signals. Research Update Meteorologists have hypothesized that ventilation, the injection of cooler and drier air into the core of a tropical cyclone, can significantly lessen the cyclone's intensity.

To further understand this phenomenon , in 2012, NCAR scientist Brian Tang and colleague Kerry Emanuel (Massachusetts Institute of Technology) published work using a hurricane model to determine how much ventilation is needed to weaken a tropical cyclone and where this is most likely to happen within it. back to top High-resolution forecasting from weather models Forecasters at NOAA’s National Hurricane Center make use of a wide array of computer models, including one called Hurricane WRF. Adopted by NOAA in 2007 , Hurricane WRF was derived from the Weather Research and Forecasting model (WRF) and configured to reveal tropical storm activity in great detail. A multiagency effort, WRF is a next-generation computer model for weather prediction that can be used by both researchers and operational forecasters. See the roster of NOAA/NCEP model output to find Hurricane WRF forecasts of current storms. NCAR has also refined research-oriented versions of WRF over the last few years, using them to study hurricanes and other forms of severe weather.

To accurately depict the small but intense features within hurricanes, a special version of the Advanced Research WRF (ARW) sharpens the detail over targeted regions to 7.5 miles (12 kilometers) for forecasts out to 120 hours, with a resolution as fine as 0.8 mi (1.33 km) near hurricanes. Since 2005, NCAR's experimental forecasts of hurricane track and intensity have ranked among the most accurate of the computer models used by researchers and forecasters to predict the season's hurricanes. See the ARW modeling page for forecasts of current storms. During August–October 2013, NCAR will be conducting 3- to 7-day hurricane forecasts using the experimental Model for Prediction Across Scales. When run on high-performance supercomputers, MPAS can simulate weather processes around the globe in very fine detail, capturing the development and evolution of individual cloud systems—a long-time goal of weather researchers. MPAS will help pave the way toward more detailed simulations by linking tropical storm and hurricane activity with global atmospheric conditions. The new simulations will provide better intensity forecasts than regional models at roughly the same cost for computing.

Experimental accuracy This side-by-side animation of Hurricane Katrina's path across the Gulf of Mexico compares the actual radar observations (left) with NCAR's Advanced Research WRF experimental forecast, issued 62 hours before landfall. In both frames, narrow rainbands can be seen pinwheeling counterclockwise into the storm's core. The model resolution for this animation was 12 kilometers (7.5 miles). The radar vantage point is stationary, on the Gulf Coast, while the model's viewpoint follows the hurricane itself. Click here or on the image to launch the animation in a new window. Anatomy of a hurricane This 3-D illustration identifies four components of a hurricane: inflow, rainbands, eye and eyewall, and outflow. Warm, moist air enters the hurricane at low levels (large orange arrows), rises through intense updrafts in the eyewall (spiral at center), and departs the storm at high altitudes (large blue arrow). A weaker downdraft makes the eye itself relatively calm and clear. Rising air also helps create concentric rainbands (short orange arrows) outside the eyewall, with sinking air (short blue arrows) in between the rainbands.

Click here or on the image for an interactive version with more detail. (Illustration © COMET , UCAR.) News media terms of use * The eyewall replacement cycle - a key event in the life of a hurricane Eyewall replacement during Katrina The peak winds of over 100 miles per hour that buffeted New Orleans during Hurricane Katrina could have been much worse had the storm made landfall at a different moment in the cycle of its eyewall . Long-lived, intense hurricanes often go through an eyewall replacement cycle that takes a day or so to complete. The result is collapse of the main eyewall and temporary weakening of the storm. Then an outer eyewall contracts and takes its place, allowing for restrengthening. Katrina appears to have been going through the weaker stage as it approached land. The first two images capture Katrina with an intact eyewall at 5:45 p.m. Eastern Daylight Time on Sunday, August 28, as it moved over warm water in the Gulf of Mexico. By 5:45 a.m. on Monday, the weakened eyewall is being further disrupted by interaction with the land surface. Click on each image to enlarge it.

( GEMPAK images by Jeff Weber, UCAR; data from water vapor and infrared bands of NOAA GOES-E satellite .) News media terms of use * Animation of 72-hour forecast on August 27, 2005, about 60 hours before landfall These animations were created from data produced by the NCAR-based Weather Research and Forecasting model (Advanced Research WRF, or WRF-ARW). The two visualizations are based on the same 72-hour forecast of Hurricane Katrina, initialized at 0000 UTC August 27, 2005, about 60 hours before landfall. The path of the hurricane in the animation predicts almost precisely the path of the actual hurricane. Click here or on the image to open a Web page where the animations can be launched in a variety of formats.(Animations ©UCAR.) News media terms of use * About UTC: The Universal Time code is based on a 24-hour clock, with 0000 UTC equal to 7:00 p.m. Central Daylight Time. Convert other UT codes to U.S. time zones with this conversion chart from the U.S. Naval Observatory .

BBC NEWS | Science & Environment | Animated guide: Hurricanes Science & Environment Printable version Animated guide: Hurricanes To view the interactive content on this page you need Javascript enabled and Flash player installed. Download Flash here Hurricanes start when strong clusters of thunderstorms drift over warm ocean waters. In the Atlantic and eastern Pacific they are called hurricanes, but in the western Pacific they are called typhoons. In the Bay of Bengal and Indian Ocean they are known as cyclones. The very warm air from the storm combines with the moist ocean surface and begin rising. This creates low pressure at the surface. As trade winds hit those within the storm, the whirling winds cause the storm to start spinning. Rising warm air leaves low pressure above the surface. Air rises faster and faster to fill this low pressure, in turn drawing more warm air off the sea and sucking cooler, drier air downwards. As the storm moves over the ocean it picks up more warm, moist air. Wind speeds start to increase as more air is sucked into the low-pressure centre. It can take hours or several days for a depression to grow into a fully-formed hurricane. Hurricanes are made up of an eye of calm winds and low pressure surrounded by a spinning vortex of high winds and heavy rainstorms.

When a hurricane hits land it often has devastating effects. The Saffir-Simpson scale was devised to measure hurricanes around the Americas and is increasingly used to categorise typhoons and cyclones, too, although some regions still use different scales. The effects: Storm surge 1.2-1.5m above normal Category 2: ï¿½ Some trees damaged ï¿½ Storm surge 1.8-2.4m above normal Category 3: How are hurricanes measured? OTHER NATURAL DISASTERS Hurricane measurement and classification A hurricane may be classified as category one if they have their usual speeds of up to 74mph. Going up to category five, they may carry extreme speeds and power, with much more destructive ability. Hurricane winds are measured using the Saffir-Simpson Hurricane scale. This scale was first developed in the late 1960s and was further developed in the early 1970s. It uses measurements in pressure, wind speed, storm surge and damage potential to put hurricanes into 5 categories. Below is a guide to hurricane categories and the damage they can cause. Sometimes even tropical storms can cause a lot of deaths than category 3 and 4 hurricanes.

At other times, category 3 hurricanes can cause more damage than category 5 hurricanes. It all depends the area that they make the landfall. If the coastal area is heavily populated and a lot of people use the beach for residential and business purposes, there is a high chance that the death toll will be high if a hurricane visits. In many cases the landfall is far from where people live and that makes the impact less destructive, even though it can affect wildlife, vegetation and other biotic and abiotic factors. ...

Question: The Saffir-Simpson scale is used to measure which natural occurrence?
Answer: HURRICANES