Weather

Earth's weather is primarily driven by rising air in three Low-pressure areas.

* The northern hemisphere polar front.
	+ Extratropical cyclones form along the front and move eastward at 12-15 m/s (43-54 km/h) and last 3-5 days. (3600-6000 km)
* The Intertropical Convergence Zone. Sometimes, a double ITCZ forms, with one north and one south of the Equator.
	+ Tropical cyclones (Hurricanes) form here and move westward.
* The southern hemisphere polar front.
	+ Extratropical cyclones form along the front and move eastward

And to a lesser extent by descending air in two High-pressure areas:

* Northern hemisphere subtropical ridge (horse latitudes)
	+ The Sahara desert is here.
* Southern hemisphere subtropical ridge (horse latitudes)
	+ The Kalahari (in south Africa), the Atacama desert (in south America), and all the deserts of Australia are here.

Average air pressure adjusted to sea level:

## Troposphere

Rising air ceases to rise when it gets to the stratosphere because the heating of ozone in the stratosphere by ultraviolet light from the sun causes the temperature within the stratosphere to increase with height and this prevents clouds and thunderstorms from rising within it.

Therefore almost all weather occurs within the troposphere which extends from Earths surface up to the tropopause which is the boundary between the troposphere and the stratosphere. (See A new method to determine the tropopause).

* The lower stratosphere has much higher ozone concentrations than the upper troposphere, but much lower water vapor concentrations.

 30
 60
 90

The troposphere holds most of the mass of Earths atmosphere.

The Atmospheric pressure at any point in the atmosphere is equal to the weight of the air above it.

* 1000 millibars would be the pressure under exactly 10 meters of water if gravity were exactly 10 m/s2
* 1 atm = 1013.25 millibars = pressure at sea level
* See Air Pressure at Altitude Calculator

Each of the six layers in the chart below holds exactly 15% of the mass of the atmosphere:

 5
 4
 3
 2
 1

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### Boundary layers

From Planetary boundary layer:

In meteorology the **planetary boundary layer** (**PBL**) is the lowest 1.5 km of the troposphere (see Boundary layer thickness). It is highly turbulent and vertical mixing is strong. Its temperature usually responds to changes in surface temperature in an hour or less (i.e. the boundary layer gets warm when the sun comes out).

* Flow is turbulent when Ri<Ric (See Kelvin–Helmholtz instability)

There are two main types of Planetary boundary layer:

* Nocturnal PBL: Turbulence is due solely to its motion over (and its contact with) the planetary surface.
	+ A Marine layer is very similar but is not limited to nighttime.
	+ The arctic PBL is also similar and also not limited to nighttime but is much thicker. It extends the full width of the PBL (1.5 km).
* Daytime PBL: Turbulence is also due to Thermals.

The atmospheric surface layer is the lowest part of the planetary boundary layer (typically the bottom 10% where the log wind profile is valid). It is the region of (approximately) 'constant flux'. The surface layer is highly turbulent and well mixed. It is deeper when the wind is strong and shallower when the air is calm. See On the Arctic Boundary Layer and Surface Layer

Above the PBL is the free atmosphere, where the wind is approximately parallel to the isobars, while within the PBL the wind is affected by surface drag and turns across the isobars.

 Daytime PBL
 Nocturnal PBL

Even in the absence of convection, the turbulence of the planetary boundary layer can generate irregular fractus clouds that form and then disappear within minutes with no discernible pattern.

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### Air masses

 In meteorology, an **air mass** is a volume of air defined by its temperature and water vapor content. Air masses cover many hundreds or thousands of miles, and adapt to the characteristics of the surface below them. The **Bergeron air mass classification** involves three letters.
* The first letter describes its moisture
	+ c: **continental** air masses (dry)
	+ m: **Maritime** air masses (moist)
* The second letter describes its thermal characteristics:
	+ T: **Tropical**
	+ P: **Polar**
	+ A: **Arctic** or **Antarctic**
	+ M: **Monsoon**
	+ E: **Equatorial**
	+ S: **Superior** air (an adiabatically drying and warming air formed by significant downward motion in the atmosphere).
* The stability of an air mass may be shown using a third letter
	+ k: **colder** than the surface below it
	+ w: **warmer** than the surface below it

Transformation:
* cA-mPk: continental arctic air transforming into marine polar air as it flows out over water.

Overrunning:
* mT/cP: Marine tropical air overrunning continental polar air at the polar front.

Polar Highs: North American High (Greenland high), Siberian High (Voeykov axis)
* Upper level lows: Polar vortex (See also: Sudden stratospheric warming)

Lows: Aleutian Low, Icelandic Low
Highs: North Pacific High (Hawaiian high), Azores High (Bermuda high)
Lows: North American Monsoon, Amazon rainforest, Congolese rainforests, Maritime Continent, Monsoon of South Asia
Highs: South Pacific High, South Atlantic High, Mascarene high, Australian high
Lows: Subpolar low pressure belt
Polar High: Antarctic high
See also: Saharan Air Layer

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## Atmospheric convection

In meteorology and physical oceanography, advection often refers to the horizontal transport of some property of the atmosphere or ocean, such as heat, humidity or salinity, and convection generally refers to vertical transport (vertical advection).

The heating of the ground by the sun produces thermals which become cumulus clouds. Like a hot air balloon, the warm air rises. The rising warm air expands and therefore cools. Large numbers of thermals can generate a thermal low.

* The **Dry Adiabatic** Lapse Rate (DALR) is a constant **0.98 °C** for every 100 meters it rises. ((°C × 9/5) + 32 = °F).

When the rising air reaches the condensation level the moisture within it condenses into droplets and releases the latent heat of condensation (2,260 Joules per gram of water) which causes the warm air to rise even further. The heat released slows the rate of cooling.

* The **Moist Adiabatic** Lapse Rate (MALR) varies with temperature but is typically about **0.5 °C** for every 100 meters (50 °C per 10 km).

The **potential temperature** of a parcel of fluid at pressure

P

{\displaystyle P}

 is the temperature that the parcel would attain if adiabatically brought to a standard reference pressure

P
0

{\displaystyle P\_0}

, usually 1000 millibars. If

P

{\displaystyle P}

 increases with height (as it does at night or in the arctic) then the air is stable. Potential vorticity works the same way but with vorticity instead of temperature.

* The International Civil Aviation Organization (ICAO) defines an international standard atmosphere (ISA) with a temperature lapse rate of 6.49 °C/km from sea level to 11 km.
* Density of air at 30 °C at 1000 millibar and 100% humidity is 1.1307 kg/m3 or 1 kg/(0.8844 m3)
* 1 kg of air at 30 °C (86 °F. 303.15 °K) at 1000 millibar and 100% humidity carries **30.326 grams** of water.
	+ 30.326 g of water holds **68,537 Joules** of energy.
		- Enough to raise the temperature of 1 kg of air about **68.5 °C** to 98.5 °C which is 371.65 °K. (The specific heat of dry air at 1 bar is 1.005 kJ per kg per °C.)
			* In the tropics the potential temperature rises about 55 degrees from the ground to the 200 millibar level above which the moist potential temperature is no longer constant.
			* The density of air at 371.65 °K is (1/371.65)/(1/303.15) = 0.816 times the density of air at 303.15 °K.
				+ Typhoon Tip had air pressure of 870 mbar and an eye temperature of 30 °C
* Each 12 °C (21.6 °F) increase in temperature doubles the amount of water vapor the air can carry.
* On a summer day, net solar energy received at a lake reaches **15 MJ** per square meter per day. See Diurnal temperature variation and Planetary boundary layer.
	+ If 80% of the energy is used to vaporize water then evaporation = 0.49 cm/day.

When a steady wind is present cumulus clouds can form in lines stretching hundreds of kilometres long called **cloud streets**. These cloud streets cover vast areas and may be broken or continuous. They form when wind shear causes horizontal circulation in the atmosphere, producing Horizontal convective rolls. The height of the rolls is the height of the planetary boundary layer and the distance between rolls is 2 or 3 times greater.

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### Updraft

When thunder roars, head indoors
In the image to the right we plot the temperature of an imaginary parcel of air as we lift it first along the **dry adiabatic** from the ground to the Lifted condensation level (LCL). Then along the **moist adiabatic** to the level of free convection (LFC).

* The buoyancy of the parcel at LFC is zero because the temperature of the parcel is equal to the temperature of the surrounding environment (at the same height).

Integrating (wrt height) the buoyancy of the parcel from A to LFC gives **Convective inhibition** (CIN). CIN is negative buoyancy. CIN inhibits convection. Overcoming CIN requires energy which must be supplied by the sun.

Once CIN is overcome a parcel is free to rise through the free convective layer (FCL). The free convective layer is a layer of positive buoyancy (PBE) and is the layer where deep, moist convection (DMC) can occur. It is the layer between the level of free convection (LFC) and the equilibrium level (EL). The equilibrium level can be as high as the stratosphere.

From Convective available potential energy:

**Convective available potential energy** (CAPE) is the opposite of CIN. CAPE is calculated by integrating vertically the local buoyancy of a parcel from the level of free convection (LFC) to the equilibrium level (EL). Convective available potential energy (CAPE), is the amount of energy a parcel of air would have if lifted through the entire free convective layer. CAPE is effectively the positive buoyancy of an air parcel and is an indicator of the strength of the storms updraft. CAPE is measured in joules per kilogram of air. If CAPE is large enough then (once CIN is overcome) cumulus clouds will continue to grow upward through the free convective layer and become thunderstorms (cumulonimbus). Extremely large values of CAPE can result in explosive thunderstorm development.
Updrafts create **low level lows** (warm-core low). In other words a low pressure area at ground level.

* Lightning flash rate (number per minute) is proportional to the 5th power of the convective velocity of the updrafts in the thundercloud. For example, a hardly noticeable 10% increase in cloud height would have a 60% change in total flash rate, which is easily observed. Storms that produce tornadoes are known to have very high lightning rates.

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### Downdraft

From Rain:

Rainfall intensity is classified according to the rate of precipitation, which depends on the considered time. The following categories are used to classify rainfall intensity:

* Light rain — when the precipitation rate is < 2.5 mm/h
* Moderate rain — when the precipitation rate is between 2.5 mm/h - 10 mm/h
* Heavy rain — when the precipitation rate is > 10 mm/h and 50 mm/h
* Violent rain — when the precipitation rate is > 50 mm/h

The falling rain creates a downdraft as it pulls cold air with it, and this cold air spreads out at the Earth's surface, causing the strong winds that are commonly associated with thunderstorms.

Dry air aloft has the effect of increasing the instability of the air and also increases the severity of the downdraft. Dry air entrained (ingested) by the downdraft causes evaporative cooling of the air which increases its density and therefore increases its rate of descent.

**Downdraft CAPE (DCAPE)**, estimates the potential strength of evaporatively cooled downdrafts. See HOW DOES DRY AIR ALOFT MAKE STORMS MORE SEVERE?.

Downdrafts entrain so much upper level air that they create **upper level lows** (cold-core low). In other words a low pressure area at the top of the troposphere.

* In the tropics upper level lows are called Tropical upper tropospheric trough (TUTT).
* Also called an Upper tropospheric cyclonic vortex

These lows continue to exist for some time even after the updrafts and downdrafts that created them cease.

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## Nonsevere storms

Tropical air is far warmer than air outside the tropics and therefore holds far more moisture and as a result thunderstorms in the tropics are much taller. Nevertheless severe thunderstorms are not common in the tropics because the storms own downdraft shuts off the inflow of warm moist air killing the thunderstorm before it can become severe. Normal thunderstorms only last about 30 minutes.

* A typical updraft is **25-50 km/h**.
	+ At that speed it takes **12-24 minutes** to go from ground to 10 km.
		- An ordinary thunderstorms lasts 30 minutes. Just long enough for the air to go up the updraft to the top and back down the downdraft to the bottom.
* The drop in atmospheric pressure under a thunderstorm is just **0.5-2 millibars**.
* Ordinary thunderstorms usually dont produce severe weather except possibly for a brief pulse.

* The average thunderstorm produces about **200,000 m3** of rain, but large storms can produce 10 times more rainfall.
	+ 200,000 m3 will cover an area 5 km in radius to a depth of 2.55 mm.
	+ The Amazon river drains an area of 7,050,000 km2 and discharges **209,000 m3** of water per second into the Atlantic ocean. (31,557,600 seconds in a year)
* Over a 30 minute period a normal thunderstorm releases **1015 Joules** of energy (442,478 m3 of water) equivalent to 0.24 megatons of TNT (ten times larger than the bomb over Nagasaki).
	+ **12.9 km3** (14,590,714,238 kg) of air at 30 °C (86 °F. 303.15 °K) at 1000 millibar and 100% humidity carries 442,478 m3 of water
		- 12.9 km3 is equal to a cylinder 1.5 km in height and **1.66 km** in radius.
		- 12.9 km3 is equal to a cylinder 4.5 km in height and **0.955 km** in radius.
		- 12.9 km3 is equal to a cylinder **41 m** in height and 10 km in radius.

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## Jet stream

From Jet stream:

Jet streams are fast flowing, narrow, meandering air currents in the atmospheres of some planets, including Earth. On Earth, the main jet streams are located near the altitude of the tropopause and are westerly winds (flowing west to east). Their paths typically have a meandering shape. Jet streams may start, stop, speed up, slow down, split into two, or combine into one stream.

Polar jet streams are rivers of air typically located near the 250 hPa (about 1/4 atmosphere) pressure level, or 7 to 12 km and are strongest in winter. The width of a jet stream is typically a few hundred kilometers or miles and its vertical thickness often less than five km. Speeds over 398 km/h have been measured (12,000 J/kg). Each large meander, or wave, within the jet stream is known as a Rossby wave. A region of wind maximum within the jet stream is called a jet streak.

The polar jet stream is created by strong updrafts along the polar front. The subtropical jet stream is created by strong updrafts associated with atmospheric rives that extend from the tropics to the polar front. Due to the circulation in the Hadley and Ferrel cells the subtropical jet becomes concentrated at the boundary between the Hadley and Ferrel cells.

Since upper level jet streams are created by (high vorticity) supercell updrafts its possible that low level jets are created by supercell downdrafts. Low level jets are stronger and more common in Oklahoma than anywhere else in the US. See Low level jets.

See also Tropical Easterly Jet and African easterly jet.

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### Tropopause folding

High vorticity jet streams are associated with tropopause folding (upper level front). A split front is actually two fronts. A low level front and an upper level front.

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## Severe storms

See Tornado emergency and Particularly Dangerous Situation
Severe thunderstorms (called supercells) are more common outside of the tropics because of the effect of the polar jet stream. The jet stream pushes against the top of the thunderstorm displacing the downdraft so that it can no longer shut off the inflow of warm moist air. As a result severe thunderstorms can continue to feed and grow for many hours. One lasted 17.5 hours. Heating of the air during the day is necessary to form thunderstorms (except along fronts) but supercells do not require it to continue to exist. Supercells can suck air up from the ground even at night. (It was a group of overnight severe storms that intensified the next day into the august 10, 2020 derecho which was part of a much bigger 988 hPa low pressure region centered over Canada.)

* A typical updraft is **120 km/h**.
	+ At that speed it takes **5 minutes** to go from the ground to 10 km.
	+ Rate of climb for modern airliners is only 40-60 km/hr.
* Updrafts of **240 km/h** are possible. (4500 J/kg = 240 km/h). See BWER
	+ All trees regardless of type or size tend to break when wind speed reaches **151 km/h** (94 mph).

**Polar jet streams** are rivers of air **7 to 12 km** above the ground.

* Speeds over **398 km/h** have been measured (12,000 J/kg).

If the jet stream is strong enough then severe weather and tornadoes can develop even in an area of low CAPE values.

* The surprise severe weather event that occurred in Illinois and Indiana on April 20, 2004 is a good example. There was strong low-level wind shear and although overall CAPE was weak (1000 J/kg), there was strong CAPE in the lowest levels (1-3 km) of the troposphere which enabled an outbreak of **minisupercells** producing large, long-track, intense tornadoes. See here for more information.

**Supercells** contain **mesocyclones**, an area of organized rotation a few miles up in the atmosphere, usually **2-10 km** across rotating at **70-90 km/h**. Most intense **tornadoes** (EF3 to EF5 on the **Enhanced Fujita Scale**) develop from supercells. In addition to tornadoes, very heavy rain, frequent lightning, strong wind gusts, and hail are common in such storms. Some supercells produce tornados and some dont. Nobody knows why. Tornados might form from dust devils (or Gustnados) that have been pulled into the storm. See also:  Mesovortices.

* The widest tornado on record is the El Reno, Oklahoma tornado with a width of **4.2 km** at its peak. (484 km/h winds)
* A probe dropped in front of an F4 (333–418 km/h) tornado near Manchester, South Dakota captured the largest drop in atmospheric pressure ever recorded. **100 millibars** in less than one minute.
	+ 100 millibars corresponds to 325 km/hr = (100 millibars \* 1 m3 / 1.225 kg)0.5
	+ The measurement is also the lowest pressure, **850 millibars**, ever recorded at Earth's surface when adjusted for elevation. 850 mbar is normal air pressure at 1.5 km altitude.
* Highest wind speed ever recorded was **480 km/h** in the 1999 Bridge Creek–Moore tornado.
* Violent tornadoes make up only 0.5% of all tornadoes but account for about half of all tornado deaths.
* Most tornadoes occur between 3pm and 9pm.

Tornado rating classifications

If supercells track to the right or left of the mean wind, they are said to be **right-movers** or **left-movers**, respectively. Supercells can sometimes develop two separate updrafts with opposing rotations and split into 2 storms. A right-mover on the right and a left-mover on the left.

* In the northern hemisphere cyclones are right-movers.
* In the southern hemisphere cyclones are left-movers.
* When the updraft is cyclonic then the downdraft is anticyconic.

A **wall cloud** is a large, localized, persistent, and often abrupt lowering of cloud that develops beneath the surrounding base of a cumulonimbus cloud. It is typically beneath the **rain-free base** (RFB) portion of a thunderstorm, and indicates the area of the strongest updraft within a storm. Rotating wall clouds are an indication of a mesocyclone in a thunderstorm; most strong tornadoes form from these.

The 100th meridian forms the eastern border of the Texas panhandle with Oklahoma and roughly marks the boundary between the **high plains** with their dry climate to the west and the humid climates to the east. The boundary between a dry air mass and a humid air mass is called a dry line. Dry air from the west, cold air from the north, and moist air from the gulf all converge in Oklahoma. Severe weather is more common in Oklahoma than anywhere else in the US. See the image below.

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## Tropical Cyclones

**Tropical cyclones** are low level lows (warm-core low).

From Tropical wave:

A **tropical wave** (also known as an easterly wave) is a type of convectively active trough. An elongated area of relatively low air pressure, oriented north to south, which moves from east to west across the tropics. Convection tends to occur on the eastern side of the tropical wave. Tropical waves in the Atlantic basin develop from disturbances, which develop as far east as Sudan in east Africa, and drift across the continent into the Atlantic Ocean. These are generated or enhanced by the midlevel (3 km) **African Easterly Jet**.

Tropical waves are the origin of approximately 60% of Atlantic tropical cyclones and of approximately 85% of intense Atlantic hurricanes (Category 3 and greater).

From **Tropical cyclogenesis**:

There are six main requirements for tropical cyclogenesis:

1. A preexisting low level focus or disturbance (usually a tropical wave)
2. Water temperatures of at least 26.5 °C (79.7 °F) are needed down to a depth of at least 50 m (160 ft)
3. Rapid cooling with height
4. High humidity in the lower to middle levels of the troposphere
5. Low vertical wind shear.
6. Enough Coriolis force to sustain a low pressure center

While these conditions are necessary for tropical cyclone formation, they do not guarantee that a tropical cyclone will form.

Depth of 26 °C isotherm on October 1, 2006:

Strong storms lose their strength very rapidly after landfall and become disorganized areas of low pressure within a day or two, or evolve into extratropical cyclones.

If a hurricane ingests dry air it can produce a low-level thunderstorm outflow boundary. See A Mesoscale Low-Level Thunderstorm Outflow Boundary Associated with Hurricane Luis.

A hurricane will have an **eye** of approximately 30–65 km (20–40 mi) across. The fastest winds are in the eyewall. A category 5 hurricane has winds in excess of 220 km/hr. Fast enough to circle the eye in 1-2 hours.

* Despite the high winds in a hurricane the updraft is usually only 20 km/h. (Except possibly for a few severe storms just inside of the eyewall. See Mesovortices)
* Over a 30 minute period a normal thunderstorm releases 1015 Joules of energy (440,000 m3 of water) equivalent to 0.24 megatons of TNT (ten times larger than the bomb over Nagasaki).
	+ A storm that lasted 24 hours would release 48 times as much energy (48 x 1015 Joules). A hurricane (a tropical cyclone) releases 52 x 1018 Joules/day equivalent to 1000 simultaneous non-stop non-severe thunderstorms.

From **Eyewall replacement cycle**:

Eyewall replacement cycles, also called concentric eyewall cycles, naturally occur in major hurricanes (Category 3 or above). When tropical cyclones reach this intensity, and the eyewall contracts or is already sufficiently small, some of the outer rainbands may strengthen and organize into a ring of thunderstorms—an outer eyewall—that slowly moves inward and robs the inner eyewall of its needed moisture and angular momentum. Since the strongest winds are in a cyclone's eyewall, the tropical cyclone usually weakens during this phase, as the inner wall is "choked" by the outer wall. Eventually the outer eyewall replaces the inner one completely, and the storm may re-intensify

Tracks of all Tropical cyclones which formed worldwide from 1985 to 2005:

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## Fronts

See also: Norwegian cyclone model
From Cold front:

A **cold front** is the leading edge of a cold dense mass of air sliding under and lifting a warmer mass of air. Temperature changes across the boundary can exceed **30 °C** (54 °F). A narrow line of thunderstorms often forms along the front.﻿ A cold front is very steep.

A cold front is considered a warm front when convection ceases and it begins to retreat ahead of the next extratropical cyclone along the frontal boundary. A warm front goes from the ground to 1.5 km over a space of about **300 km** giving a slope of about 1:200. Within the warm front nimbostratus clouds overlie the cold air.

A front is called a stationary front if it stalls.

The polar fronts are cold fronts that arises as a result of cold polar air meeting warm subtropical air at the boundary between the polar cell and the Ferrel cell in each hemisphere.

Extratropical cyclones form at and move along the polar front. To understand why we first imagine a perfectly (and unrealistically) straight polar front running east to west. North of the front cold polar air is moving toward the west parallel to the front. South of the front warm air is moving toward the east parallel to the front. At this point the front is largely inactive (**katafront**).

Then we imagine a low pressure area suddenly appearing at some random point on that front. Both the warm air south of the front and the cold air north of the front will be pulled toward the low pressure area.

The cold air moving toward the west will turn south and begin to displace the warm air at ground level not just because the cold air is denser and heavier than the warm air but also because it is actively being steered toward the warm air and actively being wedged under it. The front is now active (**anafront**).

The warm air moving toward the east will turn north but will **not** displace the cold air at ground level. Instead it will flow up over the cold air. Since the front is defined by the position of the cold air at ground level, the resulting (no longer perfectly straight) front will look like this:

Convection associated with the cold front will create a new low slightly farther east than the original low and therefore the whole system will propagate toward the east as a wave of low pressure.

As the rapidly moving cold front (blue) overruns the slow moving (or stationary) warm front (red) it generates an **occluded front** (purple). An occluded front consists of warm air at high altitude.

The dashed line is a **gust front**.

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### Gust front

The cold downdraft from a thunderstorm can create an **Outflow boundary** called a **gust front** that acts like a miniature cold front. Cool, sinking air from a storm cloud's downdraft spreads out across the land surface. This outflow cuts under warm air in the ambient environment. As the lower cooler air lifts the warm moist air, its water condenses, creating a **shelf cloud** which often rolls with the different winds above and below.

The gust front can spawn new thunderstorms out ahead of the first. Sometimes a **Squall line** (line of thunderstorms) forms along the gust front. See **Mesoscale convective complex**.

If the storms along the squall line are severe then the squall line becomes a **Derecho**. This happens on the rare occasions when the jet stream is blowing in such a way that the cold downdraft falls back behind the front where the cold descending air reinforces the front that produced the storms in the first place. See Line echo (multi-bow) wave pattern.

* 60% of derechos occur in May, June, and July.

Derechos move quickly and produce strong straight line winds. In Spanish derecho means 'straight'. According to the National Weather Service (NWS) criterion, a derecho is classified as a band of storms that have winds of at least **25.5 m/s** (92 km/h; 50 kn; 57 mph) along the entire span of the storm front, maintained over a time span of at least six hours.

* The derecho and tornado outbreak of April 4–5, 2011 with wind gusts as high as 145 km/h is reportedly one of the most prolific damaging wind events on record. The outbreak was the first in a series of devastating tornado outbreaks in the month of April 2011.
* The April 25–28, 2011 Super Outbreak was the largest, costliest and one of the deadliest tornado outbreaks ever recorded. In total, 360 tornadoes were confirmed by NOAA's National Weather Service (NWS) in 21 states from Texas to New York to southern Canada.

Tornado tracks in the US (1950-2017):

Sometimes collapsed frontal systems cease to be frontal systems and degenerate into **troughs**. But not all troughs begin as frontal systems.

Tornado outbreak of April 18, 2019:

Image courtesy of https://www.ventusky.com

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## Extratropical Cyclones

From Aleutian Low:

Cyclones (Hurricanes/Typhoons) that form in the tropical and equatorial regions of the Pacific normally start off by moving toward the west but can veer northward and get caught in the **Aleutian Low** where they become **Extratropical cyclones** which move toward the east. This is usually seen in the later summer seasons.

* Both the November 2011 Bering Sea cyclone and the November 2014 Bering Sea cyclone were post-tropical cyclones that had dissipated and restrengthened when the systems entered the Aleutian Low region. The storms are remembered as two of the strongest storms to impact the Bering Sea and Aleutian Islands with pressure dropping below **950mb** in each system.

The magnitude of the low pressure creates an extreme atmospheric disturbance, which can cause other significant shifts in weather.

* + Following the November 2014 Bering Sea cyclone, a huge cold wave, November 2014 North American cold wave, hit the US bringing record breaking low temperatures to many states.

The record lowest pressure established in the northern hemisphere is the extratropical cyclone of January 10, 1993 between Iceland and Scotland which deepened to a central pressure of **912-915 mb** (26.93”-27.02”).

* Most hurricanes have an eye below **990 millibars**.

In 2005, hurricane WILMA reached the lowest barometric pressure ever recorded in an Atlantic Basin hurricane: **882 millibars**.

Extratropical cyclones create **upper level lows** (cold-core low).

The bomb cyclone of march 2019:

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## Upper level lows

### Polar low

From Polar low:

A **polar low** is a small-scale, short-lived atmospheric low pressure system that is found over the ocean areas poleward of the main polar front in both the Northern and Southern Hemispheres, as well as the Sea of Japan.

During winter, when upper level lows (cold-core low) with temperatures in the mid-levels of the troposphere reach -45 °C move poleward over open polar waters, deep convection forms which allows polar low development to become possible. Summer lows tend to be weaker than winter lows.

The systems usually have a horizontal length scale of less than 1000 km and exist for no more than a couple of days. Polar lows have been referred to by many other terms, such as **polar mesoscale vortex**, **Arctic hurricane**, **Arctic low**, and **cold air depression**.

The most active polar lows are found over certain ice-free maritime areas in or near the Arctic during the winter. Polar lows dissipate rapidly when they make landfall. Antarctic systems tend to be weaker than their northern counterparts since the air-sea temperature differences around the continent are generally smaller.

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### Subtropical cyclones

From subtropical cyclone:

A **subtropical cyclone** is a weather system that has some characteristics of a tropical and an extratropical cyclone. Subtropical cyclones form under upper level lows (cold-core low) that drop down into the subtropics from the cold-core extratropical cyclones that created them. They require central convection fairly near the center and a warming core in the mid-levels of the troposphere. They have no weather fronts linked into their center.

Since they form from extratropical cyclones which have colder temperatures aloft than normally found in the tropics, the sea surface temperatures required for their formation are lower than the tropical cyclone threshold.

The island of Bermuda is regularly impacted by these systems. In the north Indian Ocean, the formation of this type of vortex leads to the onset of monsoon rains during the wet season.

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## Monsoons

 India
 Indoneia
 N. Australia

From **monsoon**:

Monsoon is traditionally defined as a seasonal reversing wind accompanied by corresponding changes in precipitation, but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea. The major monsoon systems of the world consist of the West African and Asia-Australian monsoons. The inclusion of the North and South American monsoons with incomplete wind reversal has been debated

(see also Subtropical Indian Ocean Dipole, Maritime Continent, Tropical Warm Pool)

The South Pacific convergence zone (SPCZ) & South Atlantic convergence zone (SACZ) are **Monsoon troughs** that branch off the The Intertropical Convergence Zone (ITCZ) at the points where the Indo-Australian monsoon and the South American monsoon occur.

* The Inter-Ocean Convergence Zone has traditionaly been called the Congo air boundary. Also called the South Indian Ocean Convergence Zone (SIOCZ) and Oceanic Tropical Convergence Zone (OTCZ).

See Asymmetry of the Intertropical Convergence Zone

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## Oscillations

The **Arctic oscillation** (AO) appears as a ringlike (or "annular") pattern of sea-level pressure anomalies centered at the poles. The presence of continents and large landmasses disrupts the ringlike structure at the Arctic pole, while anomalies surrounding the Antarctic pole are nearly circular. When the AO index is negative there tends to be high pressure in the polar region and greater movement of frigid polar air into middle latitudes. In other words the polar front moves closer to the equator.

* The **North Atlantic Oscillation** (NAO) is a weather phenomenon in the North Atlantic Ocean of fluctuations in the difference of atmospheric pressure at sea level (SLP) between the Icelandic Low and the Azores High. It is part of the Arctic oscillation.

* The **North Pacific Oscillation** (NPO) is a **teleconnection** pattern characterized by a north-south seesaw in sea level pressure over the North Pacific. During the positive (AB) phase sea level pressure is enhanced over a large region in the subtropics that extend poleward to 40N° and reduced at higher latitudes, westerlies are enhanced over the central Pacific and winter temperature are mild along much of the North America west coast but cooler than usual over Eastern Siberia and the United States South-West, precipitations are higher than usual over Alaska and the Great Plains.

The **Arctic dipole anomaly** is a pressure pattern characterized by high pressure on the arctic regions of North America and low pressure on those of Eurasia. While the Arctic Oscillation has an annular structure centered over and covering the entire Arctic, the Arctic dipole anomaly has two poles of opposite sign: one over the Canadian Arctic Archipelago and northern Greenland, the other over the Kara and Laptev Seas.

During a **La Nina**, a double ITCZ forms in the eastern Pacific, with one located north and another south of the Equator, one of which is usually stronger than the other. When this occurs, a narrow ridge of high pressure forms between the two convergence zones. Air sinking at the equator then splits in two. Half moves north to the northern ITCZ where it rises and half moves south to the southern ITCZ where it rises. Water at the surface at the equator therefore splits with half flowing north and half flowing south and deep cold water upwells at the equator.

* During an **El Niño** the opposite happens. The double ITCZ becomes a single ITCZ and the cold water stops upwelling. This can last from 9 months to 2 years. See Walker circulation.

The **Indian Ocean Dipole** (**IOD**), also known as the **Indian Niño**, is an irregular oscillation of sea surface temperatures in which the western Indian Ocean becomes alternately warmer (positive phase) and then colder (negative phase) than the eastern part of the ocean.

The **Pacific Decadal Oscillation** (PDO) is a robust, recurring pattern of ocean-atmosphere climate variability centered over the mid-latitude Pacific basin. During a "warm", or "positive", phase, the west Pacific becomes cooler and part of the eastern ocean warms; during a "cool" or "negative" phase, the opposite pattern occurs.

The **Madden–Julian oscillation** is a traveling pattern of enhanced rainfall that propagates eastward at approximately **14 to 29 km/h** (9 to 18 mph), through the atmosphere above the warm parts of the Indian and Pacific oceans (especially the South Pacific convergence zone). In the Pacific, MJO activity is typically greater during a La Niña episode and is virtually absent during the maxima of some El Niño episodes. Strong MJO activity is often observed 6 – 12 months prior to the onset of an El Niño episode. (The MJO is possibly related to the cycling of double and single ITCZ's. See also Eyewall replacement cycle.)

* The **Pacific–North American teleconnection pattern** (PNA) is a large-scale weather pattern over the North Pacific Ocean and the North American continent. The negative phase of the PNA pattern features below-average barometric pressure in the vicinity of Hawaii and over the inter-mountain region of North America, and above-average pressure located south of the Aleutian Islands and over the southeastern United States. The negative phase tends to be associated with Pacific cold episodes (La Niña). This is not so much an oscillation as much as a simple observation that much of California's rain comes from atmospheric rivers that originate near Hawaii when the region around Hawaii is extremely wet due to the MJO. (California is almost completely dry during the summer.)

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## Atmospheric rivers

See also: Great Flood of 1862, Pineapple Express, and Intensity-duration-frequency curve

Extratropical cyclones can become so large that they draw moisture up directly from the tropics in what is called an **atmospheric river**. (See the image to the right.) Atmospheric rivers are subtropical jet streams of moist air that are typically several thousand kilometers long and only a few hundred kilometers wide, and a single one can carry a greater flux of water than the Earth's largest river, the Amazon.

* The Amazon discharges more water into the ocean than the next 7 largest rivers. See Zipf's law.
	+ Like many other rivers the Amazon river valley is an Aulacogen.

During atmospheric river events both the Arctic oscillation and the Pacific–North American teleconnection pattern tend to be in the negative phase.

* The negative phase of the Arctic oscillation (AO) is associated with cold arctic air (and therefore the polar front) extending into the subtropics.
* The negative phase of the Pacific–North American teleconnection (PNA) pattern features below-average pressure (more rain) in the vicinity of Hawaii.

From Atmospheric river:

Atmospheric rivers are the major cause of **extreme precipitation events** that cause severe flooding in many mid-latitude, westerly coastal regions of the world, including the West Coast of North America, Western Europe, the west coast of North Africa, the Iberian Peninsula, Iran and New Zealand. Equally, the absence of atmospheric rivers has been linked with the occurrence of **droughts** in several parts of the world including South Africa, Spain and Portugal.

In California atmospheric rivers have contributed **30-50% of total annual rainfall** according to a 2013 study.

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## Seasonal lag

From Seasonal lag:

The amount of Solar energy reaching a location on Earth ("insolation", shown in blue) varies through the seasons but the surface temperatures will lag the primary cycle especially over the ocean.

* The top 2.5 m of the ocean holds as much heat as the entire atmosphere above it. See Mixed layer.

The length of seasonal lag varies between different climates, with extremes ranging from as little as 15–20 days (for polar regions in summer and continental interiors) to as much as 2½ months (for oceanic areas).

* San Francisco has an exceptionally long seasonal lag but this is due to the normally onshore winds weakening, and sometimes even reversing, in the autumn. See Devil winds.

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## Indices

See also: Statistical Analysis of Thunderstorms on the Eastern Tibetan Plateau Based on Modified Thunderstorm Indices
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### Lifted Index

From Atmospheric instability:

The lifted index (LI), usually expressed in kelvins, is the temperature difference between the temperature of the environment Te(p) and an air parcel lifted adiabatically Tp(p) at a given pressure height in the troposphere, usually 500 hPa (mb). When the value is positive, the atmosphere (at the respective height) is stable and when the value is negative, the atmosphere is unstable. Thunderstorms are expected with values below −2, and severe weather is anticipated with values below −6.

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### K Index

From Atmospheric instability:

 20 to 25
 26 to 30
 31 to 35
 Above 35

The K index is derived arithmetically: K-index = (850 hPa temperature – 500 hPa temperature) + 850 hPa dew point – 700 hPa dew point depression

* The temperature difference between 850 hPa (5,000 feet (1,500 m) above sea level) and 500 hPa (18,000 feet (5,500 m) above sea level) is used to parameterize the vertical temperature lapse rate.
* The 850 hPa dew point provides information on the moisture content of the lower atmosphere.
* The vertical extent of the moist layer is represented by the difference of the 700 hPa temperature (10,000 feet (3,000 m) above sea level) and 700 hPa dew point.

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### Bulk Richardson

From Atmospheric instability:

The Bulk Richardson Number (BRN) is a dimensionless number relating vertical stability and vertical wind shear (generally, stability divided by shear). It represents the ratio of thermally-produced turbulence and turbulence generated by vertical shear. Practically, its value determines whether convection is free or forced. High values indicate unstable and/or weakly sheared environments; low values indicate weak instability and/or strong vertical shear. Generally, values in the range of around 10 to 45 suggest environmental conditions favorable for supercell development..

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### Showalter index

From Atmospheric instability:

The Showalter index is a dimensionless number computed by taking the temperature at the 850 hPa level which is then taken dry adiabatically up to saturation, then up to the 500 hPa level, which is then subtracted by the observed 500 hPa level temperature. If the value is negative, then the lower portion of the atmosphere is unstable, with thunderstorms expected when the value is below −3. The application of the Showalter index is especially helpful when there is a cool, shallow air mass below 850 hPa that conceals the potential convective lifting. However, the index will underestimate the potential convective lifting if there are cool layers that extend above 850 hPa and it does not consider diurnal radiative changes or moisture below 850 hPa.

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## Snow tires

Brakes stop the tires. Tires stop the car.

In cold temperatures the rubber of a regular tire stiffens. Winter tires are designed to remain flexible, allowing the tire to grip the road better. (But winter tires can become too soft in summer.)

From Snow tire:

Attributes that can distinguish snow tires from summer tires include:

* An open, deep tread, whose *void ratio* between rubber and spaces between the solid rubber is comparatively high.
* Additional thin slits (called siping) in the rubber, that provide more biting edges and improve traction on wet or icy surfaces.
* *Shoulder blocks*—specialized tread design at the outside of the tire tread to increase snow contact and friction.
* A narrower tire to minimize resistance from the plowing effect of the tire through deeper snow.
* Hydrophilic rubber compounds that improve friction on wet surfaces

Wet-film conditions on hard-compacted snow or ice require studs or chains.

Tires should be rotated every six months or 6,000 to 8,000 miles.

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## References

1. ↑ Wikipedia:Advection
2. ↑ Wikipedia:Potential temperature
3. ↑ Wikipedia:Cumulus cloud
4. ↑ Wikipedia:Lightning detection
5. ↑ Wikipedia:Thunderstorm
6. ↑ Vertical Momentum - How Fast Does Supercell Air Go UP?
7. ↑ Indiana University
8. ↑ Vertical Momentum - How Fast Does Supercell Air Go UP?
9. ↑ Wikipedia:Wall cloud
10. ↑ Extreme Low-Level Updrafts and Wind Speeds Measured by Dropsondes in Tropical Cyclones
11. ↑ Wikipedia:Arcus cloud
12. ↑ Wikipedia:Trough (meteorology)
13. ↑ Wikipedia:Arctic oscillation
14. ↑ Wikipedia:North Atlantic oscillation
15. ↑ Wikipedia:North Pacific Oscillation
16. ↑ Wikipedia:Arctic dipole anomaly
17. ↑ Wikipedia:Pacific decadal oscillation
18. ↑ Wikipedia:Pacific–North American teleconnection pattern
19. ↑ Wikipedia:Atmospheric river
20. ↑ National Weather Service Forecast Office, Detroit, Michigan (2010-01-25). Gloassary: K. National Weather Service Central Region Headquarters. Retrieved on 2011-02-24
21. ↑ "NOAA's National Weather Service - Glossary". NOAA. http://forecast.weather.gov/glossary.php?word=SHOWALTER%20INDEX.

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## See also

Precipitable water
Madden-Julian Oscillation monitoring
National Weather Service