Chapter 10 Atmospheric Circulation And Weather Systems

As discussed in the previous chapter, the Earth's surface experiences uneven heating, which causes air to expand when warm (becoming less dense) and compress when cool (becoming denser). These density changes result in variations in atmospheric pressure across the globe. These pressure differences are the fundamental cause of air movement. Air naturally flows from areas of higher pressure to areas of lower pressure, setting the atmosphere in motion. Horizontal air movement is what we perceive as wind.

Atmospheric pressure also governs vertical air movement; air tends to rise in low-pressure areas and sink in high-pressure areas. Winds play a crucial role in redistributing heat and moisture across the planet. This constant transfer helps maintain a relatively stable average temperature for the Earth. When moist air rises, it cools, leading to the condensation of water vapour, formation of clouds, and eventually precipitation.

This chapter delves into the factors that cause pressure differences in the atmosphere, the forces that control wind patterns, the large-scale general circulation of the atmosphere, the characteristics of air masses, the weather associated with the interaction of different air masses (fronts), and the intense phenomena of tropical and extra-tropical storms.

Atmospheric Pressure

We are constantly subjected to the pressure exerted by the weight of the column of air above us. This pressure is known as atmospheric pressure. It is defined as the weight of a column of air of unit area, extending from mean sea level to the top of the atmosphere. Atmospheric pressure is commonly measured in units called millibars (mb).

At mean sea level, the average atmospheric pressure is approximately $1,013.25$ mb. Due to gravity, air is densest at the surface, resulting in the highest pressure there. Atmospheric pressure decreases with increasing altitude, as the weight of the overlying air column diminishes. Instruments used to measure air pressure include the mercury barometer and the aneroid barometer.

Variations in pressure from place to place, particularly horizontally, are the primary cause of wind, as air moves from areas of high pressure to areas of low pressure.

Vertical Variation Of Pressure

Atmospheric pressure decreases rapidly with height, particularly in the lower atmosphere. Near the surface, the pressure drops by about 1 millibar for every 10 meters ($1 \, mb / 10 \, m$) of increase in elevation. This rate of decrease slows down at higher altitudes. The following table illustrates the standard pressure and temperature at selected elevations:

While the vertical pressure gradient force (the upward force due to decreasing pressure with height) is much stronger than typical horizontal pressure gradient forces, it is almost perfectly balanced by the downward force of gravity. This balance, known as hydrostatic equilibrium, is why we don't experience constant strong upward or downward winds.

Horizontal Distribution Of Pressure

Although horizontal pressure differences are much smaller than vertical ones, even small variations are critical for driving wind. Horizontal pressure distribution is visualized on weather maps using isobars, which are lines connecting points of equal atmospheric pressure at a constant altitude (usually reduced to sea level to eliminate the effect of elevation).

To compare pressure between locations at different elevations, pressure measurements at any weather station are adjusted (reduced) to what they would be at sea level. This standardized pressure is called sea level pressure.

Weather maps show patterns of isobars that indicate pressure systems:

• Low-pressure system (Cyclone or Depression): Characterized by isobars enclosing an area with the lowest pressure at the center. Air tends to converge towards and rise in a low-pressure system.
• High-pressure system (Anticyclone): Characterized by isobars enclosing an area with the highest pressure at the center. Air tends to subside from above and diverge outwards from a high-pressure system.

Diagram illustrating isobar patterns around low and high pressure centers in the Northern Hemisphere, showing the associated wind direction and movement.

World Distribution Of Sea Level Pressure

The global pattern of sea level pressure shows distinct belts that broadly correspond to latitude. These belts shift seasonally with the apparent movement of the sun (Figures 10.2 and 10.3).

Map illustrating the distribution of average sea level pressure (in millibars) across the world for the month of January.

Map illustrating the distribution of average sea level pressure (in millibars) across the world for the month of July.

• Equatorial Low (or Doldrums): A belt of low pressure found near the equator (around 0-5° latitude), caused by intense heating and rising air.
• Subtropical Highs: Belts of high pressure located around 30-35° North and South latitudes, where air that rose at the equator descends.
• Subpolar Lows: Belts of low pressure situated around 60-65° North and South latitudes, where rising air is associated with the convergence of polar and mid-latitude air masses.
• Polar Highs: Areas of high pressure found near the North and South Poles, caused by intense cold and descending air.

These pressure belts are not static; they migrate northwards in the Northern Hemisphere summer (when the sun is overhead the Tropic of Cancer) and southwards in the Northern Hemisphere winter (when the sun is overhead the Tropic of Capricorn). This seasonal shift is more pronounced over continents than oceans due to their differential heating.

Forces Affecting The Velocity And Direction Of Wind

Wind movement is initiated by pressure differences, but its speed and direction are shaped by the combined effect of several forces:

• Pressure Gradient Force
• Frictional Force
• Coriolis Force
• Gravitational Force: While primarily acting downwards, it influences density and pressure. Its horizontal component on slopes can contribute to air movement, but it is not a primary force for large-scale horizontal winds.

Pressure Gradient Force

The pressure gradient force is the force that causes air to move from high pressure to low pressure. It arises from the difference in atmospheric pressure between two points. The greater the pressure difference over a given distance, the stronger the pressure gradient force, and the faster the air will move. The rate of change of pressure with distance is called the pressure gradient. Isobars on a weather map indicate the pressure gradient: where isobars are closely spaced, the pressure gradient is strong, and wind speed tends to be high; where isobars are far apart, the gradient is weak, and winds are lighter.

Frictional Force

Frictional force opposes the motion of the wind. It is caused by the resistance encountered as air moves over the Earth's surface and interacts with terrain, vegetation, and buildings. Friction is strongest at the surface and decreases with height, typically becoming negligible above 1-3 km. Friction slows down wind speed and, by doing so, indirectly influences the effect of the Coriolis force.

Coriolis Force

The Coriolis force is an apparent force caused by the Earth's rotation. It does not initiate wind movement but acts to deflect it. In the Northern Hemisphere, the Coriolis force deflects moving air (and other moving objects) to the right of its intended path. In the Southern Hemisphere, it deflects moving air to the left. The strength of the Coriolis force is directly proportional to the wind speed and the sine of the latitude. It is strongest at the poles (maximum latitude) and zero at the equator. Therefore, wind is deflected more at higher latitudes and when it is blowing faster. The Coriolis force acts perpendicular to the direction of wind flow.

Pressure And Wind

The actual velocity and direction of wind are the result of the combined action of the pressure gradient force, friction, and Coriolis force.

In the upper atmosphere (above the friction layer, around 2-3 km), wind is mainly controlled by the balance between the pressure gradient force and the Coriolis force. When isobars are straight and friction is absent, these two forces balance each other, and the resulting wind blows parallel to the isobars. This hypothetical wind is called the geostrophic wind (Figure 10.4).

Diagram illustrating how the balance between the pressure gradient force and the Coriolis force results in wind blowing parallel to isobars (geostrophic wind) in the upper atmosphere.

Near the Earth's surface, friction also plays a role. Friction slows down the wind, which in turn reduces the Coriolis force (as it depends on wind speed). This reduced Coriolis force no longer perfectly balances the pressure gradient force, causing the wind to blow at an angle across the isobars, towards the lower pressure area. The angle at which the wind crosses the isobars depends on the amount of friction.

Around pressure systems, the combination of pressure gradient and Coriolis forces results in characteristic wind circulation patterns (Figure 10.1):

• Around a Low-Pressure Center (Cyclonic Circulation): Air converges towards the center and spirals inwards. Due to the Coriolis force, the circulation is anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
• Around a High-Pressure Center (Anticyclonic Circulation): Air diverges outwards from the center and spirals away. Due to the Coriolis force, the circulation is clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere.

The following table summarizes the wind direction patterns around pressure systems:

Vertical air movement is linked to these horizontal patterns (Figure 10.5). In low-pressure areas, converging surface winds force air to rise. In high-pressure areas, air subsides from above and diverges at the surface. Rising air, caused by convergence, convection, uplift along fronts, orographic uplift (air forced up by mountains), is essential for cloud formation and precipitation.

Diagram showing how surface winds converge towards low-pressure centers and diverge away from high-pressure centers, leading to rising and sinking air respectively.

The absence of the Coriolis force at the equator is why tropical cyclones (which require a strong spiraling circulation) do not form very close to the equator. Air flows directly towards the low pressure instead of being deflected, preventing the necessary strong rotational development.

General Circulation Of The Atmosphere

The large-scale, persistent patterns of wind circulation around the globe are known as the general circulation of the atmosphere. This circulation is primarily driven by the uneven distribution of solar heating across different latitudes and is shaped by the resulting pressure belts, the Earth's rotation (Coriolis force), the distribution of land and oceans, and the seasonal migration of pressure and wind belts following the sun's apparent path.

Simplified diagram showing the conceptual atmospheric circulation cells (Hadley, Ferrel, Polar) and the associated pressure belts and surface winds.

A simplified model of the general circulation involves three major convection cells in each hemisphere (Figure 10.6):

• Hadley Cell: Located in the tropics. Intense solar heating at the equator causes air to rise, creating the equatorial low-pressure zone (ITCZ). This rising air moves poleward at high altitudes, cools, and sinks around 30° N and S latitudes, forming the subtropical high-pressure belts. At the surface, air flows back towards the equator from the subtropical highs, creating the Trade Winds (Easterlies). The convergence of the Trade Winds from both hemispheres forms the ITCZ.
• Ferrel Cell: Located in the middle latitudes (around 30-60° N and S). This is a less direct cell driven by the interactions between the Hadley and Polar cells. Air from the subtropical highs moves poleward as surface winds (the Westerlies). Around 60° latitude, these westerlies converge with cold air from the poles, forcing air to rise, creating the subpolar low-pressure zones. This rising air then moves equatorward aloft and sinks back in the subtropics, completing the cell.
• Polar Cell: Located in the high latitudes (around 60-90° N and S). Intense cold at the poles causes air to sink, creating the polar high-pressure zones. This cold, dense air flows equatorward at the surface, forming the Polar Easterlies. These easterlies converge with the westerlies from the mid-latitudes around 60°, forcing air to rise in the subpolar low, and then move poleward aloft to sink at the poles.

These three cells transport heat energy from the energy-surplus regions near the equator to the energy-deficit regions near the poles, preventing extreme temperature imbalances across the globe.

The formation of the subtropical high-pressure belts around 30° N and S is primarily due to the descent of air that rose convectively at the equator in the Hadley Cell. As this upper-level air moves poleward, it cools and becomes denser, causing it to sink. This sinking air compresses as it descends, warming adiabatically, but the dominant effect is subsidence, which increases pressure at the surface, forming the subtropical highs.

General Atmospheric Circulation And Its Effects On Oceans

The large-scale wind patterns of the general atmospheric circulation exert stress on the surface of the oceans, driving the major surface ocean currents. For example, the Trade Winds push water westward in the tropics, and the Westerlies drive currents eastward in the mid-latitudes.

These ocean currents play a vital role in distributing heat across the globe, similar to atmospheric circulation. Warm currents transport heat from lower to higher latitudes, and cold currents bring cooler water towards the equator. The interaction between the atmosphere and the oceans is a continuous, slow process over vast areas, with oceans providing moisture and energy input to the atmosphere.

One of the most significant interactions, with global climate implications, involves the Pacific Ocean. The cyclical warming and cooling of the central and eastern tropical Pacific Ocean surface waters are linked to fluctuations in atmospheric pressure in the western Pacific. This coupled ocean-atmosphere phenomenon is known as the El Niño-Southern Oscillation (ENSO).

• Southern Oscillation: Refers to the seesaw pattern of atmospheric pressure between the eastern and western tropical Pacific. Normally, there is high pressure in the eastern Pacific and low pressure in the western Pacific.
• El Niño: Refers to the anomalous warming of sea surface temperatures in the central and eastern tropical Pacific, associated with a weakening or reversal of the normal trade winds and a shift in the pressure pattern (lower pressure in the east, higher in the west). This warming replaces the normal cool Peruvian current off the coast of South America.

A strong ENSO event can trigger widespread weather disturbances globally, including heavy rainfall in normally arid regions like the west coast of South America, droughts in areas like Australia and India, and floods in other regions like China. Monitoring ENSO is crucial for long-range weather forecasting in many parts of the world.

Seasonal Wind

The general pattern of atmospheric circulation and the associated pressure belts and wind systems shift seasonally as the regions of maximum solar heating migrate north and south. This shift is particularly pronounced over large continental areas due to their differential heating compared to oceans.

The most significant example of a seasonal wind system resulting from this differential heating and pressure belt shift is the monsoon, particularly prominent over Southeast Asia. Monsoons involve a seasonal reversal of wind direction, typically bringing heavy summer rainfall (wind blows from sea to land) and dry winters (wind blows from land to sea).

Local Winds

In addition to the large-scale general circulation and seasonal monsoons, local differences in heating and cooling of specific surfaces can generate smaller-scale wind systems that operate daily or annually over limited areas.

Land And Sea Breezes

These are common local winds occurring along coastlines due to the different heating properties of land and water (Figure 10.7).

Diagram illustrating the daily cycle of sea breeze during the day and land breeze during the night along a coast.

• Sea Breeze (Daytime): During the day, land heats up faster and becomes warmer than the adjacent sea. The warmer air over the land rises, creating a localized low-pressure area. Over the relatively cooler sea, pressure is higher. This pressure difference drives wind from the sea towards the land, known as a sea breeze.
• Land Breeze (Nighttime): At night, land cools down faster than the sea. The cooler air over the land becomes denser, creating a localized high-pressure area. Over the relatively warmer sea, pressure is lower. This pressure difference drives wind from the land towards the sea, known as a land breeze.

Mountain And Valley Winds

These local winds occur in mountainous regions due to the differential heating and cooling of slopes and valleys.

• Valley Breeze (Daytime): During the day, mountain slopes receive direct sunlight and heat up rapidly. The air in contact with the slopes warms, becomes less dense, and flows upslope. To replace this rising air, cooler air from the valley floor moves up the valley, creating a valley breeze.
• Mountain Wind (Nighttime): At night, the mountain slopes cool down rapidly by radiation. The air in contact with the slopes also cools, becomes denser and heavier, and flows downslope into the valley floor, creating a mountain wind. This cold, descending air can accumulate in the valley bottom.

A specific type of cold, dense downslope wind, often originating from high, ice-covered plateaus or snowfields and flowing into valleys, is called a katabatic wind.

Another type of local wind is a warm, dry wind that occurs on the leeward (downwind) side of mountain ranges (e.g., the Foehn in the Alps or the Chinook in the Rockies). As moist air is forced up the windward side of the mountain, it cools and condenses, releasing precipitation. After passing over the crest, the now dry air descends the leeward side. As it descends, it is compressed and warms rapidly adiabatically (without heat exchange with the surroundings). This warm, dry wind can cause rapid melting of snow.

Air Masses

An air mass is a very large body of air that has relatively uniform temperature and humidity characteristics across its horizontal extent. Air masses acquire these characteristics by remaining stationary or moving slowly over a vast geographical region with relatively uniform surface properties for a sufficiently long period. These uniform regions are called source regions for air masses.

Ideal source regions are typically extensive, homogenous areas such as large ocean surfaces, vast plains, or large ice-covered areas. These regions allow the overlying air to take on the temperature and moisture properties of the surface below.

Air masses are classified based on their source region and associated temperature and moisture characteristics. The main types are based on whether they originate from tropical (warm) or polar (cold) regions, and from continental (dry) or maritime (moist) surfaces:

• Maritime Tropical (mT): Warm and moist air, originating over warm tropical/subtropical oceans.
• Continental Tropical (cT): Warm and dry air, originating over subtropical hot deserts.
• Maritime Polar (mP): Cold and moist air, originating over cold high-latitude oceans.
• Continental Polar (cP): Cold and dry air, originating over cold, high-latitude continental areas (e.g., Siberia, northern Canada).
• Continental Arctic (cA): Very cold and very dry air, originating over permanently ice-covered polar continents (e.g., Arctic basin, Antarctica). Sometimes considered an extremely cold variant of cP.

When air masses move away from their source regions, they carry their temperature and moisture characteristics to new areas, significantly influencing the weather of those regions.

Fronts

A front is a boundary zone that separates two air masses of different densities (which are typically related to differences in temperature and/or humidity). The process of front formation is called frontogenesis. Fronts are dynamic zones of transition where significant weather changes often occur.

Fronts primarily form in the middle latitudes (between approximately 30° and 60° latitude), where contrasting air masses from polar and tropical regions meet. These zones are characterized by relatively steep horizontal gradients in temperature and pressure across the frontal boundary, and often cause air to rise, leading to cloud formation and precipitation.

There are four main types of fronts (Figure 10.8 illustrates vertical cross-sections of fronts):

• Stationary Front: Occurs when the boundary between two air masses is not moving significantly. Winds on either side of a stationary front blow parallel to the front in opposite directions.

Diagrams showing vertical views of how different air masses interact at various types of fronts (warm, cold, occluded), and the resulting cloud formations and precipitation zones.

• Cold Front: Forms when a mass of cold air advances and displaces warmer air. Since cold air is denser, it wedges beneath the warmer air, forcing the warm air to rise rapidly. Cold fronts are typically associated with steep frontal slopes, narrow bands of intense precipitation (often heavy showers or thunderstorms), and a relatively sharp drop in temperature after the front passes.
• Warm Front: Forms when a mass of warm air advances and slides over a cooler air mass. Since warm air is less dense, it gently overrides the cooler air ahead of it. Warm fronts have gentler slopes than cold fronts and are associated with broader areas of lighter, more continuous precipitation (drizzle or steady rain or snow) over a longer period, and a gradual increase in temperature after the front passes.
• Occluded Front: Forms when a faster-moving cold front catches up to and overtakes a warm front. The warm air mass is lifted completely off the ground, sandwiched between two cooler air masses. Occluded fronts are complex and can be associated with various weather phenomena, combining characteristics of both cold and warm fronts.

Fronts are key components of weather systems like extra-tropical cyclones, which develop along these boundaries.

Extra Tropical Cyclones

Extra-tropical cyclones (also called mid-latitude cyclones or wave cyclones) are large low-pressure systems that form in the mid and high latitudes (typically between 30° and 60° latitude), outside the tropics. They are associated with frontal systems and bring significant day-to-day changes in weather.

Extra-tropical cyclones often form along the polar front, the boundary between cold polar air and warmer mid-latitude air. The development typically follows stages (Figure 10.9 shows a developed cyclone structure):

• Initially, a stationary front exists between warm and cold air masses.
• A disturbance or wave develops along the front, causing pressure to drop locally.
• This pressure drop initiates a cyclonic circulation (anticlockwise in the NH, clockwise in the SH). Warm air begins to move poleward (northwards in NH), and cold air moves equatorward (southwards in NH).
• As the circulation develops, a distinct warm front forms ahead of the advancing warm air, and a cold front forms behind the advancing cold air. A sector of warm air is wedged between these two fronts (the warm sector).
• Along the warm front, warm air glides up and over the cooler air ahead, producing a sequence of clouds (cirrus, cirrostratus, altostratus, nimbostratus) and widespread, relatively light precipitation ahead of the front.
• Along the cold front, colder, denser air rapidly lifts the warmer air ahead. This forced lifting is more vigorous, leading to the formation of cumuliform clouds (cumulus, cumulonimbus) and a narrow band of intense precipitation, often heavy showers or thunderstorms, along and just behind the front.
• Cold fronts typically move faster than warm fronts. Eventually, the cold front catches up to the warm front, lifting the warm sector completely off the ground. This stage is called occlusion, forming an occluded front. The cyclone gradually dissipates as the supply of warm, moist air is cut off from the surface.

Diagram illustrating the characteristic frontal system (warm and cold fronts), air mass distribution (warm and cold sectors), cloud patterns, and wind circulation around a well-developed extra-tropical cyclone.

Key differences between extra-tropical cyclones and tropical cyclones:

Tropical Cyclones

Tropical cyclones are intense, rotating storm systems that form over warm tropical oceans. They are among the most powerful and destructive natural hazards, characterized by extremely strong winds, torrential rainfall, and coastal flooding from storm surges.

These storms are known by different names in various regions: Cyclones (Indian Ocean), Hurricanes (Atlantic Ocean and Northeast Pacific), Typhoons (Northwest Pacific and South China Sea), and Willy-willies (off the coast of Western Australia).

Tropical cyclones originate and gain strength over warm tropical waters. Specific conditions are required for their formation and intensification:

• Large Sea Surface Temperature: The ocean surface temperature must be at least $27^\circ C$ ($80^\circ F$) to provide the necessary heat and moisture.
• Presence of Coriolis Force: A sufficient Coriolis force is needed to initiate and maintain the rotating circulation. Tropical cyclones rarely form within about 5 degrees ($5^\circ$) of the equator where the Coriolis force is negligible.
• Small Vertical Wind Shear: Minimal change in wind speed and direction with height is required to prevent the storm's vertical structure from being disrupted.
• Pre-existing Disturbance: A weak low-pressure area or a pre-existing area of low-level cyclonic circulation (like a tropical wave) is needed to act as a trigger.
• Upper-level Divergence: Air must be diverging away from the top of the storm system at high altitudes. This divergence helps draw air up from below and maintains the low pressure at the surface.

The primary energy source that fuels and intensifies a tropical cyclone is the release of latent heat from the condensation of vast amounts of water vapour. Warm, moist air rises rapidly in the towering cumulonimbus clouds surrounding the storm's center. As the water vapour condenses into cloud droplets and rain, it releases large quantities of heat, which further warms the air and causes it to rise even faster, creating a positive feedback loop that strengthens the storm.

The storm continues to gain energy as long as it is over warm ocean water, receiving a continuous supply of moisture. When a tropical cyclone moves over land or cooler water, the supply of moisture and heat is cut off, causing the storm to weaken rapidly and dissipate.

The location where the center of a tropical cyclone crosses the coastline is called its landfall. Tropical cyclones often move westward or northwestward in the tropics, steered by the trade winds. Some storms may recurve poleward (often after crossing 20° N or S latitude) and transition into extra-tropical cyclones or dissipate.

A mature tropical cyclone has a distinct vertical structure (Figure 10.10):

Diagram illustrating the vertical structure of a mature tropical cyclone, showing the central 'eye', the surrounding eyewall with strong updrafts, and outward spiraling rainbands.

• Eye: The calm, clear area at the very center of the storm. Air in the eye is typically subsiding (sinking). The diameter of the eye can range from about 10 to 100 km, but is often around 30-60 km. It is a region of very low pressure.
• Eyewall: A ring of extremely intense thunderstorms that surrounds the eye. This is where the strongest winds and heaviest rainfall occur. Air rises violently in the eyewall, reaching high altitudes (up to the tropopause). Wind speeds are highest in this region, potentially exceeding 250 km/h.
• Spiral Rainbands: Bands of thunderstorms and precipitation that spiral inwards towards the eyewall from the outer edges of the storm.

The overall diameter of a tropical cyclone can range from 150 to over 1200 km. These systems typically move relatively slowly, around 300-500 km per day.

Besides destructive winds and torrential rain, tropical cyclones are known for producing storm surges – a dangerous rise in sea level caused by the low pressure and strong winds pushing water towards the coast. Storm surges can cause extensive coastal flooding and damage. Once over land, the cyclone rapidly loses energy and dissipates.

Torrential rains and high velocity winds blow primarily in the eyewall of a tropical cyclone. This is because the eyewall is where the most vigorous updrafts of warm, moist air occur, leading to massive condensation and latent heat release (fueling the storm) and driving the strongest rotating winds around the extremely low pressure of the eye.

Thunderstorms And Tornadoes

Thunderstorms and tornadoes are severe, localized storms that are relatively short-lived but can be extremely violent.

• Thunderstorms: Are intense convective storms that develop within towering cumulonimbus clouds. They are typically triggered by strong heating of the surface on moist, unstable days, causing rapid upward motion of air. Thunderstorms are characterized by lightning (electrical discharge within or from the cloud) and thunder (the sound produced by the rapid expansion of air heated by lightning). If temperatures in the upper parts of the cloud are below freezing, precipitation can form as hail (hailstorm). In dry environments, intense convection might produce duststorms instead of thunderstorms. A thunderstorm involves strong updrafts of warm, moist air that build the cloud and intense downdrafts of cooler air and precipitation.
• Tornadoes: Are highly violent, rotating columns of air that extend downwards from the base of severe thunderstorms (specifically supercell thunderstorms) and make contact with the ground. They are characterized by extremely low pressure at their core and incredibly high wind speeds, causing immense destruction along their narrow path. A tornado over water is called a waterspout. Tornadoes are most common in the middle latitudes, particularly in certain regions like the central United States ("Tornado Alley").

These severe storms represent the atmosphere's intense and often turbulent adjustment to energy imbalances. Potential energy (stored in the air mass configuration) and latent heat energy (from condensation) are converted into kinetic energy (wind, rotation) in these storms before the atmosphere returns to a more stable state.

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