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Chapter 9 Atmospheric Circulation And Weather Systems
Chapter 8 explained that solar energy is not distributed equally across the Earth's surface. This uneven heating has significant consequences for the atmosphere. When air is heated, it expands and becomes less dense. When it cools, it contracts and becomes denser. These differences in air density lead to variations in atmospheric pressure.
Air naturally moves from areas of higher pressure to areas of lower pressure. This horizontal movement of air is what we call **wind**. Atmospheric pressure also influences vertical air movement – whether air rises or sinks.
The movement of air, both horizontal and vertical, is crucial for regulating Earth's climate. Wind helps redistribute heat and moisture around the globe, preventing tropical regions from becoming excessively hot and polar regions from becoming excessively cold. Vertically rising moist air cools, leading to condensation, cloud formation, and precipitation, which is essential for the water cycle.
This chapter explores the factors causing pressure differences, the forces that govern air circulation, the complex patterns of wind, the formation of large bodies of air with uniform properties (air masses), the weather disturbances that occur when these air masses interact, and the powerful phenomenon of tropical storms.
Atmospheric Pressure
Despite being invisible, air has weight and exerts pressure on everything it surrounds, including our bodies. This pressure is caused by the weight of the column of air extending from a given point up to the top of the atmosphere.
**Atmospheric pressure** is defined as the weight of a column of air per unit area, measured from a specific level (like mean sea level) to the top of the atmosphere. The standard unit for atmospheric pressure is the **millibar (mb)** or Pascal (Pa). At mean sea level, the average atmospheric pressure is approximately **1013.2 millibars** (or 101,325 Pa).
Due to gravity, air is densest near the Earth's surface, resulting in higher pressure at lower elevations. As altitude increases, the air becomes less dense, and the pressure decreases. Atmospheric pressure is typically measured using instruments like a mercury barometer or an aneroid barometer.
While pressure decreases with height, it also varies horizontally from place to place at any given elevation. These horizontal pressure differences are the fundamental cause of air movement (wind), which flows from high-pressure areas towards low-pressure areas.
Vertical Variation Of Pressure
In the lower part of the atmosphere (troposphere), atmospheric pressure decreases quite rapidly with increasing height. On average, the pressure drops by about **1 millibar for every 10 meters** increase in elevation. However, this rate is not constant and varies with temperature and other factors. The pressure decreases as there are fewer air molecules pressing down from above.
The decrease in pressure with height is known as the vertical pressure gradient. The force associated with this gradient is very strong – much larger than horizontal pressure gradient forces. However, this strong upward force is almost perfectly balanced by the equally strong downward force of **gravity** acting on the air molecules. This balance is why we don't constantly experience powerful upward winds, except in specific localized situations like strong thunderstorms or convection currents.
A standard atmosphere model provides average pressure and temperature values at different elevations. (This refers to Table 9.1).
| Level | Pressure in mb | Temperature °C |
|---|---|---|
| Sea Level | 1,013.25 | 15.2 |
| 1 km | 898.76 | 8.7 |
| 5 km | 540.48 | –17. 3 |
| 10 km | 265.00 | – 49.7 |
Horizontal Distribution Of Pressure
Even small differences in horizontal pressure from one location to another are crucial because they drive air movement (wind). To study horizontal pressure distribution, meteorologists create maps showing **isobars**. Isobars are lines drawn on a weather map connecting places that have the **same atmospheric pressure** at a given time.
To create comparable maps, pressure measurements taken at various altitudes are adjusted or "reduced" to their equivalent value at mean sea level. This eliminates the effect of elevation, allowing us to see pressure patterns caused by thermal differences and air circulation.
Pressure systems appear as closed isobaric patterns on weather maps:
- A **low-pressure system** (also called a depression or cyclone) is an area where the pressure is lowest in the center and increases outwards. It is represented by concentric isobars with decreasing values towards the center. (This refers to Figure 9.1 illustrating pressure systems).
- A **high-pressure system** (also called an anticyclone) is an area where the pressure is highest in the center and decreases outwards. It is shown by concentric isobars with increasing values towards the center. (This refers to Figure 9.1).
World Distribution Of Sea Level Pressure
The global pattern of average sea-level pressure reveals several significant belts that influence the general circulation of the atmosphere. These belts are not fixed but shift slightly with the seasons as the areas of maximum heating move following the sun's apparent path.
(This refers to Figure 9.2 and 9.3 showing global pressure distribution in January and July).
Key pressure belts include:
- **Equatorial Low:** Located near the equator, this is a zone of low pressure ($0^\circ$ to $5^\circ$ North and South). It forms due to intense heating and rising air (convection). This zone is also known as the Inter Tropical Convergence Zone (ITCZ).
- **Subtropical Highs:** High-pressure areas located around $30^\circ$ North and $30^\circ$ South latitudes. These form partly due to sinking air from the upper atmosphere and are zones of relatively stable, dry conditions.
- **Subpolar Lows:** Low-pressure belts found around $60^\circ$ North and $60^\circ$ South latitudes. These result from the convergence of warmer air from the subtropics and colder air from the poles, leading to rising air.
- **Polar Highs:** High-pressure zones located near the poles ($90^\circ$ North and South). These form due to the intense cold and sinking of dense air.
These pressure belts are not static. They shift 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), following the apparent movement of the sun.
Forces Affecting The Velocity And Direction Of Wind
Air movement (wind) is initiated by pressure differences, causing air to flow from high pressure to low pressure. However, the wind's speed and direction are also influenced by other forces:
Pressure Gradient Force
The **pressure gradient force** is the primary force that causes air to move. It arises from differences in atmospheric pressure over a horizontal distance. The rate at which pressure changes with distance is called the **pressure gradient**. The force acts perpendicular to the isobars, directed from higher pressure towards lower pressure.
Where isobars are closely spaced, the pressure gradient is steep, and the pressure gradient force is strong, resulting in faster winds. Where isobars are widely spaced, the gradient is gentle, the force is weak, and winds are slower.
Frictional Force
The Earth's surface exerts a **frictional force** on the moving air. This force acts opposite to the direction of wind movement and reduces wind speed. Friction is greatest near the surface (due to terrain, buildings, vegetation) and decreases with height, becoming negligible typically above 1-3 km. Over smooth surfaces like oceans, friction is minimal.
Coriolis Force
The rotation of the Earth introduces an apparent force called the **Coriolis force**. This force does not actually cause the air to move, but it deflects the path of moving objects (like wind and ocean currents) relative to the Earth's surface. Named after G.G. Coriolis, who described it in 1844, this force causes deflection to the **right** of the direction of motion in the **Northern Hemisphere** and to the **left** in the **Southern Hemisphere**.
Key characteristics of the Coriolis force:
- It acts perpendicular to the direction of motion.
- Its strength is zero at the **equator** and maximum at the **poles**.
- Its strength is proportional to the speed of the moving object and the sine of the latitude angle. Faster winds experience greater deflection.
The Coriolis force is crucial in shaping wind patterns. In the upper atmosphere, where friction is negligible, wind is primarily influenced by the balance between the pressure gradient force and the Coriolis force. The pressure gradient force pushes air from high to low pressure, while the Coriolis force deflects it. Since the Coriolis force acts perpendicular to the pressure gradient force, the resulting wind often flows parallel to the isobars.
Pressure And Wind
The actual speed and direction of wind at any point are the result of the combined effects of the pressure gradient force, frictional force, and Coriolis force.
In the upper atmosphere, typically 2-3 km above the surface, friction is minimal. Here, winds are primarily governed 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 wind flows parallel to the isobars. This theoretical wind is called the **geostrophic wind** (Figure 9.4). In reality, winds aloft are often close to being geostrophic, especially in straight flow patterns.
Near the Earth's surface, friction reduces wind speed. A slower wind experiences a weaker Coriolis force. This allows the pressure gradient force to have a greater influence, causing the wind to blow across the isobars at an angle, from high to low pressure (rather than parallel). The angle is steeper over rough terrain (more friction) and shallower over smooth surfaces (less friction).
The balance and interaction of these forces also determine the circulation patterns around pressure systems:
- Around a **low-pressure center**, air flows inwards and converges, then rises vertically.
- Around a **high-pressure center**, air sinks from aloft, then flows outwards and diverges at the surface.
These circulation patterns are called **cyclonic circulation** (around a low) and **anticyclonic circulation** (around a high). The direction of rotation differs between the hemispheres due to the Coriolis force (Table 9.2).
| Pressure System | Pressure Condition at the Centre | Pattern of Wind Direction Northern Hemisphere | Pattern of Wind Direction Southern Hemisphere |
|---|---|---|---|
| Cyclone | Low | Anticlockwise | Clockwise |
| Anticyclone | High | Clockwise | Anticlockwise |
The surface convergence in low-pressure systems (Figure 9.5) forces air to rise. Rising air is essential for cloud formation and precipitation because as air rises, it cools, and its water vapour capacity decreases, leading to condensation. Various processes can cause air to rise, including convergence (air flowing towards a central point), convection (heating from below), orographic uplift (air forced over mountains), and uplift along fronts (boundaries between air masses).
Conversely, sinking air in high-pressure systems (Figure 9.5) is associated with clear skies and stable conditions, as sinking air warms and dries.
Interestingly, the Coriolis force is zero at the equator. This means air flowing into an equatorial low-pressure area does not experience deflection and can flow directly towards the center. Without the Coriolis force to induce a rotational circulation, the low pressure tends to get filled by the incoming air rather than intensifying and forming a strong vortex. This is why tropical cyclones (which require significant Coriolis force to spin up) do not form very close to the equator.
General Circulation Of The Atmosphere
The large-scale, semi-permanent patterns of wind circulation across the globe constitute the **general circulation of the atmosphere**. This pattern is driven by several factors:
- Latitudinal variations in heating by the sun.
- The resulting pressure belts (Equatorial Low, Subtropical Highs, etc.).
- The seasonal migration of these pressure belts.
- The distribution of continents and oceans, which affects thermal patterns and pressure.
- The rotation of the Earth (Coriolis force).
The general circulation can be conceptually understood through a simplified model involving three major convective cells in each hemisphere: the Hadley Cell, the Ferrel Cell, and the Polar Cell (Figure 9.6).
- **Hadley Cell (Tropical Cell):** In the **Inter Tropical Convergence Zone (ITCZ)** near the equator (Equatorial Low), intense heating causes air to rise. This rising air flows poleward in the upper atmosphere, cools, and sinks around $30^\circ$ North and South latitudes, contributing to the formation of the **Subtropical Highs**. At the surface, air flows from these subtropical highs back towards the equator as the **Trade Winds** (easterlies).
- **Ferrel Cell (Mid-latitude Cell):** This cell is driven less directly by temperature and more by the interaction between the Hadley and Polar cells. Air sinks from the subtropical highs ($30^\circ$ N/S) and flows towards the poles at the surface as the **Westerlies**. Around $60^\circ$ N/S (Subpolar Lows), this warmer air converges with cold polar air and rises, then flows poleward aloft to sink near the poles.
- **Polar Cell:** At the poles (Polar Highs), intensely cold, dense air sinks and flows towards the mid-latitudes at the surface as the **Polar Easterlies**. Around $60^\circ$ N/S, this cold air meets the warmer westerlies and rises, then flows back towards the poles aloft.
This three-cell model explains the general pattern of pressure belts and planetary winds (Trade Winds, Westerlies, Polar Easterlies). The overall effect of this circulation is to transfer heat energy from the energy-surplus tropical regions to the energy-deficit polar regions, maintaining the Earth's heat balance and climate zones.
General Atmospheric Circulation And Its Effects On Oceans
The large-scale wind patterns of the general atmospheric circulation exert a frictional drag on the surface of the oceans, driving the large-scale **ocean currents**. These ocean currents also play a significant role in global heat distribution, transporting warm water towards the poles and cold water towards the equator, further influencing regional climates.
The interaction between atmospheric and oceanic circulation can lead to significant climate phenomena. A well-known example is the interaction in the Pacific Ocean:
- **El Niño-Southern Oscillation (ENSO):** This is a major climate pattern involving coupled atmospheric and oceanic changes in the tropical Pacific. Normally, trade winds push warm surface water towards the western Pacific. During an **El Niño** event, these trade winds weaken or reverse, causing warm water in the central Pacific to shift eastward towards the coast of South America, replacing the usual cold upwelling (Peruvian current).
- The appearance of this warm water off Peru is associated with changes in the atmospheric pressure pattern over the tropical Pacific, known as the **Southern Oscillation** (low pressure shifting from Indonesia/Australia towards the central/eastern Pacific, and high pressure shifting westward).
ENSO is the term for the combined phenomenon. Strong ENSO events can have significant global impacts, including altered rainfall patterns, droughts, and floods in various parts of the world. Monitoring ENSO is important for long-range weather and climate forecasting globally.
Seasonal Wind
The general pattern of atmospheric circulation is modified seasonally, primarily due to the north-south shift of the pressure belts and ITCZ following the sun. This shift leads to the development of **seasonal winds** that reverse direction between summer and winter. The most dramatic example is the **Monsoon** system, particularly prominent over South and Southeast Asia, where large-scale wind patterns reverse direction seasonally, bringing distinct wet and dry periods.
Apart from these large-scale seasonal shifts, there are also localized wind systems.
Local Winds
Differences in heating and cooling between adjacent surfaces (like land and sea, or mountain slopes and valleys) can create localized pressure gradients and wind systems that operate on daily or annual cycles.
Land And Sea Breezes
These are local winds that occur in coastal areas due to the differential heating and cooling of land and water. (This refers to Figure 9.7).
- **Sea Breeze:** During the **day**, land heats up faster than the sea. The warmer air over land rises, creating lower pressure. Cooler, denser air over the sea has higher pressure and flows inland to replace the rising air, creating a breeze blowing from the sea towards the land.
- **Land Breeze:** During the **night**, land cools down faster than the sea. The air over the cooler land becomes denser and has higher pressure. The sea retains heat longer, so the air over the sea is relatively warmer and has lower pressure. Air flows from the land towards the sea, creating a breeze blowing from the land towards the sea.
Mountain And Valley Winds
These local winds occur in mountainous areas due to the differential heating and cooling of mountain slopes and valley floors.
- **Valley Breeze (Anabatic Wind):** During the **day**, mountain slopes heat up faster than the valley floor. The warmer air along the slopes rises. Cooler air from the valley floor flows uphill to replace it, creating a breeze blowing up the valley.
- **Mountain Wind (Katabatic Wind):** During the **night**, mountain slopes cool rapidly. The air in contact with the slopes cools, becomes denser, and flows downslope into the valley floor as a cold, heavy air mass. This downslope flow of cold air is called a mountain wind or **katabatic wind**. Very strong, cold katabatic winds can occur where cold air drains from high plateaus or ice fields into valleys.
Some mountain winds on the leeward side of ranges can be warm and dry. When moist air is forced to rise over a mountain, it cools and releases precipitation on the windward side. As this now-dry air descends the leeward side, it warms adiabatically (due to compression), resulting in a warm, dry wind (e.g., Foehn in the Alps, Chinook in the Rockies) which can cause rapid snowmelt.
Air Masses
An **air mass** is a very large volume of air that has acquired uniform temperature and humidity characteristics from staying over a particular region, called its **source region**, for a significant period. Source regions are typically vast, homogenous areas like large oceans or extensive plains.
Air masses are classified based on the characteristics of their source region:
- By temperature: **Tropical (T)** air masses are warm, and **Polar (P)** air masses are cold. **Arctic (A)** or Antarctic (AA) are extremely cold.
- By moisture content: **Maritime (m)** air masses form over oceans and are moist. **Continental (c)** air masses form over land and are dry.
Combining these, major types of air masses include:
- Maritime Tropical (mT): Warm and moist (from tropical oceans).
- Continental Tropical (cT): Warm and dry (from subtropical deserts).
- Maritime Polar (mP): Cold and moist (from high-latitude oceans).
- Continental Polar (cP): Cold and dry (from snow-covered high-latitude continents).
- Continental Arctic (cA): Very cold and dry (from Arctic/Antarctic continents).
Fronts
A **front** is the boundary zone that forms when two air masses with different temperature and/or moisture characteristics meet. The process of front formation is called **frontogenesis**. Fronts are zones of significant weather change because the meeting of air masses with different densities often leads to the uplift of warmer, lighter air over colder, denser air, causing condensation and precipitation.
There are four main types of fronts (Figure 9.8):
- **Stationary Front:** When the boundary between two air masses remains relatively still, with neither air mass advancing.
- **Cold Front:** Forms when a colder air mass actively moves into an area occupied by warmer air. The denser cold air wedges underneath the warm air, forcing it to rise rapidly. Cold fronts are often associated with sudden temperature drops, sharp shifts in wind direction, and intense, short-lived precipitation often from cumulonimbus clouds.
- **Warm Front:** Forms when a warmer air mass advances into an area occupied by colder air. The warm air is less dense and gently slides up and over the wedge of cold air. Warm fronts are associated with gradual temperature increases, steady precipitation over a wider area, and clouds like cirrus, altostratus, and nimbostratus.
- **Occluded Front:** Forms when a faster-moving cold front catches up to and overtakes a slower-moving warm front. The warm air mass is lifted completely off the ground, trapped between the two colder air masses. Occluded fronts combine characteristics of both cold and warm fronts and represent the dissipation phase of mid-latitude cyclones.
Fronts are primarily found in the **middle latitudes** ($30^\circ$ to $60^\circ$ North and South). They are characterized by relatively steep horizontal gradients in temperature and pressure across the frontal zone. The uplift of air along fronts is a significant mechanism for producing clouds and precipitation and causing rapid changes in weather conditions.
Extra Tropical Cyclones
**Extra tropical cyclones**, also known as mid-latitude cyclones or wave cyclones, are large-scale weather systems that form in the middle and high latitudes, outside the tropics. They are associated with fronts and are characterized by a low-pressure center with converging winds and rising air. The passage of an extra tropical cyclone brings about significant and often abrupt changes in weather.
Extra tropical cyclones typically form along the **polar front**, the boundary between cold polar air and warmer mid-latitude air. The formation process (cyclogenesis) often begins with a stationary front that develops a wave-like kink. Cold air pushes southwards behind the wave, and warm air pushes northwards ahead of the wave, creating a cyclonic (anticlockwise in the Northern Hemisphere) circulation around a developing low-pressure center. This circulation intensifies, leading to distinct warm and cold fronts extending from the low center (Figure 9.9).
Ahead of the warm front, warm air gently rises over the retreating cold air, producing widespread, layered clouds (cirrus, altostratus, nimbostratus) and steady precipitation. Along the cold front, the advancing cold air forces the warm air upwards more abruptly, often resulting in tall cumulonimbus clouds and intense, showery precipitation, sometimes with thunderstorms. The cold front typically moves faster than the warm front and eventually catches up, forming an occluded front and lifting the warm air off the surface, leading to the dissipation of the cyclone.
Extra tropical cyclones differ significantly from tropical cyclones:
- They have a **clear frontal system** (warm, cold, occluded fronts), which is absent in tropical cyclones.
- They cover a **larger geographical area**.
- They can originate over **both land and sea** (tropical cyclones originate only over warm oceans).
- They are generally **less destructive** in terms of wind speed compared to mature tropical cyclones, although they can bring strong winds, heavy rain, and other hazards over a wide area.
- They typically move from **west to east** in the prevailing westerlies of the mid-latitudes.
Tropical Cyclones
**Tropical cyclones** (known by regional names like hurricanes, typhoons, cyclones, willy-willies) are powerful, violent storms that form over warm tropical oceans. They are characterized by intense low pressure at the center, very strong spiraling winds, and torrential rainfall. They can cause massive destruction when they make **landfall** (cross onto a coastal area).
Conditions required for the formation and intensification of tropical cyclones include:
- A **large sea surface with temperature greater than $27^\circ\text{C}$**. This provides the necessary heat and moisture (latent heat released during condensation) to power the storm.
- Presence of the **Coriolis force**. This force initiates the necessary rotation. Tropical cyclones therefore do not form very close to the equator ($<5^\circ$ latitude) where the Coriolis force is negligible.
- **Small variation in vertical wind speed**. Wind shear (changes in wind speed or direction with height) inhibits the formation and organization of the storm.
- A **pre-existing weak low-pressure area** or a low-level cyclonic circulation that can serve as a trigger.
- **Upper-level divergence** above the storm, which helps to remove rising air from the top of the storm, maintaining the low pressure at the surface and allowing more air to converge at the base.
The energy source for tropical cyclones is the immense amount of **latent heat** released when water vapor condenses into clouds within the towering cumulonimbus clouds surrounding the storm's center. As long as the storm remains over warm ocean water, it can continue to draw in moisture and intensify. Upon reaching land, the supply of moisture and heat from the ocean is cut off, causing the storm to weaken and dissipate.
A mature tropical cyclone has a characteristic structure (Figure 9.10).
- **Eye:** The calm, clear area at the very center of the storm, typically 15-50 km in diameter. Air in the eye slowly sinks. It is characterized by light winds and often clear skies.
- **Eye Wall:** A ring of the most intense thunderstorms and strongest winds surrounding the eye. Here, air spirals upwards rapidly to great heights (reaching the tropopause), releasing enormous amounts of latent heat and producing the heaviest rainfall and highest wind speeds (potentially exceeding 250 km/hour). This is the most dangerous part of the storm.
- **Rain Bands:** Spiral bands of thunderstorms and rain that extend outwards from the eyewall. These bands also contain strong winds and heavy rain, though less intense than in the eyewall.
The overall diameter of a tropical cyclone can be large, often 600-1200 km. They typically move relatively slowly (300-500 km per day), but their path can be unpredictable. When a tropical cyclone makes landfall, it brings damaging winds, torrential rain, and most significantly, a **storm surge** (a rise in sea level caused by the storm's winds pushing water towards the coast and the low pressure effect), which can cause extensive coastal inundation and destruction.
Tropical cyclones that move poleward and cross about $20^\circ$ N or S latitude often **recurve** (change direction, typically turning eastward) due to interactions with the mid-latitude westerly winds. These recurving cyclones can remain very powerful and destructive.
Thunderstorms And Tornadoes
In addition to large-scale cyclones, there are also intense, localized severe storms.
- **Thunderstorms:** These are relatively short-lived but violent local storms caused by intense convection in unstable, moist, warm air. They are associated with well-developed cumulonimbus clouds. Thunderstorms produce thunder (sound caused by rapid expansion of air heated by lightning) and lightning (electrical discharge). When temperatures are below freezing at high altitudes within the storm cloud, hail can form and fall as a hailstorm. If insufficient moisture is present, convection can still generate strong winds and lead to duststorms in dry regions. Thunderstorms involve strong updrafts of warm air and downdrafts of cooler air and precipitation.
- **Tornadoes:** These are the most violent and destructive of all atmospheric storms. They are rapidly rotating columns of air (a vortex) that extend downwards from the base of a severe thunderstorm (cumulonimbus cloud) to the ground. Tornadoes appear as a funnel or trunk-like shape. They are characterized by extremely low pressure at the center and incredibly high wind speeds, capable of causing catastrophic damage along their narrow path. Tornadoes are most common in the middle latitudes, although they can occur elsewhere. A tornado occurring over water is called a **waterspout**.
These violent weather events are essentially ways in which the atmosphere releases accumulated potential energy and heat energy, converting them into kinetic energy in an attempt to restore stability. After the storm passes, the atmosphere in that area returns to a more stable state.
Exercises
Multiple Choice Questions
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Answer The Following Questions In About 30 Words
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Answer The Following Questions In About 150 Words
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Project Work
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