Atmospheric Circulation And Weather Systems

Atmospheric Pressure

Vertical Variation Of Pressure

Definition: Atmospheric pressure is the force exerted by the weight of the air column above a unit area of the Earth's surface. It is a fundamental element of weather and climate.

Vertical Decrease: Atmospheric pressure decreases exponentially with increasing altitude. This is because as you go higher, there is less air above you, and therefore less weight pressing down.

Density: Air density also decreases with altitude. At sea level, air is densest; at higher altitudes, the molecules are farther apart.
Pressure Units: Pressure is typically measured in units like millibars (mb), hectopascals (hPa), or Pascals (Pa). Standard sea-level pressure is approximately 1013.25 mb or hPa.
Rate of Decrease: While the decrease is exponential, a common approximation is that pressure drops by about half for every 5.5 km increase in altitude.

Formulaic Representation: The relationship between pressure ($P$) and altitude ($z$) can be approximated by the barometric formula, which is derived from the hydrostatic equation and the ideal gas law. A simplified form is: $$ P(z) = P_0 e^{-\frac{gM}{RT} z} $$ Where:

$P(z)$ is the pressure at altitude $z$.
$P_0$ is the pressure at sea level ($z=0$).
$e$ is the base of the natural logarithm.
$g$ is the acceleration due to gravity (approx. 9.8 m/s²).
$M$ is the molar mass of air (approx. 0.029 kg/mol).
$R$ is the universal gas constant (approx. 8.314 J/(mol·K)).
$T$ is the absolute temperature (in Kelvin).
$z$ is the altitude.

Horizontal Distribution Of Pressure

Cause of Horizontal Variation: Differences in temperature cause variations in air density, which in turn create horizontal pressure gradients. Warm air is less dense and tends to rise, creating areas of lower pressure at the surface. Cold air is denser and tends to sink, creating areas of higher pressure at the surface.

Pressure Systems:

High-Pressure Systems (Anticyclones): Characterized by higher pressure at the center than in the surrounding areas. Air generally sinks in these systems, leading to fair weather conditions. Winds blow outwards from the center (anticlockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere), deflected by the Coriolis force.
Low-Pressure Systems (Cyclones/Depressions): Characterized by lower pressure at the center than in the surrounding areas. Air generally rises in these systems, leading to cloud formation, precipitation, and often stormy weather. Winds blow inwards towards the center (clockwise in the Northern Hemisphere, anticlockwise in the Southern Hemisphere), deflected by the Coriolis force.

World Distribution Of Sea Level Pressure

When mapping horizontal pressure, it's common to reduce all pressure readings to sea-level pressure. This normalizes the data, removing the effect of altitude and allowing for easier comparison of pressure patterns across the globe. Key features of the global sea-level pressure distribution include:

Equatorial Low (ITCZ - Intertropical Convergence Zone): A belt of low pressure around the equator, roughly between 5°N and 5°S latitude. This area is characterized by rising warm, moist air, convergence of trade winds, heavy rainfall, and thunderstorms.
Subtropical Highs: Belts of high pressure located around 30°N and 30°S latitude. These are areas of sinking, dry air associated with many of the world's major deserts.
Subpolar Lows: Belts of low pressure located around 60°N and 60°S latitude. These are dynamic lows formed by the convergence of air masses and are associated with stormy weather.
Polar Highs: Areas of high pressure located over the poles, associated with cold, dense air.

Forces Affecting The Velocity And Direction Of Wind

Wind is the movement of air from high pressure to low pressure. Its velocity and direction are influenced by several forces:

Pressure Gradient Force (PGF)

Definition: The force that arises from differences in atmospheric pressure over a given distance. It acts from an area of high pressure to an area of low pressure, perpendicular to isobars (lines of equal pressure).

Effect: The PGF is the primary driver of wind. The greater the pressure difference over a given distance (i.e., the steeper the pressure gradient), the stronger the PGF and the faster the wind speed.

Frictional Force

Definition: This force arises from the resistance to motion caused by the interaction between air molecules and the Earth's surface (and between air molecules themselves). It acts in the opposite direction of wind motion.

Effect:

Slowing Down Wind: Friction slows down wind speed, particularly near the Earth's surface.
Reducing Coriolis Effect: Because friction reduces wind speed, it also reduces the effect of the Coriolis force.
Directional Change: Friction causes winds to blow more directly across isobars towards low pressure, especially near the surface. Over oceans, the frictional effect is less pronounced than over land.

Coriolis Force

Definition: An apparent force that arises due to the Earth's rotation. It deflects moving objects (including air) to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. It does not affect objects moving at the equator.

Effect:

Directional Deflection: It deflects winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
Magnitude: The Coriolis force is proportional to the speed of the moving air and the sine of the latitude. It is strongest at the poles and zero at the equator.
Geostrophic Wind: In the upper atmosphere, where friction is negligible, the PGF is balanced by the Coriolis force. This balance results in winds blowing parallel to isobars at a constant speed, known as the geostrophic wind.

Pressure And Wind

The interaction of the Pressure Gradient Force (PGF), Coriolis Force, and Frictional Force determines the actual wind velocity and direction.

Upper Atmosphere (Geostrophic Flow): Where friction is minimal, the PGF pushing air from high to low pressure is balanced by the Coriolis force deflecting it. This leads to winds blowing parallel to isobars (geostrophic winds).
Surface Winds: Near the surface, friction slows the wind, reducing the Coriolis force. This allows the PGF to become more dominant, causing winds to cross isobars at an angle, from higher pressure towards lower pressure. The greater the friction, the more directly the wind blows across the isobars.

Buys Ballot's Law: A useful rule for understanding surface winds: If you stand with your back to the wind, low pressure will be to your left in the Northern Hemisphere and to your right in the Southern Hemisphere.

General Circulation Of The Atmosphere

General atmospheric circulation refers to the large-scale movement of air across the planet. It is driven by differential heating of the Earth's surface (more solar energy at the equator, less at the poles) and is modified by the Earth's rotation (Coriolis effect), distribution of land and sea, and topography.

General Atmospheric Circulation And Its Effects On Oceans

The general circulation of the atmosphere is a complex system of interconnected cells and wind belts that transport heat from the tropics towards the poles, helping to maintain a global energy balance.

Basic Model (Three-Cell Model):

In a simplified model, the Earth's atmosphere is divided into three circulation cells in each hemisphere:

Hadley Cell (0° to 30° Latitude):
- Equatorward:** Warm, moist air rises at the equator (in the Intertropical Convergence Zone - ITCZ), cools, and releases abundant precipitation.

Ferrel Cell (30° to 60° Latitude):

Poleward at Surface: Surface air from the subtropical highs flows poleward.

Mid-latitude Lows: This air rises and converges with polar air around 60° latitude, forming the subpolar lows and contributing to the polar front.

Equatorward aloft: Air flows back towards the subtropics aloft.

Surface Westerlies: Winds in this cell are generally from the west, known as the Westerlies.

Polar Cell (60° to 90° Latitude):

Equatorward aloft: Cold, dense air sinks at the poles.

Equatorward Surface: Air flows away from the poles at the surface as the Polar Easterlies.

Poleward aloft: Air rises near the subpolar lows and flows back towards the poles aloft.

Subtropical Jet Stream: A strong high-altitude eastward wind current found near the boundary between the Hadley and Ferrel cells.

Polar Jet Stream: A high-altitude jet stream found near the boundary between the Ferrel and Polar cells.

Effects on Oceans:

The general atmospheric circulation has profound effects on ocean currents, driving their patterns and influencing global heat distribution:

Surface Currents: The major surface ocean currents are driven primarily by the prevailing winds associated with the atmospheric circulation cells.

Trade Winds: Drive westward currents in the tropics (e.g., North Equatorial Current, South Equatorial Current).

Westerlies: Drive eastward currents in the mid-latitudes (e.g., North Atlantic Drift, Kuroshio Current extension).

Gyres: The prevailing winds, Coriolis effect, and the shape of ocean basins combine to create large, circular current systems called gyres. These gyres are crucial for transporting heat and nutrients.

Upwelling and Downwelling: Wind patterns can cause surface water to move away from coastlines (upwelling), bringing nutrient-rich deep water to the surface, or pile up water along coastlines (downwelling), pushing surface water downwards.

Heat Transport: Ocean currents, driven by atmospheric circulation, transport vast amounts of heat from the tropics to the poles, playing a vital role in moderating global climate. For example, the Gulf Stream transports warm water from the tropics to the North Atlantic, significantly warming the climate of Western Europe.

El Niño-Southern Oscillation (ENSO): Changes in atmospheric circulation patterns, particularly over the Pacific Ocean, can lead to significant shifts in ocean temperatures and currents, such as the El Niño phenomenon, which has global implications for weather patterns.

Seasonal Wind

Seasonal winds are winds that change their direction seasonally. The most prominent example of this phenomenon is the monsoon.

Monsoon:

Definition: A monsoon is a seasonal reversal of wind direction, typically associated with large-scale differences in temperature between continental and oceanic regions.

Mechanism:

Summer Monsoon: During summer, the landmass heats up much more rapidly than the adjacent ocean. This creates a low-pressure system over the land and a high-pressure system over the ocean. Winds then blow from the high-pressure ocean towards the low-pressure land. Because these winds originate over the warm ocean, they carry large amounts of moisture, resulting in heavy rainfall over the land.

Winter Monsoon: During winter, the land cools down more quickly than the ocean, creating a high-pressure system over the land and a low-pressure system over the ocean. Winds then blow from the high-pressure land towards the low-pressure ocean. These winds are generally dry and offshore, resulting in a dry season over the land.

Examples: The most famous monsoon system is the Indian subcontinent monsoon, which brings essential rainfall for agriculture. Other significant monsoon systems are found in East Asia, Southeast Asia, and parts of Africa and Australia.

Impact: Monsoons are critical for the climate and economy of many regions, particularly for agriculture, as they provide the necessary rainfall for crop growth. However, they can also lead to devastating floods if rainfall is excessive or prolonged droughts if they fail to materialize.

Local Winds

Local winds are winds that are influenced by local conditions, such as topography or differential heating of surfaces. They are typically less extensive than planetary or seasonal winds.

Land And Sea Breezes

These breezes occur along coastlines and are driven by the differential heating of land and sea.

Sea Breeze (Daytime):

Mechanism: During the day, the land heats up faster than the sea. The air over the land becomes warmer, less dense, and rises, creating a low-pressure area over the land. The air over the cooler sea is denser, creating a higher pressure area. Air flows from the high-pressure sea towards the low-pressure land, creating a sea breeze that blows inland from the sea.

Characteristics: Typically starts in the late morning, increases in strength during the afternoon, and can extend several kilometers inland.

Land Breeze (Nighttime):

Mechanism: At night, the land cools down faster than the sea. The air over the land becomes cooler and denser, creating a high-pressure area over the land. The air over the relatively warmer sea is less dense, creating a lower pressure area. Air then flows from the high-pressure land towards the low-pressure sea, creating a land breeze that blows offshore.

Characteristics: Usually weaker than sea breezes and occurs during the night.

Mountain And Valley Winds

These winds are influenced by the differential heating and cooling of mountain slopes and valleys.

Valley Breeze (Daytime):

Mechanism: During the day, mountain slopes heat up faster than the valley floor. The air on the slopes becomes warmer, less dense, and rises upslope, creating a low-pressure area on the slope. Cooler air from the valley floor then flows upslope to replace it, creating a valley breeze.

Characteristics: Air moves up the valley and along the slopes.

Mountain Breeze (Nighttime):

Mechanism: At night, mountain slopes cool down faster than the valley floor due to radiation. The cooler, denser air on the slopes sinks downslope into the valley under the influence of gravity, creating a mountain breeze.

Characteristics: Air moves down the mountain slopes and into the valley. This can lead to the accumulation of cold air in the valley bottom, potentially causing frost.

Air Masses

Definition: An air mass is a large body of air (hundreds or thousands of kilometers across) that has relatively uniform temperature and humidity characteristics horizontally. It develops when air remains stagnant over a particular region (source region) for a sufficient period, allowing it to acquire the temperature and moisture properties of that surface.

Classification: Air masses are classified based on their source region and temperature characteristics:

Temperature:

Polar (P): Originating from high latitudes, typically cold.

Tropical (T): Originating from low latitudes, typically warm.

Equatorial (E): Originating from near the equator, typically very warm and humid.

Arctic (A) / Antarctic (AA): Originating from the polar regions, typically very cold.

Humidity (Moisture Content):

Maritime (m): Originating over oceans, typically humid or moist.

Continental (c): Originating over land, typically dry.

Common Air Mass Types: Combining these classifications gives us common air mass types:

cP (Continental Polar): Cold and dry, originates over northern Canada and Siberia.

mP (Maritime Polar): Cool and moist, originates over the North Pacific and North Atlantic.

cT (Continental Tropical): Hot and dry, originates over the southwestern US and northern Mexico in summer.

mT (Maritime Tropical): Warm and humid, originates over the Gulf of Mexico, Caribbean Sea, and tropical Atlantic.

mE (Maritime Equatorial): Very warm and very humid, originates over equatorial oceans.

Modification: As an air mass moves away from its source region, it gradually modifies its characteristics by interacting with the underlying surface and the surrounding atmosphere. For example, a cP air mass moving over the warmer southeastern United States will gradually become modified, warming and picking up moisture.

Influence on Weather: Air masses are the building blocks of weather. Their interaction, particularly along boundaries called fronts, leads to significant weather changes.

Fronts

Definition: A front is a boundary or transition zone between two air masses with different temperature and/or humidity characteristics. These boundaries are not sharp lines but rather zones, often hundreds of kilometers wide, where significant weather changes occur.

Types of Fronts:

Cold Front:

Description: The boundary where a colder, denser air mass advances and replaces a warmer air mass. The colder air acts like a wedge, forcing the warmer air upwards.

Associated Weather: As the warm, moist air is rapidly lifted, it cools, condenses, and forms cumulonimbus clouds. This often results in a narrow band of intense precipitation (showers, thunderstorms), strong winds, and a rapid drop in temperature after the front passes. A steep pressure gradient often forms along the front.

Symbol: A blue line with blue triangles pointing in the direction of movement.

Warm Front:

Description: The boundary where a warmer, less dense air mass advances and overrides a colder air mass. The warm air gradually slides up over the cold air.

Associated Weather: The gentle lifting of warm, moist air leads to the formation of layered clouds (cirrus, cirrostratus, altostratus, nimbostratus). This typically brings widespread, steady precipitation, often light to moderate, that can last for many hours. Temperatures rise after the front passes.

Symbol: A red line with red semicircles pointing in the direction of movement.

Stationary Front:

Description: A boundary between two air masses where neither air mass is advancing significantly. The front essentially remains in place, although the air masses on either side may flow parallel to it.

Associated Weather: Can bring prolonged periods of clouds and precipitation, similar to a warm front, but without significant temperature changes.

Symbol: A line with alternating blue triangles pointing towards the warm side and red semicircles pointing towards the cold side.

Occluded Front:

Description: Forms when a faster-moving cold front catches up to and overtakes a slower-moving warm front. The cold front lifts the warm air mass completely off the ground, creating a boundary between two cold air masses, one of which is colder than the other.

Associated Weather: Often brings a complex mix of weather, including precipitation from both warm and cold front conditions, and can be associated with mature low-pressure systems.

Symbol: A purple line with alternating purple triangles and semicircles pointing in the direction of movement.

Frontogenesis and Frontolysis:

Frontogenesis: The process by which a new front forms or an existing front intensifies, typically due to increasing horizontal temperature gradients.

Frontolysis: The process by which a front weakens and dissipates, usually when the temperature contrast across the boundary decreases.

Extra Tropical Cyclones

Definition: Extra tropical cyclones (also known as mid-latitude cyclones or temperate cyclones) are large-scale low-pressure systems that form in the middle and high latitudes (outside the tropics). They are the primary drivers of weather changes in these regions.

Formation:

Polar Front Theory (Norwegian Cyclone Model): Extra tropical cyclones are believed to form along the polar front, the boundary between polar and tropical air masses.

Wave Development: Initially, the polar front may be relatively straight. However, disturbances (waves) can develop along this front due to upper-level wind patterns.

Cyclogenesis: As a wave develops, a low-pressure center forms. Cold air advances westward and southward behind the low, while warm air advances eastward and northward ahead of the low. This process creates distinct boundaries – a cold front and a warm front – emanating from the low-pressure center.

Mature Stage: The cyclone intensifies, and the cold front begins to catch up to the warm front. An occluded front forms when the cold front completely overtakes the warm front.

Dissipation: Eventually, the cyclone loses its energy as the temperature contrast between the air masses diminishes and the pressure gradient weakens.

Structure:

Low-Pressure Center: Located at the heart of the system.

Fronts: Typically feature a well-defined cold front and warm front, and eventually an occluded front.

Air Masses: Composed of contrasting polar and tropical air masses.

Cloud and Precipitation Bands: Associated with the fronts. Warm fronts produce widespread stratiform clouds and precipitation, while cold fronts bring more localized convective clouds and intense showers.

Associated Weather:

Wind: Strong winds blow inwards and counter-clockwise in the Northern Hemisphere (clockwise in the Southern Hemisphere) around the low-pressure center.

Precipitation: Varies depending on the type of front and location within the cyclone, ranging from steady rain and snow to intense thunderstorms.

Temperature Changes: Significant temperature shifts occur as different air masses pass over a location.

Pressure Changes: A steady drop in pressure usually indicates an approaching cyclone, while rising pressure suggests fair weather.

Importance: These cyclones are responsible for much of the day-to-day weather experienced in the mid-latitudes, including significant precipitation and wind events.

Tropical Cyclones

Definition: Tropical cyclones are intense, rotating low-pressure storm systems that form over warm tropical or subtropical ocean waters. They are characterized by a well-defined eye, an eyewall with the strongest winds and heaviest rainfall, and spiral rainbands.

Nomenclature: They are known by different names depending on the region:

Hurricanes: North Atlantic Ocean, Northeast Pacific Ocean.

Typhoons: Northwest Pacific Ocean.

Cyclones: South Pacific Ocean, Indian Ocean.

Formation Requirements:

Warm Ocean Waters: Sea surface temperatures must be at least 26.5°C (80°F) down to a depth of about 50 meters to provide the necessary heat energy.

Coriolis Force: Sufficient distance from the equator (typically at least 5° latitude) is needed for the Earth's rotation to impart spin to the system.

Low Vertical Wind Shear: Little change in wind speed or direction with height is required to allow the storm structure to remain intact.

Pre-existing Disturbance: Often develop from a tropical wave or disturbance.

Moist Mid-Troposphere: To support cloud and thunderstorm development.

Structure:

Eye: A calm, clear region at the center of the storm where air sinks.

Eyewall: A ring of intense thunderstorms surrounding the eye, containing the storm's strongest winds and heaviest precipitation.

Rainbands: Bands of thunderstorms that spiral outward from the eyewall, producing heavy rain and gusty winds.

Stages of Development:

Tropical Disturbance: A cluster of thunderstorms with no organized circulation.

Tropical Depression: A closed circulation forms with wind speeds less than 39 mph (63 km/h).

Tropical Storm: Wind speeds reach 39-73 mph (63-118 km/h). At this stage, it is given a name.

Tropical Cyclone (Hurricane/Typhoon): Wind speeds exceed 74 mph (119 km/h).

Associated Hazards:

High Winds: Can cause widespread structural damage and uproot trees.

Heavy Rainfall: Leads to inland flooding, landslides, and mudslides.

Storm Surge: A dangerous rise in sea level caused by the storm's winds pushing water towards the coast and the low pressure at the center. This is often the most deadly aspect of a tropical cyclone.

Tornadoes: Can be spawned within the rainbands of landfalling tropical cyclones.

Dissipation: Tropical cyclones weaken when they move over cooler waters, lose their supply of warm, moist air, encounter high wind shear, or make landfall.

Thunderstorms And Tornadoes

Thunderstorms

Definition: A thunderstorm is a storm characterized by the presence of lightning and its acoustic effect, thunder. It is produced by a cumulonimbus cloud, often accompanied by strong winds, heavy rain, and sometimes hail or tornadoes.

Formation: Thunderstorms require three ingredients:

Moisture: Sufficient water vapour in the lower atmosphere to form clouds and precipitation.

Instability: A condition where the atmosphere becomes unstable, meaning that a parcel of air, once lifted, continues to rise on its own because it is warmer and less dense than its surroundings. This is often caused by warm, moist air near the surface and cooler, drier air aloft.

Lifting Mechanism: A process that initiates the upward movement of air, such as:

Convection (heating of the surface).

Orographic lift (air forced up by mountains).

Frontal lifting (warm air forced over cold air).

Convergence of air at the surface.

Life Cycle of a Thunderstorm (Cumulonimbus Cloud):

Developing Stage (Cumulus Stage): Dominated by updrafts. Warm, moist air rises rapidly, cools, and condenses to form towering cumulus clouds. There is no precipitation or lightning yet.

Mature Stage: Characterized by both updrafts and downdrafts. Precipitation begins to fall, dragging air down with it (downdraft). Lightning and thunder occur during this stage. This is the most intense stage.

Dissipating Stage: Dominated by downdrafts. The downdraft spreads out and cuts off the supply of warm, moist air to the updraft, causing the storm to weaken and eventually dissipate.

Types of Thunderstorms:

Single-cell: Short-lived, isolated storms (often called "air-mass thunderstorms").

Multi-cell: A cluster of thunderstorms, where individual cells are in different stages of development, leading to a longer-lasting storm system.

Supercell: A long-lived, highly organized thunderstorm with a deep, persistent rotating updraft (mesocyclone). Supercells are responsible for most significant tornadoes and large hail.

Tornadoes

Definition: A tornado is a violently rotating column of air that is in contact with both the surface of the Earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. They are the most violent storms on Earth.

Formation:

Tornado Genesis: Tornadoes most commonly form within supercell thunderstorms, although they can also form in other thunderstorm types.

Conditions for Supercells: Require significant atmospheric instability, strong vertical wind shear (a change in wind speed and/or direction with height), and a lifting mechanism.

Mesocyclone: The strong vertical wind shear can cause the air within a thunderstorm to rotate, forming a mesocyclone (a rotating column of air within the updraft).

Tornado Development: The exact process by which a mesocyclone descends and forms a tornado is still an area of active research, but it involves complex interactions of updrafts, downdrafts, and rotation near the ground.

Characteristics:

Appearance: Often visible as a condensation funnel extending from the cloud base to the ground, or as a debris cloud at the surface.

Wind Speeds: Can range from about 65 mph (105 km/h) to over 200 mph (322 km/h), with some exceptionally violent tornadoes exceeding 300 mph (480 km/h).

Path: Typically travel in a curved path, often associated with the movement of the parent thunderstorm.

Size: Widths can range from a few meters to over a kilometer.

Measurement of Intensity: The Enhanced Fujita (EF) Scale is used to rate tornado intensity based on the damage caused, with EF0 being the weakest and EF5 being the strongest.

Geographic Occurrence: While tornadoes can occur almost anywhere in the world, they are most frequent in the "Tornado Alley" region of the central United States, where conditions for supercell formation are often met.