Atmospheric Circulation and Weather Systems
1. Learning Objectives
After reading these notes, you will be able to:
2. Atmospheric Pressure
📊 Standard Pressure and Temperature at Selected Levels
| Level | Pressure (mb) | Temperature (°C) |
|---|---|---|
| Sea Level | 1,013.25 | 15.2°C |
| 1 km | 898.76 | 8.7°C |
| 5 km | 540.48 | –17.3°C |
| 10 km | 265.00 | –49.7°C |
🌍 World Distribution of Sea Level Pressure
- Equatorial Low (0°): Near the equator, sea level pressure is low — called the equatorial low.
- Subtropical Highs (30°N & 30°S): High-pressure areas along 30°N and 30°S — called subtropical highs.
- Sub-Polar Lows (60°N & 60°S): Low-pressure belts along 60°N and 60°S — called sub-polar lows.
- Polar Highs (poles): Near the poles, pressure is high — called polar high.
- These pressure belts are not permanent — they oscillate with the apparent movement of the Sun. In Northern Hemisphere, they move southward in winter and northward in summer.
3. Forces Affecting Velocity and Direction of Wind
Rate of change
of pressure with
distance
Due to Earth’s
rotation — deflects
wind direction
Affects wind speed
Max at surface
upto 1–3 km
Acts downward
Prevents strong
upward winds
🔴 1. Pressure Gradient Force
Differences in atmospheric pressure produce a force. The rate of change of pressure with respect to distance is the pressure gradient. Pressure gradient is strong where isobars are close to each other and weak where isobars are far apart. This force drives air from high to low pressure.
⚡ 2. Frictional Force
It affects the speed of the wind. It is greatest at the surface and its influence generally extends upto an elevation of 1–3 km. Over the sea surface, friction is minimal. Friction slows down wind speed especially near the ground.
🌀 3. Coriolis Force
The rotation of Earth about its axis affects the direction of wind. Named after French physicist Coriolis (1844). Deflects wind to the right in Northern Hemisphere and to the left in Southern Hemisphere. Deflection is more when wind velocity is high. Maximum at poles, absent at equator.
🌍 4. Geostrophic Wind
Winds in the upper atmosphere (2–3 km above surface) are free from frictional effect. When isobars are straight and there is no friction, the Pressure Gradient Force is balanced by Coriolis Force → resultant wind blows parallel to the isobar. This wind is called Geostrophic Wind.
🌀 Cyclonic and Anticyclonic Circulation
| Pressure System | Pressure at Centre | Northern Hemisphere | Southern Hemisphere |
|---|---|---|---|
| Cyclone | Low | Anticlockwise | Clockwise |
| Anticyclone | High | Clockwise | Anticlockwise |
4. General Circulation of the Atmosphere
🔄 Three Circulation Cells
☀️ Hadley Cell (Tropics)
Air at the ITCZ (Inter Tropical Convergence Zone) rises due to convection caused by high insolation. Winds from tropics converge here. The converged air rises, reaches top of troposphere (14 km altitude), moves towards poles. Causes accumulation at 30°N and S → forms subtropical high. Near land surface, air flows back to equator as easterlies.
🌀 Ferrel Cell (Middle Latitudes)
Circulation in middle latitudes — sinking cold air from poles meets rising warm air from subtropical high. At the surface, these winds are called westerlies. The cell is known as the Ferrel Cell.
❄️ Polar Cell (Polar Latitudes)
Cold dense air subsides near the poles and blows towards middle latitudes as polar easterlies. This cell is called the Polar Cell. The transfer of heat energy from lower latitudes to higher latitudes maintains the general circulation.
🌊 Effect on Oceans
General circulation also affects the oceans. Large-scale winds initiate large, slow-moving ocean currents. Oceans in turn provide input of energy and water vapour into the air. These interactions take place slowly over large parts of the ocean.
Southern Oscillation: Change in pressure condition over Pacific — linked to El Niño.
ENSO = Combined phenomenon of Southern Oscillation + El Niño. In strong ENSO years — arid west coast of South America receives heavy rainfall; drought in Australia (and sometimes India); floods in China. Closely monitored for long-range weather forecasting.
5. Seasonal and Local Winds
🌊 Land and Sea Breezes
🌅 Sea Breeze (Day)
During the day, land heats up faster and becomes warmer than the sea. Over land → air rises → low pressure. Sea is relatively cool → high pressure. Pressure gradient: sea to land → wind blows from sea to land = Sea Breeze.
🌃 Land Breeze (Night)
During night, land loses heat faster and becomes cooler than the sea. Over sea → relatively warmer. Pressure gradient: land to sea → wind blows from land to sea = Land Breeze.
⛰️ Mountain and Valley Winds
🌄 Valley Breeze (Day)
During the day, slopes get heated → air moves up the slope. To fill the gap, air from valley blows up the valley. This upslope wind is called Valley Breeze.
🌙 Mountain Wind / Katabatic Wind (Night)
During night, slopes get cooled → dense cold air descends into the valley as Mountain Wind. Cool air draining from high plateaus and ice fields into the valley is called Katabatic Wind.
6. Air Masses and Fronts
🌫️ Air Masses
📋 Five Major Source Regions and Air Mass Types
| Air Mass Type | Symbol | Source Region | Nature |
|---|---|---|---|
| Maritime Tropical | mT | Warm tropical and subtropical oceans | Warm and moist |
| Continental Tropical | cT | Subtropical hot deserts | Hot and dry |
| Maritime Polar | mP | Relatively cold high latitude oceans | Cool and moist |
| Continental Polar | cP | Very cold snow-covered continents in high latitudes | Very cold and dry |
| Continental Arctic | cA | Permanently ice-covered Arctic/Antarctica | Extremely cold |
🌪️ Fronts
❄️ Cold Front
When cold air moves towards warm air mass — the contact zone is called the Cold Front. Cold air pushes warm air upward → cumulus clouds develop along cold front. Cold front moves faster than warm front.
🌡️ Warm Front
When warm air mass moves towards cold air mass — the contact zone is a Warm Front. Warm air glides over cold air → sequence of clouds appear ahead of warm front → cause precipitation.
⏸️ Stationary Front
When the front remains stationary without moving — it is called a Stationary Front.
🔼 Occluded Front
When an air mass is fully lifted above the land surface, it is called an Occluded Front. Cold front overtakes warm front → warm air completely lifted up → cyclone dissipates.
7. Extra Tropical Cyclones
🌀 Formation of Extra Tropical Cyclone
- Extra tropical cyclones form along the polar front. Initially, the front is stationary.
- In Northern Hemisphere, warm air blows from the south and cold air from the north of the front.
- When pressure drops along the front, warm air moves northwards and cold air moves southwards → sets in motion an anticlockwise cyclonic circulation.
- This leads to a well-developed extra tropical cyclone with a warm front and a cold front.
- Warm air glides over cold air → sequence of clouds ahead of warm front. Cold front pushes warm air up → cumulus clouds along cold front.
- Cold front moves faster → overtakes warm front → warm air completely lifted → front is occluded → cyclone dissipates.
⚖️ Extra Tropical vs Tropical Cyclone — Comparison
| Feature | Extra Tropical Cyclone | Tropical Cyclone |
|---|---|---|
| Frontal System | Present (clear frontal system) | Absent |
| Origin | Over land and sea both | Only over seas |
| Area covered | Much larger area | Smaller area |
| Wind velocity | Lower | Very high (up to 250 km/hr) — more destructive |
| Direction of movement | West to East | East to West |
| On reaching land | Continues | Dissipates |
8. Tropical Cyclones
✅ Conditions Favourable for Formation of Tropical Cyclones
- Large sea surface with temperature higher than 27°C — provides energy through evaporation.
- Presence of Coriolis force — needed to initiate rotation (absent at equator, so tropical cyclones don’t form near equator).
- Small variations in vertical wind speed — uniform wind direction with height.
- Pre-existing weak low-pressure area or low-level cyclonic circulation.
- Upper divergence above the sea level system — allows air to escape at the top.
🌀 Structure of a Mature Tropical Cyclone
👁️ The Eye
The centre of the cyclone — a region of calm with subsiding air. The diameter of the circulating system varies between 150 and 250 km. Eye is the calmest part.
🌪️ Eye Wall
Around the eye is the eye wall — there is a strong spiralling ascent of air reaching the tropopause. Wind reaches maximum velocity here — up to 250 km/hr. Torrential rain occurs here. This is the most destructive part.
🌧️ Rain Bands
From the eye wall, rain bands radiate outward. Trains of cumulus and cumulonimbus clouds drift into the outer region. The diameter of the storm over Bay of Bengal, Arabian Sea and Indian Ocean is 600–1200 km.
⚡ Energy Source
Energy that intensifies the storm comes from condensation process in towering cumulonimbus clouds. With continuous supply of moisture from the sea, the storm is further strengthened. On reaching land, moisture supply is cut off → storm dissipates.
Recurvature: Cyclones crossing 20°N latitude generally recurve and are more destructive.
Storm Surge: Cyclone creates storm surges that inundate coastal low lands. System moves slowly — 300–500 km per day. The storm peters out on reaching land.
9. Thunderstorms and Tornadoes
⚡ Thunderstorm
Caused by intense convection on moist hot days. A thunderstorm is a well-grown cumulonimbus cloud producing thunder and lightning. When clouds extend to heights where sub-zero temperature prevails → hailstorm forms. If insufficient moisture → thunderstorm can generate duststorms. Characterised by intense updraft of rising warm air → clouds grow bigger → precipitation → later downdraft brings cool air and rain.
🌪️ Tornado
From severe thunderstorms, sometimes spiralling wind descends like a trunk of an elephant with great force, with very low pressure at the centre — causing massive destruction. Such a phenomenon is called a Tornado. Tornadoes generally occur in middle latitudes. The tornado over the sea is called Water Spout.
Summary — Quick Revision
Atmospheric Pressure = weight of air column per unit area from sea level to top of atmosphere. Unit = millibar (mb). Average at sea level = 1,013.2 mb. Decreases with height — about 1 mb per 10 m.
Pressure Belts: Equatorial Low (0°) → Subtropical High (30°N/S) → Sub-Polar Low (60°N/S) → Polar High (90°). Not permanent — shift with Sun’s apparent movement.
Three forces on wind: Pressure Gradient Force (drives wind), Coriolis Force (deflects direction — right in NH, left in SH; absent at equator, max at poles), Frictional Force (reduces speed, upto 1–3 km).
Geostrophic Wind: In upper atmosphere (2–3 km), when isobars are straight, pressure gradient force balanced by Coriolis force → wind blows parallel to isobar.
Three Circulation Cells: Hadley Cell (tropics, ITCZ), Ferrel Cell (middle latitudes, westerlies), Polar Cell (polar regions, polar easterlies). These set pattern for general circulation.
ENSO: El Niño (warm water off Peru coast) + Southern Oscillation (pressure changes over Pacific) = ENSO. Effects: heavy rain in South America, drought in Australia/India, floods in China.
Sea Breeze (day, sea → land), Land Breeze (night, land → sea), Valley Breeze (day, up the slope), Mountain Wind/Katabatic Wind (night, down the slope).
5 Air Mass Types: mT (maritime tropical), cT (continental tropical), mP (maritime polar), cP (continental polar), cA (continental arctic). Tropical = warm; Polar = cold.
4 Fronts: Cold, Warm, Stationary, Occluded. Extra tropical cyclones have clear frontal system (tropical cyclones do not). Extra tropical: west to east movement. Tropical: east to west.
Tropical Cyclone: Indian Ocean = Cyclone, Atlantic = Hurricane, W. Pacific = Typhoon, W. Australia = Willy-willies. Needs sea surface temp > 27°C. Eye = calm, Eye Wall = most destructive, Wind up to 250 km/hr. Dissipates on reaching land.
Important Terms to Remember
- Atmospheric Pressure: The weight of a column of air contained in a unit area from the mean sea level to the top of the atmosphere. Expressed in millibar (mb). Average at sea level = 1,013.2 mb.
- Millibar (mb): Unit of measuring atmospheric pressure. 1 mb = 100 Pascals. Mercury barometer and aneroid barometer are used to measure pressure.
- Isobars: Lines connecting places having equal pressure on a weather map — all values reduced to sea level for comparison.
- Pressure Gradient Force: The force produced due to differences in atmospheric pressure. Rate of change of pressure with distance. Drives wind from high to low pressure. Strong when isobars are close; weak when isobars are apart.
- Coriolis Force: Force due to Earth’s rotation about its axis — deflects wind to the right in Northern Hemisphere and to the left in Southern Hemisphere. Named after French physicist Coriolis (1844). Maximum at poles, absent at equator.
- Geostrophic Wind: Wind in upper atmosphere (2–3 km), free from friction, where pressure gradient force is balanced by Coriolis force → wind blows parallel to isobar.
- Cyclonic Circulation: Wind circulation around a low pressure area. Anticlockwise in Northern Hemisphere, clockwise in Southern Hemisphere.
- Anticyclonic Circulation: Wind circulation around a high pressure area. Clockwise in Northern Hemisphere, anticlockwise in Southern Hemisphere.
- ITCZ (Inter Tropical Convergence Zone): The zone near the equator where easterlies from both hemispheres converge. Air rises here due to convection → forms Hadley Cell.
- Hadley Cell: Circulation cell in the tropics — air rises at ITCZ, moves poleward at altitude, sinks at 30°N/S forming subtropical highs, returns to equator as easterlies.
- Ferrel Cell: Mid-latitude circulation cell. Sinking cold air from poles meets rising warm air from subtropical high. Surface winds = westerlies.
- Polar Cell: Cold dense air subsides near poles and moves towards middle latitudes as polar easterlies.
- El Niño: Appearance of warm water off the coast of Peru when warm water of central Pacific drifts towards South American coast, replacing the cool Peruvian current.
- Southern Oscillation: Change in pressure conditions over the Pacific Ocean, associated with El Niño.
- ENSO: Combined phenomenon of Southern Oscillation and El Niño. In strong ENSO years, large-scale weather variations occur worldwide.
- Sea Breeze: During daytime — land heats faster → low pressure over land → high pressure over sea → wind blows from sea to land.
- Land Breeze: During night — land cools faster → high pressure over land → low pressure over sea → wind blows from land to sea.
- Valley Breeze: During day in mountainous regions — slopes heat up → air moves upslope, valley air also moves upward.
- Katabatic Wind: Cool air from high plateaus and ice fields draining into valleys at night under gravity. A type of mountain wind.
- Air Mass: A large body of air having little horizontal variation in temperature and moisture — acquires characteristics of its source region.
- Source Region: The homogeneous surface (vast ocean or plain) over which an air mass forms and acquires its characteristics.
- Front / Frontogenesis: The boundary zone between two different air masses is called a front. Process of its formation = frontogenesis. Four types: cold, warm, stationary, occluded.
- Cold Front: Contact zone when cold air moves towards warm air mass. Cold air pushes warm air upward → cumulus clouds → heavy precipitation. Moves faster than warm front.
- Warm Front: Contact zone when warm air mass moves towards cold air mass. Warm air glides over cold air → sequence of clouds → precipitation.
- Occluded Front: When cold front overtakes warm front — warm air is completely lifted above land surface. Cyclone dissipates after occlusion.
- Extra Tropical Cyclone: Systems developing beyond the tropics, in mid and high latitudes. Have a clear frontal system. Move west to east. Can originate over land and sea. Cover a larger area but wind velocity is lower than tropical cyclones.
- Tropical Cyclone: Violent storm originating over warm tropical oceans. Known as Cyclone (Indian Ocean), Hurricane (Atlantic), Typhoon (Western Pacific), Willy-willies (Western Australia). Needs sea surface temp > 27°C. Dissipates on reaching land.
- Eye of Cyclone: Region of calm with subsiding air at the centre of a mature tropical cyclone. Diameter 150–250 km.
- Eye Wall: Around the eye — region of strong spiralling ascent of air. Maximum wind velocity (up to 250 km/hr). Torrential rain occurs here. Most destructive zone.
- Landfall: The place where a tropical cyclone crosses the coast. Cyclones crossing 20°N generally recurve and are more destructive.
- Storm Surge: The rise of sea level created by a tropical cyclone — inundates coastal low lands.
- Thunderstorm: A well-grown cumulonimbus cloud producing thunder and lightning — caused by intense convection on moist hot days. Characterised by intense updraft of warm air.
- Tornado: A spiralling wind that descends from a severe thunderstorm like a trunk of an elephant — very low pressure at centre — causes massive destruction. Generally occurs in middle latitudes. Over sea = Water Spout.
- Water Spout: A tornado occurring over the sea.