The atmosphere is in constant motion. Air circulates around the globe in a complex system of winds, and this circulation distributes heat and moisture, drives weather systems and influences climate. The primary driving force for atmospheric circulation is the unequal heating of the Earth's surface: warm air at the equator rises, while cold air at the poles sinks. This sets up global wind systems that are modified by the rotation of the Earth, the distribution of continents and oceans, and seasonal changes.
In this chapter we study the general circulation of the atmosphere, the forces that affect wind, the global pressure belts, the planetary wind systems, and the weather systems such as cyclones and anticyclones. We also examine local winds, fronts and the mechanisms that produce our daily weather. Understanding atmospheric circulation is essential for explaining rainfall patterns, storms and the distribution of climates over the globe.
Air pressure is the weight of air on the Earth's surface. The pressure gradient force arises from the difference in atmospheric pressure between two points. Air moves from high pressure to low pressure, and the rate of change of pressure over distance is the pressure gradient. The steeper the gradient, the faster the wind.
Friction is the retarding force that the Earth's surface exerts on the moving air. Friction is greatest near the ground and decreases with height. It slows down the wind and reduces the effect of the Coriolis force near the surface.
The Coriolis force is the apparent deflection of moving objects (including winds and ocean currents) caused by the rotation of the Earth. It deflects winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The deflection is strongest at the poles and zero at the equator. The Coriolis force acts at right angles to the wind direction and does not change wind speed, only its direction.
The unequal heating of the Earth and the dynamics of the atmosphere create a pattern of pressure belts: 1. Equatorial Low Pressure Belt (Doldrums): Around the equator (0 to 5 degrees N and S), intense heating causes air to rise, creating low pressure. This belt is characterised by calm winds and heavy rainfall. 2. Subtropical High Pressure Belts (Horse Latitudes): Around 25-35 degrees N and S, descending air creates high pressure. These belts are dry and calm. 3. Subpolar Low Pressure Belts: Around 60-65 degrees N and S, where warm subtropical air meets cold polar air, creating low pressure. 4. Polar High Pressure Belts: Near the poles, cold dense air sinks, creating high pressure.
The pressure belts generate global wind systems: - Trade winds: From the subtropical highs towards the equator. In the Northern Hemisphere they blow as north-easterlies and in the Southern Hemisphere as south-easterlies. They are steady and reliable, historically used by sailing ships. - Westerlies: From the subtropical highs towards the subpolar lows. They blow from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere. - Polar easterlies: From the polar highs towards the subpolar lows. The boundary where the westerlies meet the polar easterlies is the polar front, a zone of low pressure and stormy weather.
The general circulation describes the average large-scale motion of air over the globe. The classic model divides each hemisphere into three cells: 1. Hadley cell: Air rises at the equator, moves poleward at high altitude, sinks in the subtropics, and returns towards the equator at the surface as the trade winds. 2. Ferrel cell: A mid-latitude cell between the Hadley and polar cells, in which air rises at the polar front and sinks in the subtropics. The westerlies blow within this cell. 3. Polar cell: Air sinks at the poles, flows equatorward at the surface, rises at the polar front and returns to the pole aloft. This three-cell model explains the general pattern of winds and pressure belts.
The pressure belts and wind systems shift with the apparent movement of the Sun. In summer, the belts shift poleward and in winter they shift equatorward. The most dramatic seasonal wind is the monsoon, particularly the South Asian monsoon, which is caused by the differential heating of land and sea, the shift of the Inter-Tropical Convergence Zone (ITCZ), and the presence of the Himalayas.
Local winds are smaller-scale winds caused by local temperature and pressure differences: - Land and sea breezes: During the day, the land heats faster, so air rises over the land and the sea breeze blows from sea to land; at night the reverse happens. - Mountain and valley breezes: During the day, valley sides heat up and air rises up the slopes (valley breeze); at night, cold air drains downslope (mountain breeze). - Foehn and Chinook: Warm, dry winds that descend the leeward side of mountains. - Loo: A hot, dry, dusty wind that blows over northern India in summer.
Cyclones are systems of low pressure in which winds spiral inwards and upwards. They are of two types: - Tropical cyclones: Form over warm tropical oceans where sea surface temperature is above 27 degrees Celsius. They are known by different names in different regions: hurricanes in the Atlantic and eastern Pacific, typhoons in the western Pacific, and cyclones in the Indian Ocean. They bring violent winds, torrential rain and storm surges. - Temperate (extra-tropical) cyclones: Form along the polar front in mid-latitudes where cold polar air meets warm subtropical air. They are associated with fronts and bring moderate rain and cloudy weather.
Anticyclones are systems of high pressure in which winds spiral outwards and downwards. Air descends and warms, so anticyclones are associated with clear skies, dry weather and calm conditions. In the Northern Hemisphere, winds rotate clockwise around an anticyclone and anticlockwise around a cyclone.
Thunderstorms are local storms produced by strong convection, usually in humid tropical and subtropical regions in the afternoon. They bring heavy rain, lightning and thunder. Tornadoes are small, intense, rotating columns of air extending from a thunderstorm to the ground. They are the most violent storms, with wind speeds that can exceed 500 km/h, and occur mainly in the USA's "Tornado Alley".
| Pressure Belt | Location | Character |
|---|---|---|
| Equatorial Low (Doldrums) | 0-5 deg N/S | Rising air, calm, heavy rain |
| Subtropical High (Horse Latitudes) | 25-35 deg N/S | Descending air, dry, calm |
| Subpolar Low | 60-65 deg N/S | Meeting of warm and cold air |
| Polar High | Near poles | Sinking cold air, dry |
| Wind System | Direction (NH) | From | To |
|---|---|---|---|
| Trade winds | North-easterly | Subtropical high | Equatorial low |
| Westerlies | South-westerly | Subtropical high | Subpolar low |
| Polar easterlies | North-easterly | Polar high | Subpolar low |
| Feature | Tropical Cyclone | Temperate Cyclone |
|---|---|---|
| Region | Warm tropical oceans | Mid-latitudes (polar front) |
| Source of energy | Latent heat of warm water | Contrast of air masses |
| Winds | Very violent | Moderate |
| Names | Hurricane, typhoon, cyclone | None specific |
Atmospheric circulation is the mechanism by which the atmosphere redistributes heat and moisture around the globe. Wind is generated by the pressure gradient force, modified by friction, and deflected by the Coriolis force. The global pressure belts and planetary winds, the trade winds, westerlies and polar easterlies, are organised into the three-cell circulation of the Hadley, Ferrel and polar cells. Seasonal shifts of these systems produce monsoons, and local temperature differences create land and sea breezes and other local winds. Weather systems such as tropical and temperate cyclones, anticyclones, thunderstorms and tornadoes are the dramatic expressions of this circulation. Together, these systems explain the day-to-day weather and the general climate patterns of the Earth.
Keywords: Pressure gradient, Coriolis force, trade winds, westerlies, Hadley cell, ITCZ, monsoon, cyclone, anticyclone, doldrums.