The Atmosphere, Winds and Cyclones
The thin envelope of gases that surrounds the Earth is called the atmosphere. It makes life possible, protects us from harmful solar radiation, and is the stage on which all weather happens. For competitive examinations, this topic is important because it combines direct facts - the percentage of nitrogen, the name of a layer, the local name of a cyclone - with concepts such as air pressure, wind systems and the formation of storms. Examiners ask about the composition and layers of the atmosphere, the pressure belts, the planetary winds, the difference between cyclones and anticyclones, and the local names of tropical cyclones in different parts of the world.
Composition of the atmosphere
The atmosphere is a mixture of gases. Nitrogen is the most abundant gas, making up about 78 per cent, followed by oxygen at about 21 per cent. Argon forms about 0.93 per cent, and carbon dioxide only about 0.04 per cent, yet carbon dioxide and water vapour are extremely important because they absorb heat and cause the greenhouse effect. Dust particles and water vapour, though present in very small amounts, are essential for cloud formation and rainfall.
Layers of the atmosphere
The atmosphere is divided into five layers. The troposphere is the lowest layer, extending from the surface to about 8 kilometres at the poles and 18 kilometres at the equator. It contains most of the water vapour and dust and is where all weather - clouds, rain, storms - occurs. Above it lies the stratosphere, which contains the ozone layer that absorbs harmful ultraviolet rays; jet aircraft fly in the lower stratosphere. Next is the mesosphere, where meteors burn up and where the temperature is the lowest. Above that is the thermosphere (which includes the ionosphere), where the temperature rises sharply and where the aurora occurs and radio waves are reflected. The outermost layer is the exosphere, which gradually merges into space.
Air pressure and winds
Air pressure is the weight of the air column above a point, and it decreases with altitude. Differences in air pressure cause air to move from a high-pressure area to a low-pressure area, and this moving air is called wind. The main pressure belts are the equatorial low-pressure belt, the sub-tropical high-pressure belts, the sub-polar low-pressure belts and the polar high-pressure belts. Because of the rotation of the Earth, winds are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere; this is the Coriolis effect. The permanent or planetary winds are the trade winds, which blow from the sub-tropical highs towards the equator; the westerlies, which blow towards the poles; and the polar easterlies. Seasonal winds such as the monsoon of South Asia, and local winds such as the loo, mistral and chinook, are also important.
Cyclones and anticyclones
A cyclone is an area of low pressure in which winds blow inwards and rise, producing clouds, rain and storms. In the Northern Hemisphere cyclonic winds rotate anticlockwise, and in the Southern Hemisphere they rotate clockwise. An anticyclone is an area of high pressure in which winds blow outwards and descend, bringing clear and dry weather. A tropical cyclone is a violent storm that forms over warm oceans and is known by different local names: it is called a hurricane in the North Atlantic and the eastern Pacific, a typhoon in the western Pacific and the China Sea, a cyclone in the Indian Ocean, and a willy-willy in Australia. A tornado is a smaller but extremely violent rotating column of air that touches the ground, common in the United States.
Clouds and types of rainfall
Clouds are formed when rising moist air cools and its water vapour condenses into tiny droplets. There are three main types of rainfall. Convectional rainfall occurs when hot air rises in the equatorial regions and heavy thunderstorms follow in the afternoon. Orographic or relief rainfall occurs when moist air is forced to rise over a mountain range, giving heavy rain on the windward side and a dry rain-shadow on the leeward side. Cyclonic or frontal rainfall occurs when warm and cold air masses meet along a front, as in the temperate regions.
Quick revision
Remember the composition - nitrogen about 78 per cent and oxygen about 21 per cent - and the five layers from bottom to top: troposphere (weather), stratosphere (ozone), mesosphere (meteors), thermosphere (aurora and radio) and exosphere. Air pressure decreases with height, and the Coriolis effect deflects winds right in the north and left in the south. The three planetary winds are the trade winds, westerlies and polar easterlies. Cyclones are low-pressure systems with inward, rising winds, named hurricane, typhoon, cyclone and willy-willy in different regions, while anticyclones are high-pressure systems with clear weather.
How to prepare this topic
- Memorise the composition percentages and the order of the five atmospheric layers.
- Connect each layer with its key feature - weather, ozone, meteors, aurora or space.
- Understand why winds blow and how the Coriolis effect changes their direction.
- Learn the local names of tropical cyclones region by region.
- Distinguish the three types of rainfall with one example each.
Common exam traps
- The troposphere, not the stratosphere, is where weather occurs.
- The ozone layer lies in the stratosphere.
- Cyclones are low-pressure areas with inward winds; anticyclones are high-pressure with outward winds.
- Tropical cyclones rotate anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
- Nitrogen, not oxygen, is the most abundant gas in the atmosphere.
Frequently Asked Questions
What is the composition of the atmosphere?
The atmosphere is a mixture of gases in which nitrogen is the most abundant at about 78 per cent, followed by oxygen at about 21 per cent. Argon forms about 0.93 per cent and carbon dioxide about 0.04 per cent. Although carbon dioxide and water vapour are present in small amounts, they are very important because they absorb heat and cause the greenhouse effect, and water vapour is essential for cloud formation and rainfall.
What are the layers of the atmosphere and which is the most important for weather?
The atmosphere has five layers - the troposphere, stratosphere, mesosphere, thermosphere and exosphere. The troposphere is the lowest layer, extending about 8 kilometres at the poles and 18 kilometres at the equator, and it is where all weather such as clouds, rain and storms occur. The stratosphere contains the ozone layer, the mesosphere is where meteors burn up, and the thermosphere contains the ionosphere where the aurora occurs.
What causes winds and what are the planetary winds?
Winds are caused by differences in air pressure, as air moves from a high-pressure area to a low-pressure area. The rotation of the Earth deflects these winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, which is called the Coriolis effect. The three permanent or planetary winds are the trade winds, which blow towards the equator; the westerlies, which blow towards the poles; and the polar easterlies.
What is the difference between a cyclone and an anticyclone?
A cyclone is an area of low pressure in which winds blow inwards and rise, producing clouds, rain and storms; in the Northern Hemisphere cyclonic winds rotate anticlockwise and in the Southern Hemisphere clockwise. An anticyclone is an area of high pressure in which winds blow outwards and descend, bringing clear and dry weather. A tornado is a smaller but extremely violent rotating column of air.
What are the local names of tropical cyclones in different regions?
A tropical cyclone is a violent storm that forms over warm oceans and is known by different local names. It is called a hurricane in the North Atlantic and the eastern Pacific, a typhoon in the western Pacific and the China Sea, a cyclone in the Indian Ocean, and a willy-willy in Australia. The three main types of rainfall are convectional, orographic or relief, and cyclonic or frontal rainfall.