Besides keeping us warm, the Sun also supplies the energy to drive the Earth’s powerful and complicated weather systems. Wind, rain, snow, hail, fog, frost, heat and cold are the ground-level effects of the Sun’s heat on our planet’s air, water and land at different latitudes.
The Sun radiates colossal amount of energy. A great deal of this radiation is visible light. The Sun also pours out other types of electromagnetic radiation, such as ultraviolet rays, radio waves and x-rays. When this radiation hits the Earth’s outer atmosphere, it has the power of about 1 KW per square metre. If perfectly collected, the solar radiation on a square with sides of 1 metre would boil the water in a 1.7 litre kettle in about ten minutes. The total power reaching the Earth from the Sun is about 100 million million KW. That is more than one hundred thousand times the power produced by all the power stations of the world.
Nearly half of this radiation is either absorbed by the atmosphere or reflected by it back into space. When the radiation that reaches ground level hits land, sea, a tree, building, sunbather or whatever, it is absorbed and changed into the infra-red (heat) radiation that we feel. When the Sun is directly overhead, as it in tropics, on average each square metre receives about half a kilowatt – enough to power about eight reading lamp light bulbs. Closer to the poles, where the Sun is shining at an angle to the surface, the Earth receives only about one third as much energy on average.
Balancing heat
If there were no winds, the poles would get very much colder and the equator would get incredibly hot. Fortunately, the winds carry hot air away from the tropics and cold air away from the poles. How the winds come to blow is the key to understanding the weather.
The troposphere
Almost all the activity happens in the bottom layer of the atmosphere, which is called the troposphere. This is about 18 km deep over the equator and 8 km deep over the poles. In the troposphere, the air is swirling about and this is where most of the heat redistribution takes place. The atmosphere is held down to the Earth by the force of gravity, and the weight of all the air makes up atmosphere pressure.
The first stage of heat redistribution occurs when air near the equator heats up. Air expends and becomes less dense when it heats up and so it rises. This leaves a reduced pressure on the surface of the Earth bellow. As a result, cooler air from nearer the poles is sucked in to replace the air that has risen. This movement of air is felt as wind. The large volumes of air that rises from the areas around the equator eventually reach the top of the troposphere. They then move towards the poles, becoming cooler all the time. Meanwhile, the cool air arriving near the equator heats up and starts to rise.
In the tropics, warm air rises in a band extending about 10 degrees either side of the equator. After moving away from the equator, this air descends around latitudes 30 degrees north and south of the equator. The cool air replaces the warm air does not come directly from the north and south, as the rotation of the Earth deflects the winds. This is called the Coriolis Effect.
Wind belts
When the Coriolis Effect acts on the cool air approaching the equator, it gives rise to the winds known as the north-east and south-east trades. The air that descends around 30 degrees north and south of the equator causes regions of high pressure. Some of this air returns to the equator, while some moves towards the poles. The latter gives rise to winds which, because of the Coriolis Effect, blow from the south-west and north-west. These winds are known as the westerlies. In the northern polar region, cold winds blow from the north-east. These are called easterlies.
