General Circulation of the Atmosphere

It has been shown in previous notes, that there is a surplus of energy towards the Equator and a deficit nearer to the poles. In theory, this surplus energy should be transferred to areas with a deficiency of energy by means of a single convective system. This would be the case for a non-rotating Earth.

The Sun’s rays hit the Equator directly, causing a consistent flow of warm, rising air in the tropics. This rising air is rapidly cooled to produce the towering cumulonimbus clouds, frequent afternoon thunderstorms and low pressure characteristic of the equatorial climate. This is the reason why rain forests exist at the Equator. As the rising air cools to the temperature of the surrounding air, uplift ceases and it begins to move away from the Equator. Further cooling, increasing density and diversion by the Coriolis force cause the air to slow down and to subside, forming the descending limb of the Hadley Cell https://fr-libido.com/. In looking at the northern hemisphere, it can be seen that the air subsides at 30°N of the Equator to create the sub-tropical high pressure with its clear skies and dry, stable conditions. This is the reason why extensive deserts exist at this latitude. On reaching the Earth’s surface, the cell is completed as some of the air returns to the Equator as the north-east trade winds. The remaining air is diverted pole ward forming the warm south-westerlies which collect moisture when they cross areas of sea. The Maltese Islands are located within the south-westerlies zone. This is why the westerlies are the most common winds direction locally. These warm winds meet cold Arctic air at 60°N of the Equator. This forces the air to rise and form an area of low pressure and the rising limb of both the Ferrel and Polar cells. The resultant unstable conditions produce the heavy-cyclonic rainfall associated with mid-latitude depressions. These depressions are another mechanism by which surplus heat is transferred. While some of this rising air returns to the tropics, some travels towards the poles where, having lost its heat, it descends to form another stable area of high pressure at 90°N of the Equator. This is why the north pole is considered as one of the driest deserts on Earth. Air returning to the polar front is referred to as the cold easterlies.