♥ Wednesday, March 4, 2009 ♥
Today we learned the model of global air circulation. Mr heah started the lesson with two questions: Where can deserts be found in the world and why are they there. As we know, the hottest zone in the world is the equatorial part, but the desert cannot be found in the equator and a kind of desert known as cold desert also exists. Therefore, the desert doesn’t depend on the temperature very much. The deserts are form mainly because of the dry climate.
As we can see in the world map of Koppen-Geiger climate classification, the deserts can always be found in the subtropical zone. This is because this zone is the driest in the world due to the global air circulation.
We want to find the general rule of the air circulation, so that we have to control some factor. We assume that the earth is complete uniform and the global reception and loss of energy can cause the temperature gradient of air in equator and poles. If we also ignore the rotation of the earth, we can get the single cell model (shown in figure 1p-1). Since the temperature of the air in the equator is higher than that in the poles, the pressure of the air in the equator is lower than that in the poles. So the air in the equator will flow vertically to the upper level of atmosphere and flow towards the poles. When the air reaches the poles, it will sink and flow back to the equator again.
However, the single cell is too simple to describe the air circulation. Now we can think about the factor of rotation of the earth, and we can get the three cell model. (Shown in 7p-2) Since the Coriolis force affects the direction of the air flowing, the air will be deflected no matter in the bottom or upper level of the atmosphere. By about latitude of 30°, the direction of air flow is nearly parallel to the latitude. This will cause the accumulation of air, so that some air is compelled to sink and move to different directions so that some air can flow back to equator to complete the Hadley cell. The polar cell is similar to the Hadley cell but the air in the pole sink to the bottom to start the circulation. However, the Ferrel cell which Mr heah taught us is a bit different from the one described in the online readings. The online readings say in latitude of 30°, the air in both of the upper and bottom of atmosphere flows towards the poles and the air in the bottom will rise in around 60°of latitude. However, Mr heah taught us the upper air in the Ferrel cell will flow towards latitude of 30°. I think the online readings are correct, since if the upper air in the Ferrel cell flow back to latitude of 30°, the polar jet stream cannot be formed.
These are the main things we learned today. At the end of lesson, Mr heah ask as to think why the deserts can be found in the subtropics. I think it is because the zone is in the subtropical high pressure zone which is created by the upper air sinking by around latitude of 30°. It is also in the trade-wind zone. So the air sinks to cause the high pressure and temperature and the relative humidity decreases so that the cloud and rain cannot form easily. That is why there are so many deserts.
(Source of pictures: PhysicalGeography.net)
These are the main things we learned today. At the end of lesson, Mr heah ask as to think why the deserts can be found in the subtropics. I think it is because the zone is in the subtropical high pressure zone which is created by the upper air sinking by around latitude of 30°. It is also in the trade-wind zone. So the air sinks to cause the high pressure and temperature and the relative humidity decreases so that the cloud and rain cannot form easily. That is why there are so many deserts.
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