9. Circulation Patterns of the Atmosphere

Photo 16: I'm trying to open an unripe coconut on a beach near Cancun, Mexico. 2008-12-14

Equatorial Low and Polar High

The two causes of air circulation on the Earth is unequal heating by the Sun and Earth’s spinning around its axis. The area from the equator to about 35° latitude north and south of the equator receives a surplus of heat respective to the area beyond that. Therefore, the heat transfer on the Earth occurs from the Equator outwards to the poles, creating an equatorial low pressure area (Inter-Tropical Convergence Zone) and a polar high pressure area. Air circulates from the poles to the equator occurs in the lower altitudes and from the equator back to the poles at higher altitudes in the Atmosphere. The ITCZ is the rainiest latitudinal region in the world, because as air rises it cools and can hold less moisture. This results in much lush green vegetation in the Caribbean area (photo 16).

Subtropical High and Upper Mid-Latitude Low

The cool wind from the poles that cooled off the equatorial area now returns to the poles at higher latitudes, but falls downward at about 30° latitude, forming the Subtropical High areas. These areas are most dessert like (south-western areas of the USA and northern Africa for example). The winds from these areas to the equator are called Trade Winds; the winds moving towards the poles are called Westerlies (because they come from the west in both Hemispheres). Winds from the poles, due to the Polar High, are called Polar Easterlies.  These two winds meet each other at the Polar Front, an area of convergence at 60° latitude called Upper Mid-latitude Low. This is a conceptualized model; in reality many other factors play a part in the circulation patterns of the Atmosphere. In Alberta we experience much Westerlies, wind coming from the Pacific Ocean over the Rocky Mountains. This brings precipitation as rain in the summer and snow in the winter (photo 17).

Photo 17: My first experience with large amounts of snow on my birthday. 2006-02-28

Figure 18: A Model of Atmospheric Circulation, added to this post to accompany the text with added clarity. Image from  Encyclopædia Britannica, Inc.