The change in temperature or temperature differential, which causes atmospheric circulation can be compared to the temperature differences produced in a pan of water placed over a gas burner. As the water is heated, it expands and its density is lowered. This reduction in density causes the warmer, less dense water to rise to the top of the pan. As it rises, it cools and is forced to the edges of the pan. Here it cools further and then sinks to the bottom, eventually working its way back to the center of the pan where it started. This process sets up a simple circulation pattern due to successive heating and cooling.

Ideally, the air within the troposphere may be compared to the water in the pan. The most direct rays of the Sun hit Earth near the equator and cause a net gain of heat. The air at the equator heats, rises, and flows in the upper atmosphere toward both poles. Upon reaching the poles, it cools sufficiently and sinks back toward Earth, where it tends to flow along the surface of Earth

1. Earth rotates, resulting in an apparent force known as the Coriolis force (a deflecting force). This rotation results in a constant change to the area being heated.

2. Earth is covered by irregular land and water surfaces that heat at different rates. Regions under the direct rays of the Sun absorb more heat per unit time than those areas receiving oblique rays. The heat produced by the slanting rays of the Sun during early morning may be compared with the heat that is produced by the slanting rays of the Sun during winter. The heat produced by the more direct rays at midday can be compared with the heat resulting from the more direct rays of summer.

The length of day, like the angle of the Suns rays, influences the temperature. The length of day varies with the latitude and the season. Near the equator there are about 12 hours of daylight with the Suns rays striking the surface more directly. Consequently, equatorial regions nor-mally do not have pronounced seasonal temperature variations.

During the summer in the Northern Hemi-sphere, all areas north of the equator have more than 12 hours of daylight. During the winter the situation is reversed; latitudes north of the equator have less than 12 hours of daylight. Large seasonal variation in the length of the day and the seasonal difference in the angle at which the Suns rays reach Earths surface cause seasonal temperature differences in middle and high latitudes.

The weak temperature gradient in the sub-tropical areas and the steeper gradient poleward can be seen in figures 3-1-2A and 3-1-2B. Note