CHAPTER 3 ATMOSPHERIC CIRCULATION

CHAPTER 3 ATMOSPHERIC CIRCULATION To understand large-scale motions of the atmosphere, it is essential that the Aerographer’s Mate study the general circulation of the atmosphere. The sun’s radiation is the energy that sets the atmosphere in motion, both horizontally and vertically. The rising and expanding of the air when it is warmed, or the descending and contracting of the air when it is cooled causes the vertical motion. The horizontal motion is caused by differences of atmospheric pressure; air moves from areas of high pressure toward areas of low pressure. Differences of temperature, the cause of the pressure differences, are due to the unequal absorption of the Sun’s radiation by Earth’s surface. The differences in the type of surface; the differential heating; the unequal distribution of land and water; the relative position of oceans to land, forests to mountains, lakes to surrounding land, and the like, cause different types of circulation of the air. Due to the relative position of Earth with respect to the Sun, much more radiation is absorbed near the equator than at other areas, with the least radiation being absorbed at or near the poles. Consequently, the principal factor affecting the atmosphere is incoming solar radiation, and its distribution depends on the latitude and the season. GENERAL CIRCULATION LEARNING OBJECTIVE: Recognize how temperature, pressure, winds, and the 3-cell theory affect the general circulation of Earth’s atmosphere. The general circulation theory attempts to explain the global circulation of the atmosphere with some minor exceptions. Since Earth heats unequally, the heat is carried away from the hot area to a cooler one as a result of the operation of physical laws. This global movement of air, which restores a balance of heat on Earth, is the general circulation. WORLD TEMPERATURE GRADIENT Temperature gradient is the rate of change of temperature with distance in any given direction at any point. World temperature gradient refers to the change in temperature that exists in the atmosphere from the equator to the poles. 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 back to the equator. (See fig. 3-1). Simple circulation of the atmosphere would occur as described above if it were not for the following factors: 3-1