Tham khảo tài liệu 'the earth’s atmosphere contents part 4', khoa học tự nhiên, vật lý phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Measuring Air Pressure 141 WARM FIGURE It takes a shorter column of cold air to exert the same pressure as a taller column of warm air. Because of this fact aloft cold air is associated with low pressure and warm air with high pressure. The pressure differences aloft create a force that causes the air to move from a region of higher pressure toward a region of lower pressure. The removal of air from column 2 causes its surface pressure to drop whereas the addition of air into column 1 causes its surface pressure to rise. The difference in height between the two columns is greatly exaggerated. rapidly with height simply because you climb above fewer molecules in the same vertical distance. In Fig. move up the warm column until you come to the letter H. Now move up the cold column the same distance until you reach the letter L. Notice that there are more molecules above the letter H in the warm column than above the letter L in the cold column. The fact that the number of molecules above any level is a measure of the atmospheric pressure leads to an important concept Warm air aloft is normally associated with high atmospheric pressure and cold air aloft is associated with low atmospheric pressure. In Fig. the horizontal difference in temperature creates a horizontal difference in pressure. The pressure difference establishes a force called the pressure gradient force that causes the air to move from higher pressure toward lower pressure. Consequently if we remove the invisible barrier between the two columns and allow the air aloft to move horizontally the air will move from column 2 toward column 1. As the air aloft leaves column 2 the mass of the air in the column decreases and so does the surface air pressure. Meanwhile the accumulation of air in column 1 causes the surface air pressure to increase. In summary heating or cooling a column of air can establish horizontal variations in pressure that cause the air to move. The net accumulation of air .
