The formation conditions and weather characteristics of air masses

1. The formation of air masses. The heat of the atmosphere mainly comes from the earth's surface, and the water vapor in the air also comes from the evaporation of water on the earth's surface. Therefore, the underlying surface is the most direct heat source of the air and the most important moisture source. The conditions for the formation of air masses first require a large-scale underlying surface with relatively uniform properties. Vast oceans, snow-covered continents, endless deserts, etc., can all be used as sources for the formation of air masses. In addition, the formation of air masses should also have appropriate flow field conditions, so that a wide range of air can stay over the source for a long time or move slowly, and pass through the turbulence, convection, radiation, evaporation and condensation of various scales in the atmosphere and large-scale Physical processes such as vertical movement of a range exchange water vapor and heat with the earth's surface, thereby obtaining relatively uniform temperature and humidity characteristics corresponding to the underlying surface.

2. An appropriate flow field usually refers to a quasi-stationary large-scale high-pressure flow field. Under the control of quasi-stationary high pressure, the divergent subsidence motion in the high pressure can reduce the horizontal gradient of temperature and humidity in the atmosphere, increase the horizontal uniformity of temperature and humidity characteristics in the atmosphere, and at the same time, the stable circulation can make the air cooler. Move slowly for a long time on the underlying surface with relatively uniform temperature and humidity characteristics, so that the air has enough time to obtain the temperature and humidity characteristics of the underlying surface. For example, the Siberian region is dominated by a low-moving high pressure in winter, which is the source of dry and cold air masses. The Pacific high pressure often exists on the vast ocean southeast of China, which is the source of warm and moist air masses.