Formation conditions and weather characteristics of air mass

1. The heat of air mass forming the atmosphere mainly comes from the surface, and the water vapor in the air also comes from the evaporation of surface moisture, so the underlying surface is the most direct heat source and the most important source of water vapor. The condition of air mass formation requires a wide range of underlying surfaces with relatively consistent properties. The vast ocean, the ice-covered continent and the endless desert can all be used as the source of air mass formation. In addition, the formation of air mass should also have suitable flow field conditions, so that a large range of air can stay over the source for a long time or move slowly, and exchange water vapor and heat with the surface through physical processes such as turbulence, convection, radiation, evaporation and condensation in the atmosphere and large-scale vertical movement, so as to obtain relatively uniform temperature and humidity characteristics corresponding to the underlying surface.

2. The suitable flow field usually refers to the quasi-static large-scale high-pressure flow field. Under the control of quasi-static high pressure, the divergent subsidence movement in high pressure can reduce the horizontal gradient of temperature and humidity in the atmosphere and increase the horizontal uniformity of temperature and humidity characteristics in the atmosphere. At the same time, the stable circulation can make the air move slowly on the underlying surface with relatively uniform temperature and humidity characteristics for a long time, so that the air has enough time to obtain the temperature and humidity characteristics of the underlying surface. For example, Siberia is occupied by a high pressure that does not move much in winter, which is the source of dry and cold air mass. Pacific high often exists in the vast ocean of southeast China and is the source of warm and humid air mass.