Why is it so cold sometimes?

The ever-changing atmospheric state is not only the atmospheric state in a certain time and space, but also the continuous change of atmospheric state in a certain time interval. So it can be understood as a general term for weather phenomena and weather processes. Weather phenomenon refers to various natural phenomena in the atmosphere, that is, the comprehensive expression of the spatial distribution of various meteorological elements in the atmosphere (such as wind, clouds, fog, rain, snow, frost, thunder, hail, etc.). ) in an instant. The weather process is the change process of weather phenomena with time in a certain area. Weather is the general name of atmospheric conditions (such as cold and warm, wind and rain, dry and wet, sunny and so on). ) and its changes in a short time in a certain area. Weather system usually refers to the atmospheric motion system with typical characteristics, such as high pressure, low pressure, high pressure ridge and low pressure trough, which cause the change and distribution of weather. All kinds of weather systems have certain time and space scales, and they are intertwined and influence each other. The combination of many weather systems constitutes a large-scale weather situation and hemispheric and even global atmospheric circulation. The weather system is always in the process of rebirth, development and extinction, and there are corresponding weather phenomena in different development stages. Therefore, the weather and weather changes in a region are related to the weather system and its development stage, and are the comprehensive results of atmospheric dynamic and thermal processes. All kinds of weather systems are formed, developed and evolved in a certain atmospheric circulation and geographical environment, and all reflect the environmental characteristics of a certain region. For example, the polar regions and their surroundings are covered with ice and snow all year round, and the air is cold and dry. This unique geographical environment has become the background condition for the formation and development of polar low-altitude cold high pressure and high-altitude polar vortex and low trough. It is a necessary condition for the emergence and development of convective weather system that the equatorial and low latitudes are hot and humid all the year round and the atmosphere is unstable. Mid-high latitudes are areas where cold and warm air currents often cross. Cold and warm air masses not only come and go frequently, but also have unstable baroclinic, which is an important basis for the formation and development of frontal and cyclone systems. The formation and activities of weather system will in turn have a profound impact on the structure and evolution of geographical environment. Therefore, understanding the formation, structure, movement and change of weather system and its relationship with geographical environment is of great significance for understanding the formation, characteristics and change of weather and climate and predicting the evolution of geographical environment.