Distribution of Drought and Frozen Disasters of China in Yuan Dynasty

Drought

Drought usually refers to a climatic phenomenon in which the total amount of fresh water is small enough to meet the needs of human survival and economic development. Generally, it is a long-term phenomenon. Drought has been a major natural disaster facing mankind since ancient times. Even with today's advanced science and technology, its catastrophic consequences can still be found everywhere. It is particularly noteworthy that with the economic development and population expansion of human beings, the shortage of water resources has become increasingly serious, which has directly led to the expansion of arid regions and the increase of the degree of aridization. The trend of aridization has become a global concern.

Frozen disasters

Cold air is the main cause of low-temperature freezing disasters. In the spring and autumn seasons, the cold air in the north and the warm and humid air in the south frequently intersect, often causing low temperature and rainy weather. However, the strong cold air, especially the southward eruption of the cold wave, as a result, the temperature drops sharply, causing "cold spring", "frost" and other disasters.

There is a process of accumulation, strengthening, southward movement and outbreak of cold air, and it is supported by corresponding weather conditions. Under the effect of strong radiative cooling, large-scale cold air masses will form in the Arctic Ocean and Siberia, and adiabatic expansion cooling will occur due to the ascending movement of the air in the cold reactor, which will continue to strengthen. When the cold air reactor reaches a certain intensity, it will erupt southward under a favorable circulation situation, and a cold wave will occur.

For frost, it usually occurs on cold, sunny (no clouds or few clouds), no wind or light breeze, and low humidity at night. Mostly, the ground surface and plant surface radiate a lot of heat outward, making it near the ground. The air is cooled below 0 °C. Frost is related to ground radiative cooling, and the amount of cloud is one of the most important factors affecting the amount of radiation. The presence of clouds is not conducive to the formation of frost. Wind can promote the mixing of near-surface and upper-layer air, thereby reducing the degree of cooling on the ground, so strong winds are not conducive to frost.