What is the reason why South Asia is divided into three seasons?

According to the advance and retreat of monsoon, South Asia can be divided into three seasons: cool, hot and rainy. /kloc-From October to February, the northeast monsoon blowing from the mainland to the ocean brings sunny, dry and cool weather, which is a cool season. In the cool season, the daily temperature difference is large, there is some cyclone rain in the northwest, and there is also more precipitation on the east coast of the Indian Peninsula. From March to May, as the direct point of the sun moves northward, the temperature rises rapidly and the climate is dry and hot, which is called hot season. The temperature in the central Deccan Plateau reaches 35℃, and the absolute maximum temperature in thar desert reaches above 50℃, and sandstorms sometimes occur. From June to September, due to the continuous high temperature in South Asia, there was a thermal depression in northwest India, which strongly attracted the southwest monsoon deflected by the southeast trade wind to cross the equator, thus further strengthening the influence of the southwest monsoon. The southwest monsoon passes through the vast and warm tropical ocean and is full of water vapor, which brings abundant precipitation to most parts of South Asia and forms the rainy season. 90% of the annual precipitation is concentrated in the rainy season. The rainy season in the west of the peninsula comes first, and the rainy season in the northwest of India comes last. The late arrival and early departure of southwest monsoon is one of the main reasons for the large precipitation variability in South Asia. 10 ~ 1 1 means that the rainy season ends, the temperature begins to decrease gradually, and the air pressure in the northern region also increases gradually, so the pressure gradient between land and sea gradually weakens, so the southwest monsoon begins to contract, the precipitation decreases, and it soon turns back to the cool season.

Tropical rain forest and tropical monsoon forest occupy a vast area in South Asia, and their distribution is closely related to precipitation. West to Shanxi, east to the southern foothills of the Himalayas, Assam and most parts of Sri Lanka Island, with an annual precipitation of more than 2000 mm, it belongs to a tropical rainforest area; The annual precipitation in most areas of Deccan Plateau is 1, 000 ~ 2,000 mm, and the defoliation of trees in dry season reduces evaporation in hot season. The annual precipitation in Deccan Plateau and the northwest of Indian Peninsula is between 500 ~ 1 000 mm, which is mostly shrub and grassland vegetation. There is little rainfall in thar desert and its surrounding areas, which is desert and semi-desert, and only sparse herbs and prickly shrubs grow. The strength of southwest monsoon is early or late, which has an important impact on agricultural production in South Asia.