Riyadh is a tropical desert, but precipitation is concentrated in winter. Why?

The atmospheric environment is the first, which is controlled by subtropical high. The subtropical high is a dynamic high, and the formation of the desert boat is due to the continuous flow of air over the equator through the subtropical zone, which leads to the downward flow and the increase of low-level air pressure, which belongs to warm high pressure. The subtropical weather is sunny and dry, mainly because of the prevailing downdraft. Second, it is controlled by the dry trade wind. Trade winds are directional winds blowing from subtropical high to equatorial low. Due to the rotation of the earth, the wind direction in the northern hemisphere shifts to the right and becomes the northeast trade wind. It turns left in the southern hemisphere and becomes the southeast trade wind. In the subtropical region of the northern hemisphere, the west coast of the mainland is on the leeward side of the northeast trade wind, and the east coast of the mainland is on the windward side of the northeast trade wind. Similarly, in the subtropical region of the southern hemisphere, the west coast of the mainland is on the leeward side of the southeast trade wind, and the east coast of the mainland is on the windward side of the southeast trade wind. In this way, there is little precipitation on the west coast of subtropical areas in the northern and southern hemispheres, with dry climate and desert; But there is more precipitation on the east coast of the mainland, not a desert. Third, affected by the cold current. Ocean currents have a great influence on the climate. Cold current can reduce the temperature and humidity of the area it flows through, while warm current can increase the temperature and humidity of the area it flows through. From the world climate and ocean current map, it can be seen that the west coast of subtropical continent is affected by cold current, with scarce precipitation and dry climate; The east coast of the mainland is affected by the warm current, and there is more precipitation. As for the formation of tropical desert climate in subtropical areas, it is not only affected by subtropical high or dry trade winds, but also related to being far away from the ocean and being less affected by the ocean. Temperature and humidity Tropical desert climate Tropical desert climate appears in the subtropical high pressure area and trade wind area on the west coast of the mainland and within the mainland. It is controlled by the descending airflow of subtropical high all the year round. Seen from the whole earth, the formation of dry climate zone (arid zone) is mainly related to latitude, atmospheric circulation and other factors. Between latitude 15 ~ 35, it is controlled by subtropical high pressure area (also known as reflux high pressure area), and it is an area that is hit by trade winds all the year round. The air in the high pressure area has a sinking effect. When the air sinks, it forms adiabatic heating, which reduces the relative humidity and makes the air very dry. The trade wind is a stable wind direction blowing from the subtropical high pressure belt to the equatorial low pressure belt, and it keeps heating up in the process of blowing to the equator; The hotter the air, the more water is consumed, which forms a very dry dry wind. In this way, in the subtropical high zone control area, the atmosphere is very stable, the humidity is low, and it is cloudy and rainy locally, which is an arid area with little rainfall on the earth. Most of the great deserts in the world are distributed here, such as the Sahara Desert in North Africa, the Arabian Desert in Southwest Asia and the Atacama Desert in South America. Therefore, it is called "returning to the desert". The formation of tropical desert climate varies slightly from place to place. The tropical desert climate in the west of South America is long and narrow. The reasons are as follows: ① The Andes are located on the west coast of the South American continent, which makes the distribution area of tropical desert climate narrow. (2) There is a cold current in Peru on the west side, which reduces temperature and humidity. ③ It is on the leeward slope of the southeast trade wind, with little precipitation. The causes of the tropical desert climate in Pakistan are almost unique in the world. Its causes are mainly affected by the following two aspects: one reason: serious damage to virgin forests and other vegetation. The second reason is that the southwest monsoon is difficult to reach or go deep because of the blocking of mountains on the edge of the Iranian plateau. Reference: Baidu Encyclopedia hopes to help you.