Why are all the places at 30 degrees north latitude desert?

This is a systemic problem. To put it simply, the formation of desert will inevitably lead to less rainfall, because the earth's rotation >: the earth's atmospheric flow > is a large-scale circulation, and 30 degrees north and south latitude is the subtropical high control area. The airflow is dominated by subsidence, so it is difficult to rain. The desert with latitude of 30 degrees is relatively concentrated, but the surface of the earth is uneven. Three-circle cycle is only an ideal state, and there will be some other situations in some areas. For example, China is located in the east of Eurasia, and the Pacific Ocean is in the east, so the atmosphere corresponding to China generally flows in opposite directions in Xia Dong in two seasons > the near-surface atmosphere flows from the ocean to the mainland in summer >: the air from the ocean is very humid > it brings abundant rainfall > there is no desert at 30 degrees in China, and the northwest is far from the ocean > the water vapor from the ocean is difficult to reach the inland > the northwest is very dry, and a large desert appears.