The main reason that the subtropics at 30 degree north and south of the equator contain deserts is because of that .

of rising air that is part of cell that originates at the equator.

Explanation:

Deserts are regions of the world that receives little to no precipitation all year round. Most deserts are known to be hot during the day and very cold at night. Deserts are typically lifeless with little no biodiversity of organisms.

Most deserts of the world are usually found around 30 degrees north and south of the equator because of the rising air masses which are a part of the global circulation. At the equator warm air laden with moisture rises and moves away due to their density and pressure. As they leave they equatorial region, they drop their moisture to form clouds. The air then becomes cold and dry as it moves towards the pole from the equator. It eventually sinks as it approaches the tropics of the cancer and capricorn. This dry and cold air is what leads to the formation of desert. Dry and cold air moves on the surface  and does not provide rainfall.