21. Major hot deserts in northern hemisphere are located between 20-30 degree north and on the western side of the continents. Why?

    Deserts are created due to arid and semi-arid conditions because of the lack of moisture. It is accelerated by intense heating.

    Between 20-30 degree north latitude the insulation is relatively direct and less scattered and reflected by the atmosphere. Intense heating takes place in this zone. 

The North East trade winds carry moisture from the warm oceans and cause intense rainfall on the eastern regions of the continents. However, when they reach the western side of the continents they lose most of their moisture and are similar to off shore winds(land breeze) thereby causing arid conditions. This creates a rain shadow region in the western region.

Presence of anti-cyclonic conditions between 20–30 degree latitudes on western margins of continents . The region is locsted between equiotral low pressure belt and sub-tropical high pressure belt. The air while descending is adiabitically heated by compression. Thus, it's moisture holding capacity is less.

Incidence of cold currents along the western coastal regions cause the on shore winds to be dry. They are also not rain bearing.

Thus, major hot deserts in northern hemisphere such as  Thar desert, Rajasthan in Indian sub continent, Sahara  desert in Africa,  Great Basin desert in North America, Arabian desert in Arabian Peninsula are all located between 20-30 degree north and on the western side of the continents.