Question

On average, more solar energy falls in the equatorial regions than in desert regions. Why then are deserts hotter on average – as measured by the dry bulb temperature - than the tropics?

Answer 1

This discrepancy between the temperatures of a desert region and the equatorial areas can not be described only with the help of incoming solar radiation. Rather there are many other factors which interplay to cause this difference. In the equatorial areas, the incoming solar radiation causes extensive evaporation from the surface of water. This water can hold a lot of heat coming from the solar radiation. This decreases the amount of available heat for the above lying atmosphere. So, there is lesser amount of heat for increasing the temperature of the atmosphere. When the evaporated water vapour rises upward through the atmosphere, at some level it starts to condense and form clouds. These clouds again reflect the incoming solar radiation and decrease the amount of available solar heat for heating the atmosphere. So, this for that decreases the temperature of the atmosphere of the equatorial region. Equatorial regions are areas of dense forest. This dense canopy absorbs a major part of the incoming solar radiation. This causes a decrease in the available heat and temperature of the atmosphere. This canopy again takes heat from the atmosphere for the transpiration process. This further cools the atmosphere. These are some of the reasons why the equatorial atmosphere remains cool in comparison to the desert regions. At the desert regions there is no canopy available for the absorption of the incoming solar radiation. Further more, there is no available water at these areas which will get evaporated by the solar radiation. So, all the incoming solar radiation is used for heating the atmosphere at these locations. This great amount of heat then increases the temperature of the atmosphere to a great degree at the desert regions.

These are some of the reasons for the difference in temperature of these two locations in spite of getting similar amounts of incoming solar radiation.