Explanation:

Both A and R are true and R explains A

• Desert surfaces receive a little more than twice the solar radiation received by humid regions and lose almost twice as much heat at night. Many mean annual temperatures range from 20-25° C. The extreme maximum ranges from 43.5-49° C. Minimum temperatures sometimes drop to -18° C.
• Tropical deserts are located in regions between 5 and 30 degrees latitude. For eg. The climate of the Sahara desert is scorching hot and parched dry. It has a short rainy season. Since the sky is cloudless and clear, the rate of water evaporation is greater than the rate of water accumulation. Hence, there is water scarcity. However, the nights may be freezing cold with temperatures nearing zero degrees Celsius.