Because the sand and bare rocks radiate heat, warming the surroundings
Explanation
The thermal regime of hot deserts is primarily dictated by the physical properties of the surface material and the lack of atmospheric moisture. The interaction between solar insolation and the desert surface determines the extreme diurnal temperatures.
Option Analysis
- Due to the presence of extensive volcanic activity is incorrect: While the Sahara contains volcanic structures (such as the Tibesti and Ahaggar mountains), active volcanic activity is not the source of the general climatic heat experienced across the vast desert region.
- Because the sand and bare rocks radiate heat, warming the surroundings is correct: The desert surface is dominated by sand and bare rock, which have low specific heat capacities. This allows them to absorb solar insolation very quickly during the day, raising the surface temperature significantly. This heat is then radiated into the overlying air, causing extreme daytime temperatures. At night, the absence of cloud cover allows this heat to escape rapidly, leading to cool nights.
- Due to the absence of winds during the daytime is incorrect: The Sahara is not characterized by a lack of wind. Strong winds are a common feature (e.g., the Harmattan), often causing sandstorms. The presence or absence of wind is not the primary driver of the high insolation-based heat.
- Because of the low altitude of the entire desert region is incorrect: The Sahara is not uniformly low in altitude. It possesses a varied topography, including deep depressions and high mountain ranges. Thus, altitude is not the sole or primary factor for the heat across the entire region.
Key Takeaway: The extreme daytime heat in deserts results from the rapid heating of sand and rocks due to their low specific heat capacity, combined with clear skies that allow maximum solar insolation to reach the surface.