When air over a region cools and descends toward the surface, which pressure condition is most likely to form?

Cool air is denser and heavier than warm air, so it tends to sink rather than rise. When air over a region cools, it contracts slightly, its density increases, and it begins to descend toward the surface. This sinking motion increases the amount of air packed into the column above the ground, which raises the surface air pressure in that locality.

In the situation described, descending cool air is moving down into the lower layers of the atmosphere. As this air settles, it adds mass to the column above the surface and therefore increases the weight of air pressing on the ground. This creates a high pressure area, which is why the correct option states that a high-pressure system forms when cool air sinks toward the surface.

Option B is incorrect because cool air does not rise to create low pressure; it behaves opposite to warm air and tends to descend. Option C talks about a trough of low pressure with very weak winds, which is not the direct outcome of widespread sinking cool air. Option D is wrong because systematic cooling and subsidence do change the pressure pattern, rather than merely altering humidity without affecting pressure.

For UPSC, remember the broad rule that rising warm air is associated with low pressure, while sinking cool air is associated with high pressure. This principle helps you understand why subtropical high-pressure belts form where air from the upper troposphere descends, and why many deserts are located under such subsiding, high-pressure regions.