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

Introduction / Context:
High pressure and low pressure systems are basic building blocks of weather and climate. They are closely related to the vertical movement and temperature of air masses. When air cools and sinks, it has a different effect on surface pressure than when air warms and rises. This question explores why descending cool air is associated with high pressure conditions.

Given Data / Assumptions:

• Air over a region cools, becomes denser, and starts to descend toward the surface.
• We consider a sufficiently large area so that a pressure system can form.
• External disturbances such as strong cyclones are not dominating the situation.
• Basic gas laws and gravity apply.

Concept / Approach:
Cool air is denser than warm air when pressure is similar. As air cools aloft and sinks, it compresses slightly and increases the mass of air present in the column over a given area. More mass in the column means more weight pressing on the surface. The result is an increase in surface pressure and the development of a high pressure area. Such regions often show divergence of air near the ground and generally stable weather conditions.

Step-by-Step Solution:

Verification / Alternative check:
Typical subtropical high pressure belts, such as those around 30 degrees latitude, are explained by large scale descending air within the Hadley circulation. Cold anticyclones over continents in winter also demonstrate how cooling and sinking of air create strong high pressure cells. Observations consistently show that these regions are associated with higher surface pressure values.

Why Other Options Are Wrong:
Option B is incorrect because cool air does not rise; it sinks. Option C confuses the system with a trough of low pressure, which is associated with rising air rather than sinking air. Option D is wrong because temperature and vertical motion clearly affect pressure; changes in humidity alone cannot explain the system. Option E is unrealistic, as natural systems almost always have some pressure gradient and motion.

Common Pitfalls:
Learners sometimes mix up the roles of warm and cool air, assuming that any movement creates low pressure. Remember that rising warm air usually produces low pressure, while sinking cool air produces high pressure. Another pitfall is ignoring the idea of mass in the air column and focusing only on temperature without linking it to density and weight.

Final Answer:
The correct condition is that a high-pressure area forms because descending cool air adds more weight to the surface column and increases surface pressure.