Explanation:
Concept overview / definition
When air is heated or cooled, its density changes and this strongly influences vertical motion. A parcel of air that is warmed by insolation becomes less dense than the surrounding air, so it tends to rise. A parcel that cools becomes denser and heavier than its surroundings, so it sinks. These rising and sinking motions help create low-pressure and high-pressure areas in the lower atmosphere.
Why the correct option is correct
Statement 1 is correct because heating provides energy to a parcel of air, its molecules move faster, it expands and its density falls below that of nearby air, so it moves upward. Statement 2 is also correct, since cooling removes energy, the parcel contracts, becomes more dense, and therefore tends to descend toward the surface layers. Both statements capture the basic density logic that underpins the link between temperature, density and vertical air movement in climatology.
Why the other options are incorrect
Statement 3 is incorrect because changes in density due to heating and cooling are central to the formation of high and low pressure areas. If air did not become lighter when heated or heavier when cooled, it would not rise or sink in the way described in the transcript, and the familiar pattern of surface low and high pressure systems would not form. Hence, any claim that density changes are irrelevant to pressure patterns contradicts the core physical explanation.
UPSC exam tip / common confusion
A common confusion is to memorise that warm air rises and cold air sinks without linking it to density and pressure. In the exam, always think in steps: temperature change leads to density change, density change leads to rising or sinking motion, and that in turn modifies surface pressure. This chain of reasoning helps you eliminate options that talk about temperature but ignore density or vertical motion, especially in conceptual questions on pressure belts and winds.
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