If pressure decreases by 10 millibars per 10 metres in the lower atmosphere, what change occurs on ascending 200 metres?

Concept overview / definition Key idea: A common near-surface rule of thumb used in some exam contexts is that pressure decreases by about 10 millibars for every 10 metres of ascent in the lower atmosphere. This is an approximation for quick elimination and rough calculations, not a precise physical law.

Why the correct option is correct At 200 metres, there are 20 blocks of 10 metres. If each 10 metres reduces pressure by about 10 millibars, the total reduction is 20 × 10 = 200 millibars, so pressure would be 200 millibars lower than the given sea-level value under this stated approximation.

Why the other options are incorrect A 100 millibar drop corresponds to only 100 metres under the same rule. A 20 millibar drop corresponds to only 20 metres. An increase is impossible under the stated rule because ascent reduces the air column weight above the observer.

UPSC exam tip / common confusion Always check what approximation the question explicitly provides. Use it even if it feels oversimplified. Elimination: Convert height to “number of 10 m steps” first, then multiply by the stated millibar change.