Discussion
Assertion (A): Severe pollution episodes occur under stagnant high pressure systems.
Reason (R): Descending air forms a subsidence inversion while wind speeds fall.
Select the correct answer from the options below.
Answer: B
Correct answer: Both A and R are true, and R is the correct explanation of A
Both statements are true and the reason explains the assertion. Severe pollution episodes do occur under stagnant high pressure systems, and the mechanism is the one given: air descending within the high is compressed and warms adiabatically, forming a subsidence inversion that caps the mixed layer, while the weak pressure gradients of a stagnant high leave the wind light.
Both factors reduce the ventilation coefficient - the product of mixing height and wind speed - so a given emission is diluted into a much smaller volume. And because such systems can persist for days, concentrations accumulate rather than clearing overnight.
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