Humidity, evaporation, and condensation

At the same temperature, (e_s) remains nearly same and (RH) rises as (e) increases. Exam tip: solve constant-temperature cases separately.

Water vapour helps retain heat like a greenhouse gas. Exam tip: connect humid nights with warmer nights.

H₂O is the chemical formula for water, consisting of two hydrogen atoms bonded to one oxygen atom. When water exists in the gaseous state, it is called water vapour. Water vapour is naturally present in the atmosphere and is also an important greenhouse gas because it absorbs and re-emits infrared radiation. CO₂ is carbon dioxide, O₃ is ozone, and N₂ is nitrogen, so H₂O is the only correct answer.

Relative humidity is calculated as RH = (e/e_s) × 100, where e is the actual vapour pressure and e_s is the saturation vapour pressure at the same temperature. If e_s remains constant and e increases, the numerator of the fraction becomes larger, so the value of RH increases. The air is therefore holding a greater proportion of the moisture required for saturation. It may reach 100% if e becomes equal to e_s. Thus, option B is correct.

Relative humidity is calculated as RH = (actual vapour pressure ÷ saturation vapour pressure) × 100. Substituting the given values gives RH = (12 ÷ 24) × 100 = 0.5 × 100 = 50%. Thus option B is correct. The units cancel because both pressures are measured in the same unit.