Paper 1 · climatology / energy budget
Which of the following correctly explains why water vapour condenses in the atmosphere?
AMoist air is heated well above its dew point as it descends, which causes water vapour to evaporate more rapidly onto condensation nuclei suspended in the surrounding air.
BMoist air is cooled below its dew point, becomes saturated, and water vapour changes to liquid water on condensation nuclei such as dust or salt particles.
CDry air absorbs water vapour from condensation nuclei as it descends and warms.
DMoist air expands and warms as it rises, causing water vapour to disperse evenly through the atmosphere.
Explanation: Condensation happens when moist air is cooled below its dew point, for example through adiabatic cooling as air rises or radiative cooling of the ground overnight, so the air becomes saturated and water vapour changes to liquid water on condensation nuclei such as dust, salt or pollution particles. Rising air cools rather than warms, so options describing warming or evaporation reverse the actual process.
Derived from ZIMSEC Geography Paper 1, November 2003, Q3
