Humidity, evaporation, and condensation

Direct answer: A, wind removes the moist boundary layer near the clothes. Water on wet fabric evaporates into the thin layer of air touching the fabric. This layer quickly becomes humid, so the difference between the vapour pressure at the wet cloth and in the nearby air becomes smaller. Strong wind mixes away that humid air and replaces it with comparatively drier air. The larger moisture gradient allows more water molecules to escape from the fabric, so drying is faster. Wind does not destroy water vapour; it transports and mixes it. B is wrong because wind does not normally turn the water on clothes into ice. C is scientifically impossible: vapour remains matter in the air. D is wrong because wind does not always make temperature fall below freezing, and drying occurs at ordinary temperatures too. Temperature, humidity, exposed surface area, and airflow can all affect drying. Memory cue: wind renews the air at the surface; it does not remove water by magic.

Direct answer: A. Here e_s means saturation vapour pressure, the maximum vapour pressure that air can support at a particular temperature. This maximum decreases when temperature falls. Suppose the actual vapour pressure changes little while the air rises and cools. Because the saturation limit becomes lower, the actual value gets closer to that limit. Relative humidity therefore increases, and the air may reach saturation. Once saturation is reached, excess water vapour can condense on condensation nuclei to form cloud droplets or ice particles. B is wrong because cooling does not automatically make vapour pressure zero; actual vapour pressure may remain present. C is wrong because water vapour does not become solid rock. D is the opposite of the process: cooling toward saturation often helps cloud formation, although moisture, nuclei, and suitable atmospheric conditions are also needed. Memory cue: cooling lowers the moisture capacity of air; the same vapour then fills the smaller capacity more completely.

Correct answer: A. Condensation nuclei are tiny particles, such as dust or salt aerosols, on which water vapour can condense to form droplets. If suitable nuclei are scarce, there are fewer sites for droplets to begin forming, so initial cloud development can be hindered under otherwise similar conditions. This does not make clouds impossible, since sufficiently strong supersaturation and other processes can also support condensation. Nuclei occur over land as well as over oceans.

Correct answer: A. Droplets may grow inside a cloud, but they still have to travel through the air before reaching the ground. When the air below the cloud is very dry, it can take up water vapour from the falling droplets. The droplets therefore become smaller, and some may disappear completely before reaching the surface. Precipitation that evaporates before reaching the ground is often called virga when it is visible as streaks beneath a cloud. B is impossible in this atmospheric process because water droplets do not turn into rocks. C is wrong because dry air tends to remove water from droplets rather than strengthen rainfall. D is wrong because evaporation and condensation are directly connected with precipitation. Memory cue: cloud growth does not guarantee ground rain; always check the air beneath the cloud.

Correct answer: A. Moist air rises on the windward side of a mountain, expands, cools, condenses, and often produces rain. Because some water has fallen out, the air crossing the ridge contains less water vapour. As this air descends on the leeward side, pressure increases and the air is compressed and warmed. Warmer air can hold more water vapour before becoming saturated, while the actual amount of vapour may remain low. Relative humidity is the amount of vapour present compared with the maximum amount the air could hold at its temperature, so it decreases. This dry descending air produces a rain-shadow region. B is wrong because descending air is usually warming and is not automatically saturated. C is wrong because vapour does not increase merely through descent. D is incomplete: the mountain causes uplift and rain, but it does not simply stop the air. Memory cue: wet windward ascent, dry leeward descent.

Correct answer: A. A raindrop must travel through the air between the cloud and the ground. When this lower air is very dry, it can take up water vapour from the falling drops. As a result, the drops may become smaller, partly evaporate, or disappear before reaching the surface. The visible precipitation below the cloud can therefore decrease; precipitation that evaporates before reaching the ground is often called virga. Option A is correct because it connects dry air with evaporation during descent. Option B is wrong because water drops do not turn into rocks. Option C is wrong because dry air generally removes water from drops rather than increasing the amount reaching the ground. Option D is wrong because evaporation and water vapour are directly involved in precipitation. Memory cue: a cloud can be raining above while dry air makes the ground receive little or no rain.

Correct answer: A. Moist air rises on the windward side of a mountain, cools, condenses, and often loses water through precipitation. After crossing the summit, the air descends on the leeward side. Descending air is compressed by increasing atmospheric pressure and therefore warms. At the same time, much of its moisture has already been removed as rain or snow on the windward slope. Warmer air can hold more water vapour before becoming saturated, while the actual moisture supply is reduced, so relative humidity falls. Option A correctly combines both causes. B is wrong because descending air is not always saturated. C is wrong because moisture generally does not increase after windward rainfall. D is incomplete and incorrect; the mountain forces uplift and changes airflow but does not simply stop the air. Memory cue: wet ascent, dry descent—the rain-shadow sequence.