Forms of condensation: dew, frost, fog, and mist

Correct answer: C. Fog and mist consist of tiny liquid water droplets suspended in air; they can reduce visibility without falling as rain. Dew forms on a surface, frost is ice deposited on a surface, and snowfall consists of falling ice crystals or snowflakes. The suspended liquid droplets described in the question therefore indicate fog or mist.

Correct answer: D. Condensation is the change of water vapour into liquid water when moist air cools to saturation, often on or around tiny particles that help droplets form. In fog and mist, the resulting tiny liquid droplets remain suspended in the air near the ground. Their main difference is usually the degree to which they reduce visibility, not the basic process that creates them. A is wrong because both require moisture, not dry air. B is unrelated to their formation. C is wrong because these are atmospheric water droplets, not rocks. Memory cue: fog and mist share the same process—airborne droplets; visibility distinguishes their intensity.

Correct answer: B. Fog consists of tiny liquid water droplets suspended in the air near the ground. It may reduce visibility while remaining airborne. Frost, in contrast, is ice deposited on a surface when conditions are cold enough for freezing. Therefore A describes frost, not fog. C describes dew because liquid water is deposited on grass; it does not identify suspended droplets in air. D is wrong because dry air contains too little water vapour for fog or frost formation. The key diagnostic is location: fog is in the air, whereas dew and frost are on surfaces. Memory cue: fog floats; frost forms on a surface.

Correct answer: B. Mist is made of small water droplets suspended in air, like fog, but it is generally less dense and reduces visibility less strongly. If road signs remain readable from a moderate distance, the visibility reduction is limited, so mist is the best choice. A is less suitable because dense fog usually produces a much greater obstruction to vision. C is frost, a solid deposit on surfaces, so it does not mainly describe reduced visibility through airborne droplets. D is dew, liquid water deposited on a surface, and it also does not normally fill the air to obstruct distant signs. Memory cue: mist is a lighter veil in the air; fog is the denser veil.

Direct answer: C, because rising air expands as pressure decreases upward. Atmospheric pressure is greater near the surface and generally becomes lower with height. An air parcel that rises moves into this lower-pressure environment. It expands because the surrounding pressure is reduced. Expansion requires energy, and the parcel’s internal energy is used in doing this work, so its temperature falls when there is no major external heat supply. This is called adiabatic cooling. If the air contains enough moisture, cooling may bring it to saturation; water vapour then condenses and clouds can form. A is meaningless and does not describe a physical process. B is wrong because air does not turn into soil. D is wrong because rising air moves upward, not because it falls into the sea. The cooling is not simply because height is cold; the pressure decrease and expansion are the key steps. Memory cue: rising air expands and cools; sinking air compresses and warms.