Humidity, evaporation, and condensation

Correct answer: A. Because water molecules need latent heat to become vapour.

The correct answer is A. A humidity deficit means that the surrounding air is capable of receiving more water vapour, so it can promote evaporation. However, evaporation is not controlled by humidity deficit alone. Each water molecule that escapes from the liquid surface requires latent heat to change from liquid to vapour. If the available energy supply is small—for example, because of weak heating or limited energy transfer—only a limited number of molecules can make that phase change per unit time. The process is then energy-limited even though the air is dry. A is correct because it identifies the necessary energy requirement. B is wrong because dry air generally accepts additional vapour readily. C is wrong because humidity deficit does not eliminate atmospheric pressure. D is wrong because evaporation is the conversion of water into vapour and therefore requires a liquid water source. The deficit provides demand, while energy provides the ability to meet that demand. Memory cue: dry air may invite evaporation, but latent heat pays for it.

Correct answer: A. Because a limited amount of water vapour is shared among many particles

Correct answer: A. Condensation nuclei are tiny particles on which water vapour can condense. If the amount of available moisture is limited but many nuclei are present, the moisture is shared among many particles, so each can acquire only a small amount of water and the droplets may remain small. This assumes other conditions are comparable; more nuclei do not make condensation impossible.

Correct answer: A. Because heat released during condensation keeps rising air relatively warm

Correct answer: A. Rising moist air expands and cools. Once it becomes saturated, water vapour condenses and releases latent heat. This slows the air parcel’s cooling, so it may remain warmer and less dense than the surrounding air and continue to rise. This can support greater instability, although the outcome also depends on the surrounding atmosphere.

Correct answer: A. It will generally decrease

Correct answer: A. Relative humidity is RH = (e/e_s) × 100, where e is actual vapour pressure and e_s is saturation vapour pressure. If e_s rises proportionally much faster than e, the ratio e/e_s becomes smaller, so RH generally decreases. For example, if e rises from 10 to 11 while e_s rises from 20 to 30 in the same pressure units, RH falls from 50% to about 37%.

Correct answer: A. Because the vapour-pressure gradient decreases

Correct answer: A. Evaporation is driven in part by the difference between the saturation vapour pressure at the water surface and the actual vapour pressure in the air. As evaporation makes nearby air more humid, its vapour pressure rises and this difference can shrink. The weaker gradient can slow evaporation, though wind, temperature, and other conditions also affect the rate.

Correct answer: A. Because slower evaporation of sweat reduces body cooling

Correct answer: A. Sweat cools the body as it evaporates, taking energy from the skin. Humid air already contains substantial water vapour, so sweat generally evaporates more slowly, especially when the air is still. This reduces evaporative cooling and can make warm conditions feel hotter. Humidity does not directly increase the body’s internal heat production.

Correct answer: A. Because vertical mixing is weak, and particles can act as condensation nuclei

Correct answer: A. In a temperature inversion, warmer air lies above cooler air near the ground, creating a stable layer that suppresses vertical mixing. Pollutants and moisture can then remain concentrated near the surface. Particles in the polluted air may provide condensation nuclei; if the air is sufficiently moist and cool, fog can form around them.

Correct answer: A. The air is warming, and its actual water-vapour content is likely increasing

Correct answer: A. Air temperature indicates how warm the air is. Dew point is the temperature to which air must cool, at unchanged moisture content, to become saturated. A rising dew point generally indicates more actual water vapour. Thus, rising temperature and dew point together suggest warming air with increasing moisture. They do not, by themselves, show that relative humidity is 100%; that depends on how close the air temperature is to the dew point.

Correct answer: A. Because water vapour is linked to radiation, clouds, and precipitation

Correct answer: A. Water vapour absorbs and emits some longwave radiation, influencing atmospheric heating and cooling. It can condense to form clouds, which affect incoming sunlight and outgoing radiation, and it is involved in precipitation. Evaporation absorbs energy and condensation releases latent heat, helping redistribute heat and moisture. These processes can therefore influence local climate.

Correct answer: A. In warm clouds, small droplets collide and form larger drops

Coalescence is the joining of small cloud droplets. In a warm cloud, droplets of different sizes move through the cloud, collide, and merge. Repeated collisions produce larger and heavier drops. When a drop becomes large enough for gravity to overcome the upward air currents and resistance, it can fall as rain. This is an important mechanism of warm-cloud or collision-coalescence rainfall.

Option A is correct because it describes small droplets colliding and forming larger drops in warm clouds. Dry air cannot create drops without water, and cloud droplets do not all instantly change into vapour. Earthquakes are not the normal cause of this process. The essential sequence is collision, merging, growth, and eventual fall as precipitation.

Correct answer: A. Actual vapour pressure has moved closer to saturation vapour pressure

The correct answer is A. Relative humidity is the actual vapour pressure divided by the saturation vapour pressure at the same temperature, multiplied by 100. Since the temperature is unchanged, the saturation vapour pressure is essentially unchanged. An increase from 40% to 80% therefore means that actual vapour pressure is a larger fraction of the saturation value and is closer to it.

Correct answer: A. Because water vapour absorbs longwave radiation and reduces surface heat loss

The correct answer is A. At night, the surface loses energy by emitting longwave radiation. Water vapour absorbs some of this outgoing radiation and re-emits energy in different directions, including toward the surface. This can reduce the surface’s net heat loss, so under otherwise similar conditions a moist night may have a relatively higher minimum temperature than a very dry night.