Humidity, evaporation, and condensation

Correct answer: A. Most atmospheric water vapour enters the atmosphere at or near Earth’s surface through evaporation from oceans, lakes, rivers, soil, and wet vegetation, together with plant transpiration. As air rises, temperature generally decreases through the lower atmosphere. Colder air has a lower capacity to hold water vapour, so some vapour condenses into cloud droplets or ice crystals and may later return as precipitation. In addition, the main moisture sources are concentrated near the surface, so less vapour is normally found at greater heights. B is wrong because oceans are not more common aloft. C reverses the distribution of moisture sources. D is scientifically false; atmospheric water vapour mainly comes from surface water and biological processes, not from outer space. Memory cue: near-surface sources plus colder upper air explain the vertical moisture decrease.

Correct answer: A. Fog consists of tiny water droplets suspended near the ground. After sunrise, heating of the surface can warm the nearby air. At a given amount of water vapour, warmer air has a higher saturation vapour pressure, so its relative humidity falls and it moves away from saturation. The droplets can then evaporate, thinning the fog and improving visibility. This depends on local conditions; warming does not always clear fog quickly.

Correct answer: A, 20. Relative humidity is defined as RH = (actual vapour pressure / saturation vapour pressure) × 100. Let actual vapour pressure e = 15 and RH = 75%. Rearranging gives saturation vapour pressure, e_s = (e × 100) / RH = (15 × 100) / 75 = 1500/75 = 20. Thus the air currently contains vapour pressure equal to 75% of the saturation value. B simply repeats the actual pressure and ignores the formula. C is the percentage, not the requested pressure. D results from an incorrect operation. Memory cue: to find the saturation value, multiply actual pressure by 100 and divide by RH.

Correct answer: A. During evaporation, liquid water becomes water vapour and moves from a surface into the air, so matter is transferred. The molecules that escape take in latent heat, which is transported with the water vapour and can be released later during condensation. Thus, evaporation transfers both water and energy.

Correct answer: A. Relative humidity is the actual water-vapour pressure divided by the saturation vapour pressure at the same temperature, expressed as a percentage. Since saturation vapour pressure changes with temperature, the same relative humidity can correspond to different actual amounts of water vapour in warm and cool air. Measures such as dew point or specific humidity can provide additional information about moisture.

Direct answer: B is correct. Clear skies and calm conditions can help a surface lose heat at night, but cooling alone does not guarantee dew. Dew forms only when air close to the surface contains enough water vapour and becomes saturated as the surface cools to its dew point. Very dry air contains little moisture, so even if the surface cools, there may not be enough vapour to produce visible liquid droplets. A is wrong because surfaces can cool on clear nights. C is wrong because calm air usually reduces mixing rather than automatically removing moisture. D is wrong because a dew point exists whenever air has water vapour. Memory cue: dew needs cooling plus moisture.

Direct answer: A is correct. The dew point is the temperature at which air becomes saturated with its existing water vapour, assuming the pressure and moisture content are considered in the usual way. As air cools to this temperature, relative humidity reaches about 100 percent, and excess vapour can condense on tiny particles into microscopic liquid droplets. When these droplets remain suspended close to the ground, fog forms. A correctly describes this process. B is wrong because air becomes saturated, not completely dry. C is impossible because vapour does not turn into stone. D is wrong because air pressure does not disappear at the dew point. Memory cue: reach dew point, reach saturation, then droplets can appear.

Direct answer: A is correct. Mist consists of tiny water droplets suspended in air near the ground. As the Sun rises, the land and nearby air warm. Warmer air can hold more water vapour, and the droplets may evaporate back into the air. Daytime heating can also promote mixing, which disperses the moist layer. If the air remains sufficiently humid or cooling continues, mist may persist, so clearing is common but not guaranteed. A correctly identifies evaporation caused by warming. B, C, and D are wrong: mist does not become stone, air does not disappear, and water vapour does not become lava. Memory cue: morning warming often turns suspended droplets back into vapour.

Correct answer: A. Moisture is necessary for clouds, but air usually also needs to cool to its dew point or become saturated before water vapour condenses into droplets or ice crystals. Without suitable cooling, the vapour may remain invisible in the air, so cloud formation is hindered. Moisture alone does not guarantee rain or hail.

Correct answer: A, condensation into tiny droplets or ice crystals. Cooling reduces the amount of water vapour that air can hold. When air reaches saturation, further cooling or the presence of suitable particles allows water vapour to change into liquid droplets or ice crystals. These small particles collect around condensation nuclei and together form a cloud. B is wrong because air does not become rock during cloud formation. C is wrong because saturation does not destroy moisture; it changes some vapour into liquid or solid particles. D is unrelated because the sea does not need to dry for a cloud to form. Memory cue: cooling air reaches saturation, then condensation builds cloud particles.

Correct answer: A. Dew is a form of surface condensation. During suitable conditions, a surface cools to or below the dew point, and nearby water vapour condenses directly into small liquid drops on grass, leaves, or other objects. Rain is precipitation: cloud droplets or ice particles grow within a cloud and then fall through the atmosphere to the ground. Thus both contain liquid water, but their place and process of formation differ. B is wrong because dew forms on surfaces rather than falling from clouds. C is wrong because rain develops in clouds, although it may change while falling. D is wrong because the two processes are not identical: dew is deposited locally at a surface, while rain is falling cloud precipitation. Memory cue: “Dew stays on the surface; rain falls from the sky.”

Correct answer: D. Tiny particles such as suitable dust, salt, or smoke can act as condensation nuclei. Water vapour can attach to these microscopic surfaces, and under suitable cooling and humidity conditions, the attached water can grow into cloud droplets. Thus A, B, and C describe helpful or scientifically reasonable roles: particles provide surfaces and support condensation. D is the wrong understanding because dust does not always destroy water vapour. Its role is not automatically harmful; it can help water vapour change into liquid droplets. The word “always” is an important warning in the option. Dust may affect cloud properties in different ways depending on its composition, size, and atmospheric conditions, but it is not correct to say that it always destroys vapour. Memory cue: condensation nuclei are “seeds” for droplets, not enemies that remove all vapour.

Correct answer: A. A low cloud base often indicates that rising near-surface air needs only a small amount of cooling to reach saturation. This is commonly associated with relatively humid air and a small temperature–dew-point difference near the ground. Cloud-base height also depends on temperature and how the air is lifted, so this is a likely association, not an absolute rule.

Correct answer: A. Descending air is compressed by higher pressure and warms. Although warmer air can hold more water vapour before becoming saturated, its actual water-vapour content does not automatically increase. Its relative humidity therefore tends to fall, making condensation and cloud formation less likely and reducing the chance of rain.

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.