Adiabatic Processes and Lapse Rates Questions and Answers

Answer: C

Because the saturation vapour pressure curve is convex, a mixture of two saturated samples at different temperatures is supersaturated and condensation occurs. This is why the breath condenses on a cold day and why steam fog forms over warm water in cold air. Radiation, advection and upslope fog all form by cooling a single air mass rather than by mixing two, so the mechanism is quite different even though the result looks similar. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: D

Cold air flowing over much warmer water is rapidly moistened and mixed, and because the saturation curve is convex the mixture becomes supersaturated and fog forms. Arctic sea smoke is the extreme case. Advection fog forms by the opposite arrangement, warm air over a cold surface, which is a useful contrast to draw explicitly.

Answer: A

Advection fog forms where air is cooled from below to its dew point, most commonly where warm maritime air crosses a cold ocean current or a snow covered surface. The Grand Banks and the California coast are classic locations. Unlike radiation fog it can persist in moderate winds and can form at any time of day.

Answer: B

The cooling is supplied continuously by the surface as fresh air is advected over it, so moderate wind sustains rather than destroys the fog. Radiation fog by contrast requires calm conditions because wind mixes it away. The distinction explains why sea fog can persist for days while land fog often clears within hours.

Answer: C

The ground must cool efficiently, which requires clear skies, and the air must be nearly calm so that only a shallow layer is cooled, yet moist enough to reach saturation. Very light wind of one or two metres per second actually helps by mixing the cooling through a sufficient depth. Cloud cover prevents the radiative cooling entirely.

Answer: D

Perfectly calm conditions produce only dew or a very shallow ground fog, while a light wind of one or two metres per second distributes the cooling through several tens of metres and produces a deeper fog. Stronger winds mix in warm dry air and disperse it. The optimum is narrow, which is part of why fog forecasting is so difficult.

Answer: A

Sunlight penetrates the fog and heats the ground, which then warms the air from below, mixing the fog upward until it evaporates. Dense fog reflects much of the sunlight and so persists longer, which is why some winter fogs survive the whole day. Droplet settling is far too slow to clear a fog layer, and drier air arriving from elsewhere would be a change of air mass rather than a clearance of the existing one. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Answer: B

A dense fog reflects much of the incident sunlight, and in winter the available energy is small in any case, so anticyclonic gloom can persist for days. The feedback between fog density and solar penetration makes the situation bistable and hard to forecast. Summer fogs almost always clear within a few hours of sunrise.

Answer: C

By international convention the obscuration is called fog when the horizontal visibility falls below one kilometre and mist when it lies between one and five kilometres. Both consist of water droplets, whereas haze consists of dry particles and is reported separately. Aviation practice adds further categories for very low visibility, since the operational consequences differ sharply below two hundred metres. Knowing the thresholds matters because the reported category rather than the measured value drives many procedures.

Answer: D

Mist is a suspension of water droplets at high relative humidity, while haze is dry aerosol and occurs at humidities well below saturation. Haze often has a brownish tint from its composition. The distinction matters for forecasting because haze requires a source of particles and a stable layer while mist requires only sufficient cooling.