Adiabatic Processes and Lapse Rates Questions and Answers

Q: Which term describes the mixing of two saturated air masses?

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.

Q: Which fog forms over warm water in very cold air?

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.

Q: Which fog forms as warm moist air crosses a cold surface?

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.

Q: Which condition allows advection fog to persist in wind?

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.

Q: Which condition is required for radiation fog to form?

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.

Q: Which effect does a very light wind have on radiation fog?

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.

Q: Which process clears radiation fog in the morning?

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.

Q: Which situation makes fog particularly persistent?

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.

Q: Which visibility threshold defines fog rather than mist?

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.

Q: Which distinction separates mist from haze?

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.

91.

Which term describes the mixing of two saturated air masses?

A.Advection fog
B.Upslope fog, which forms where air is forced to ascend a gentle slope and cools adiabatically to saturation
C.Mixing fog can result from combining two saturated samples
D.Radiation fog, which forms instead when the ground cools the air immediately above it below its dew point

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.

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92.

Which fog forms over warm water in very cold air?

A.Advection fog, which forms instead where warm moist air moves across a surface that is colder than its dew point
B.Radiation fog
C.Frontal fog, which forms where precipitation falling from warm air aloft saturates the colder air beneath it
D.Steam fog

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.

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93.

Which fog forms as warm moist air crosses a cold surface?

A.Advection fog
B.Steam fog, which instead forms where very cold air moves across water that is considerably warmer than it is
C.Radiation fog
D.Valley fog, which forms where cold air drains into a hollow overnight and cools below its dew point there

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.

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94.

Which condition allows advection fog to persist in wind?

A.The wind prevents the fog droplets from settling out, which would otherwise clear the layer within an hour
B.Cooling from below continues as long as the air keeps moving over the cold surface
C.The wind mixes the fog upward so that it becomes deeper and therefore harder for the sun to burn off later
D.There is no such condition

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.

Discuss in Forum

95.

Which condition is required for radiation fog to form?

A.Warm dry air
B.A strong temperature inversion already present at the surface before the sun sets in the evening beforehand
C.Clear skies light winds and moist air
D.Overcast skies with strong winds, which allow the surface to cool rapidly by radiating to the cloud above

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.

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96.

Which effect does a very light wind have on radiation fog?

A.It prevents the fog from forming at all by mixing warmer air down from above the shallow cooled layer
B.It has no effect
C.It causes the fog to lift into a layer of low stratus cloud within an hour of its initial formation
D.It deepens the fog by mixing the cooling through a greater depth

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.

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97.

Which process clears radiation fog in the morning?

A.Solar heating of the ground beneath the fog
B.The advection of drier air from the surrounding region, which reduces the humidity below saturation again
C.Increased radiative cooling
D.The settling of the fog droplets under gravity once they have grown large enough to fall out of the layer

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.

Discuss in Forum

98.

Which situation makes fog particularly persistent?

A.A fog forming over a warm surface, from which heat is continuously supplied to the layer of air above it
B.A dense fog in winter with a low sun angle
C.A shallow fog in summer, when the strong solar heating is able to penetrate to the ground within minutes
D.A fog in strong wind

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.

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99.

Which visibility threshold defines fog rather than mist?

A.One hundred metres
B.Five kilometres, which is the threshold used in aviation reporting to distinguish haze from clearer conditions
C.One kilometre
D.Ten kilometres, below which the visibility is described as reduced but the obscuration is not yet called fog

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.

Discuss in Forum

100.

Which distinction separates mist from haze?

A.Mist occurs at night and haze during the day, the two being otherwise identical in their physical composition
B.Haze is denser
C.Mist is reported only over the sea and haze only over land, according to the surface beneath the observer
D.Mist consists of water droplets and haze of dry particles

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.

Discuss in Forum

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