Which effect does a subsidence inversion have on the boundary layer?
Answer: A
The inversion acts as a lid, preventing thermals from penetrating and confining moisture and pollutants to the layer below. This is why stratocumulus forms so readily beneath it and why haze accumulates. Descending air warms rather than cools by adiabatic compression, which is precisely what makes the inversion.
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Which cloud type forms beneath a subsidence inversion over the ocean?
Answer: B
The cool moist marine boundary layer capped by a subsidence inversion produces the extensive stratocumulus decks of the eastern subtropical oceans, which are among the most important cloud features in the global radiation budget. Their brightness and extent make small changes in their coverage a large climate feedback. Deep convection is precisely what the inversion prevents. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.
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Which region shows the most extensive marine stratocumulus?
Answer: C
Cold upwelling water beneath strong subsidence off California, Peru, Namibia and Australia produces the most persistent stratocumulus decks on Earth. Their high albedo over a dark ocean makes them a large negative radiative influence. The warm pool supports deep convection instead, and the storm tracks produce transient frontal cloud rather than persistent decks. Most errors here come from carrying a result across from a neighbouring case where it does not hold.
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Which process warms air descending the lee slope of a mountain?
Answer: D
Descent through increasing pressure compresses the air and raises its temperature at the dry adiabatic rate once the cloud has evaporated, which is why foehn winds are so warm and dry. Having lost moisture on the windward side, the air descends along a dry adiabat rather than the saturated one it ascended. That asymmetry is the whole mechanism. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.
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Which asymmetry produces the foehn warming?
Answer: A
Air rising and condensing cools at about 5 kelvin per kilometre, but once the moisture has fallen out as precipitation the descending air warms at 9.8, so it arrives warmer and much drier. The temperature difference across a 3 kilometre barrier can exceed 10 kelvin. The precipitation on the windward side is essential to the mechanism.
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Which names are given to the foehn in different regions?
Answer: B
The chinook east of the Rockies and the zonda east of the Andes are the same phenomenon as the Alpine foehn. The mistral and bora are cold downslope winds in which the air was already cold before descending, so the adiabatic warming is insufficient to make them warm. Distinguishing warm from cold downslope winds is worth doing carefully.
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Which feature distinguishes the bora from the foehn?
Answer: C
In a bora, cold continental air spills over a coastal mountain barrier and, although it warms adiabatically as it descends, it arrives still colder than the air it replaces. The foehn by contrast arrives warmer. The distinction lies in the initial temperature of the source air rather than in any difference of mechanism.
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Which effect does the foehn have on humidity?
Answer: D
Having lost much of its water as precipitation on the windward side, and warming at the dry adiabatic rate on descent, the air arrives with a much reduced relative humidity. Falls from 80 per cent to 20 per cent within an hour are recorded. The dryness is as characteristic of the foehn as the warmth and contributes to the wildfire risk associated with it. Most errors here come from carrying a result across from a neighbouring case where it does not hold.
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Which hazard is associated with foehn conditions?
Answer: A
Warm dry wind melts snow rapidly, contributing to spring flooding, and dries vegetation to the point where fires spread readily, as in several Alpine and Colorado events. The chinook is known locally as the snow eater for exactly this reason. Fog and icing require moisture, which the foehn has largely removed.
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Which quantity is the lifting condensation level?
Answer: B
Lifting a parcel dry adiabatically raises its temperature towards its dew point until the two meet, and that level is the lifting condensation level, corresponding closely to convective cloud base. The level of free convection lies above it wherever a stable layer must be penetrated. Distinguishing the two is essential when assessing convective potential. Checking the units of each quantity catches a wrong formula before the arithmetic begins.
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