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At night, since the air is no longer being heated from below, convective mixing shuts off. Without mixing, the air in contact with the ground will cool off as the ground itself cools off. Since the air becomes more dense as it cools, it stays near the ground and keeps cooling off. The result is a shallow layer of very cold air at the ground, beneath a layer of warmer air that hasn't come in contact with the ground. This temperature inversion is called a nocturnal inversion.
This pattern can be disrupted if it's windy. The wind generates turbulence and produces mechanical mixing which carries some of the colder air aloft and brings some of the warmer air down to the surface. Thus, the cooling effect of the ground is felt over a deeper layer of the atmosphere, and no one level cools off as much. The really strong nocturnal inversions are found near the center of high pressure systems, where winds are especially light.
Usually, there's a sort of balance between two competing effects: the ground trying to cool off the lowest layer of the atmosphere, and the wind trying to mix the cooler air with some warmer air. Usually there's still a temperature inversion; it's just not as strong as it would be if there were no wind.
Alas, it's difficult to forecast things like this, because temperature and wind are interrelated. If an inversion forms, that helps reduce the wind at the surface. Usually the winds at night are weaker than the winds during the day, for this reason, and the wind will frequently become calm at night at the surface as the lowest layer of air cools. The stronger the inversion, the weaker the turbulence, and the more difficult it is for winds to penetrate all the way down to the surface.
Nighttime conditions also tend to be dependent on the topography and vegetation near the observing station. If the station is in a valley, the cold air that forms at night can pool within the valley and form a strong inversion with very cold surface air. On the other hand, if the station is near the top of a hill, the cold air flows down the hill away from the station and the station can potentially remain warmer. Also, stations in the middle of a forest can more easily go calm near the ground than stations in the middle of grasslands.
FORECASTING TECHNIQUE: How will the wind affect the nighttime temperature?
Unless a cold front or some other major weather feature is coming through, you should forecast minimum temperatures by estimating the amount of nighttime cooling and subtracting that number from the previous maximum temperature.
For example, suppose a given station cools 25 F under ideal conditions. If the daytime high was 75 F, the nighttime low would be expected to be 50 F.
Although I have given some rough rules for which stations are most susceptible to temperature inversions, every station is different, and the best way to predict how much the temperature will cool under given wind conditions is by using experience. Check back to a previous night with similar conditions. How much did it cool then? If you can't find a matching day, you can still sometimes get a good sense of what might happen from a station's recent weather history.
Building up some experience on how much a station cools at night is one of the main reasons for watching a station's weather one or two weeks before one has to forecast for it.
For some very vague guidelines, which I wouldn't even follow because they vary so much from station to station, try this:
Calm conditions: temperature will drop 20-30 F from the afternoon high to the overnight low.
Light to moderate winds: temperature will drop 15-20 F from the afternoon high to the overnight low.
Strong winds: temperature will drop 10-15 F from the afternoon high to the overnight low.
I repeat: these numbers vary a lot from station to station, and there are lots of other things which affect minimum temperature, such as soil type, soil moisture, and clouds (see module on clouds, moisture and temperature). But the wind speed often has the strongest effect, particularly on clear nights.
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