Does the wind direction affect the presence or absence of clouds? Why?
1 Answer
Only if there is a slope.
Explanation:
Generally speaking, clouds form when the air becomes saturated with water vapor, in other words the air is holding all the water vapor it can. The amount of water vapor in the air is referred to as the humidity. The amount of water vapor in the air as a percentage of the maximum amount of water vapor that the air can hold is called the relative humidity. When the relative humidity reaches 100% you get cloud.
There are 2 ways to get the relative humidity to reach 100%. The first is by simply adding more water vapor. Wind direction has no affect on this.
The second way that air can reach 100% relative humidity is by cooling the air, since the warmer the air is the more humidity it can hold. The wind can have an affect on this indirectly.
Contrary to what some believe, the wind does not lower the temperature. If the wind is bringing new cooler air to a location the original air is not cooled (well not cooled much) it is merely pushed away by the cooler air.
However, in the situation where the air is being blown uphill the change in elevation will lower the temperature. As air rises the temperature has to drop. This is due to the fact that as you go up in the atmosphere there is less atmosphere above you and therefore the pressure drops. According to Gay-Lussac's law, as the pressure drops the temperature also drops.
So as wind moves air up a slope, and the temperature cools the air 100 % relative humidity can be reach and cloud can form.
The reverse is true of wind blowing air down a slope. In that case the air is heated and the humidity drops, causing clouds to dissipate.
