Previously, Cecil had mentioned that a cloud that comes down to ground level is fog. Fine.
But what if ground level is 2500 feet above sea level, is it fog or cloud?
My wife and I live in the Sierra Nevada foothills at 2500 feet of elevation and work at sea level. Frequently, when driving home we will enter a fog bank (or cloud) and then, driving further we leave it behind. Did we drive through a low cloud or was it fog?
My wife says it is a cloud because of the elevation, I insist it is fog if it is at ground level, regardless of where ground level is: sea level, 2500 feet or Denver.
It really doesn’t matter. Clouds and fog are the same thing. It is just called fog when it meets the ground, and it doesn’t matter what ground elevation you are talking about. Fog is a low cloud.
<hijack>
why are some places so prone to fog? I’m not talking about cities like London or anything, but there’s a cemetary next to the local high school that is shrouded in fog every fall/winter morning. It makes it kinda creepy, and I always wondered why the kids walking to school walked through that cemetary. Seems very possible to trip over low headstones because of the poor visibility. Not to mention that you’d be hard-pressed to find me walking through a cemetary in any circumstances (I’m a bit superstitious). But I’ve wondered why that place had fog nearly every morning, when fog in my city isn’t exactly commonplace.
</hijack>
There are several ways fog can form. But it comes down to one of two things: you either have to add moisture to the air, or cool the air to its dewpoint.
This could happen by forcing warm, humid air across a cool surface, moving cool air across a warm, wet surface, radiational cooling of wet ground, forcing moist air upward and providing adiabatic cooling, etc.
Some locations are going to be more prone to fog, just by being wet more often, or whatever. Just depends on the specific location.
You are likely to have fog if the temperature is within 4°F of the dewpoint. Basically, warm air hold more moisture than cool air. If the air mass is saturated, then there is a point at which it “can’t hold any more water” (the dewpoint). When you lower the temperature to or near the dewpoint, the water condences into fog or cloud.
Bodies of water tend to retain heat, and cool down at a slower rate than the land. If you have a light onshore breeze (light winds are best for fog) that moves the air mass from over the relatively warm water to over the relatively cool land, fog can occur.
One reason that a cemetary might have fog when the surrounding area doesn’t, is that cemetaries tend to be watered. Air over a cemetary is likely to contain more water than air outside of it.
Another possible difference between clouds and fog: clouds (the ones in the sky) appear when moist air rises hundreds of feet and its temperature falls. If you find some mist on the ground, chances are that it formed by some other means than by the air being cooled by rising hundreds of feet.
Of course if you’re on top of an actual mountain, genuine clouds formed elsewhere may collide with the mountain and touch the “ground” there.