Does the fog descend or rise from the ground?

Does the fog descend or rise from the ground?

     This morning at sunrise., Andy beheld a filmy fog, hovering low above the ground. It was a lovely sight. But in a short while the sunbeams pierced it like poking fingers and the soft gauzy veil disappeared. Just by looking at it, there was no way to tell whether it seeped up from the earth or descended through the air. Only scientists can give us the answer    and their answer is a firm "Neither."

Obviously a soft filmy fog comes from somewhere. It's logical to suppose it arrives from the ground below or the air above. Since scientists insist that neither of these answers is correct, then we had better prepare ourselves for a mighty com¬plicated scientific answer. As a matter of fact, the formation of fogs has a complicated explanation involving temperature and what it does to air loaded with gaseous water vapor. The same basic routine takes place in the formation of clouds, high above the ground.

Actually a fog is a cloud, sitting comfortably on the ground. The inside of a fog is just like the inside of a very gentle kind of cloud. Both are made of the same material    miniature droplets of water suspended in the gaseous air. Originally this moisture was evaporated from the oceans, lakes and rivers. Evaporation caused the water molecules to separate and become vapor that mingled with the other gases of . the air. Warm air is thirsty, so the water vapor tends to spread out as it rises aloft. Temperatures up there are cooler. This forces the gaseous vapor to become droplets of misty moisture and create a billowy cloud.

This happens because warm, thristy air is able to contain a lot of extra water vapor. As the air chills, its peppy molecules of gas slow down and crowd closer together. This causes the gaseous vapor to change back into their liquid form. However, the separate molecules are too far apart to form rivers of running water. The best they can do is to form misty droplets of moisture, small enough to hang suspended in the air.

Fogs form when vapor ladened air near the surface is chilled by a drop in the temperature. Andy's mornin.  fog formed because the warm west wind brought in a load of invisible vapor, evaporated from the Pacific Ocean. The ground cooled off fast during the night. As the air chilled, some of the invisible gaseous vapor was forced to form a visible fog of filmy moisture. It did not rise from the ground or descend from above    but formed right there in a surface layer of warm air that turned cool.

Fogs also form when warm, vapor filled air blows up the cool slopes of a mountain. Others form along shores where warm and cool currents bring masses of warm and cool air into collision, or when mild moist ocean breezes collide with cold air over the land.

The rules that govern the formation of cloudy fogs are very precise. At a certain temperature, air can contain so much vapor    and no more. This is its saturation point. If this air turns a few degrees cooler, it has a surplus of gaseous vapor that must be changed back into liquid water droplets. And the change takes place right there in the air where the fog is born.

 

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