Susan Martin, Age 11, Of Florence, Ore., For Her Question:
What makes the fog come and go?
The weather is created in the lowest level of the atmosphere, in the turbulent air that sits upon continents and oceans around the globe. Mists and hazy fogs can appear and disappear almost anywhere. But they are most common along shore lines, where the lands meet the seas.
A fog is the daughter of the sun and air that hugs the surface of our weathery planet. The filmy veil is made of the same kind of misty moisture that forms the high flying and the low flying clouds. For a fog is no more than a cloud sitting on the ground or on the surface of the sea. The material is liquid water in the form of microscopic droplets, small enough to float in the air. It takes a million of these droplets to form a medium size drop of rain, and in a fog large enough to blanket a city block there is not enough liquid water to fill a teacup.
The lower atmosphere drifts in breezes and blows in gusty winds around the face of the earth. And this air is always thirsty. The warmer it is, the more water it can drink up from the seas and other moist places around the world. The beaming sun warms the land and sea, and they warm the air above them. The warm air evaporates the liquid water and moisture on the earth and changes it to vapor. This water vapor is a gas that floats up and mingles with the other invisible gases of the air.
there are strict rules about the amount of water vapor in the air. A cubic meter of air at 86 degrees fahrenheit can hold 30.4 grams of vapor and no more. At this point it is saturated with humidity. The same amount of air at 68 degrees is saturated to the limit with only 17.31 grams of vapor. When warm saturated air is chilled, it is left with a surplus of vapor. This surplus gas is changed back into liquid moisture.
When warm, vapor loaded air chills high above the ground, its surplus moisture changes into clouds. When warm, vapor loaded air is chilled near the ground, its surplus vapor may form a misty fog. This often happens in coastal regions where warm, wet sea winds clash with colder land breezes. The early mornings are pearly gray with filmy fogs.
As the smiling sun climbs higher in the sky, the earth and the air above it become warmer. And, remember, warm air can hold more vapor. It changes the cloudy droplets back into gassy water vapor and the morning mist disappears. Fog forms when warm wet air becomes cool, and it disappears as the cooled air becomes warm again.
Misty fogs tend to appear and disappear in valleys and on lofty mountain s1opes. The thirsty air is warmed by the daytime sun and chilled during the night, and these mists of the mountains and valleys often burn off by noon. The thickest fogs tend to form at sea where warm and cool air masses meet and clash above merging warm and cool ocean currents. The sunbeams find it hard to penetrate through to warm the surface of the sea, and these stubborn fogs sit for days like dense gray blankets upon the chilly ocean.