Linda McBroom, age 11, of Nashville, Tenn., for Her question:

Does frost come up from the ground or down from the sky?

The crisp white frost is first cousin to the pearly dew

 and neither of them rises out of the ground or falls down from the sky. Both dew and frost are formed directly from the air when temperature and moisture conditions are dust right.

Many of our weather conditions occur because warm air can hold more water vapor then cool air. Vapor is the gaseous form of water and, like the other gases in the air, it is invisible. Nevertheless, it is moisture and can readily be turned back into drops of liquid water, such as the dew, or fragments of solid ice, such as the frost. always thirsty and ready to drink up water by evaporating it into gassy vapor. But there are very strict rules about how much of this vapor the air can hold. At a temperature of 86 degrees Fahrenheit, one cubic meter of air can hold a little more than 30 grams of water. At 50 degrees, a cubic meter of air can hold less than ten grams of vapor. At 32 degrees, which is the freezing point of water, a cubic meter of air can hold only 4.85 grams of vapor. The cooler the air, the less vapor it can hold.

Vapor in the air is called humidity and when air of any temperature has its full quota of vapor, the humidity is said to be 100 per cent. Now suppose that the temperature drops when the air already has 100 per cent humidity which is all the vapor it can hold. The surplus vapor must be unloaded, It may condense into tiny droplets and become clouds, or into pearly drops of dew. Under certain conditions this surplus vapor freezes into fine fragments of ice.

The crisp white frost which dresses the twigs in fuzzy overcoats and paints fairy foliage on the windowpanes is formed from countless fragments of frozen vapor,

For sometimes when the temperature is below freezing point, the tiny particles of vapor become solid ice before first condensing into droplets of water.

Before sunrise on a chilly morning, the air is slightly warmer than the ground, the trees and other solid surfaces. The surplus vapor in the air is already cold enough to turn into ice but it needs to touch a colder solid surface before it does so. The air which touches the twigs and the ground deposits countless fine fragments of icy crystals and bit by bit they build up into a frothy white frosting and the sun rises on a white wonderland.

Frost on the window panes is formed in the same way. But because of the warmer air inside the house it tends to melt. Then it freezes again and the runs and rivulets are set in designs of fairy foliage

 Jack Frost, we say, has been painting the window panes.