Barbara Grimmer, age 11, of Spokane, Washington, for her question:
How does frost form?
This is the season when the frost etches dainty pictures of ferny foliage on the window panes. Sometimes it adds crisp white icing to the grasses or sparkles on the shingles of roofs. In regions where winter brings long spells of frosty weather, day and night, the garden soil is frozen too hard for hoeing. All these weathery wonders require coolness and moisture but the formation of frost is not quite as simple as one would expect.
Frost cannot form unless the temperature drops as low as 32 degrees Fahrenheit. This is the freezing point of water, when the runny liquid may change to solid ice. However, sometimes moisture remains in its liquid form until super cooled air is several degrees colder than the normal freezing, point of water. Many other weather conditions cause variations in the basic frost forming recipe.
When the weatherman predicts frost, he means that the outdoor temperature is expected to drop at least to the freezing point of water. This is sure to convert all the exposed surface water to solid ice. Fragile leaves become frostbitten because the liquid in their tissues becomes tiny ice crystals. These mini daggers tear their tender cell walls. Sometimes a killing frost of this sort wipes out citris, lettuce and other crops where winters are expected to be mild. In colder regions, long spells of below freezing weather turn the ponds into skating rinks and freeze the trapped moisture in the ground.
All these types of frost occur rather simply, when freezing temperatures change exposed water to solid ice. The showier types of frost are formed from moisture in the air and these operations are far more complex. The atmosphere always contains a quota of moisture. It may be in the form of gaseous vapor or minuscule droplets of liquid. This air borne moisture uses several recipes to convert itself into feathery hoarfrost, icy pictures on window panes and various other frosty decorations.
The limit to the vapor in the atmosphere depends on a sliding temperature scale. A cubic meter of air at 50 degrees Fahrenheit can hold up to 9.41 grams of gaseous water. If it chills to 32 degrees, it has 4.85 grams more than its quota. This surplus may be converted into liquid droplets of freezing fog. When this chilly mist touches objects near the ground, it encrusts them with feathery white rime frost.
However, sometimes a temperature drop changes surplus vapor directly into tiny ice crystals. They encrust the winter scenery with feathery white hoarfrost.
As the temperature drops, surfaces near the ground cool fast and chill the air that touches them. Then frosty films of ice crystals may form on cold window panes. But warm drafts keep melting the ice and miniature rivers run through the frosting. Cool drafts keep re freezing the runny liquid. And by morning, the window panes are adorned with frosty pictures of ferry foliages.