Janne Patterson, age 12; of Jackson, , Miss,,, for her question:
What causes frost on window panes?
A clear, calm frosty morning is a sure sign that winter has arrived. W© wake up tingling all over and full of pep. The sky is blue, the sun is bright and somebody has been very busy during the chilly night: Jack Frost has painted the window panes with dainty pictures of feathers and foliage ‑ or so it seems.
Actually we know that‑ Jack Frost is merely an imaginary person. The dainty frost on the window panes is caused by certain factors in the weather. The temperature must be just right, as must be the wind and the moisture in the air. Then, while we sleep, frost will form on the windows in delicate designs to delight us when we awake.
The air must contain moisture in the form of water vapor. This vapor is a gas, like other gases in the; air. We cannot seep smell or feel it and it is made of water particles too small for us even to imagine. It must be present in the air before frosty art work can be painted on the windows.
There are rules about how much vapor the air can hold. Warm air can hold a lot of vapor, cool air holds loss. When warm air loaded with vapor becomes cool it must give up some of its vapor quota. The frost on the window comes from this surplus quota.
In order to form frost on the windows, the air outdoors must be below the freezing point of water, which is 32 degrees Fahrenheit. The outside of the window pane, then, is at least as cold as ice.
Pressing against this cold, smooth surface is the air. And the cold air is all reedy to unload countless tiny particles of water vapor. As these particles touch the cold glass, they become tiny, tiny fragments of ice.
The sir must be calm or the wind would not glow the tiny ice crystals to settle on the window panes. When alt these conditions are present in the wea.thor we can expect a display of delis^tc: white art work. The air must be laden with moisture. There must be no wind. The temperature outdoors must be freezing or lower.
The first layers of frosty icing on the glass must melt a little. The icing oozes or flows in mushy little rivers ‑ but not for long. The icecold air soon freezes it solid again. This window pang is now covered with icing, frozen into feathery patterns The air presses against it, adding still more icy fragments. The design grows. The edges become still more feathery. The leafy designs seem to grow scalloped edges and delicate wins. By morning the frost on the windows is a wonderful work of Mother Nature's art.