Carol Bronson, age 14, of Linden, Alberta, Canada, for her question:
What causes frost patterns on windows?
The frosty season is over and we are ready to welcome the different worlds of spring and summer. A few of us give a stray thought to the next snowy season of skating and winter sports. And a lot of us plan to track down that frosty window pane artist before the time comes to admire his handiwork again.
Even small toddlers notice the frosty patterns that form on the winter windows. They gaze with wonder at the ferny foliage and the frothy frills, the fox's tails and the filmy feathers. Their bright eyes see pictures etched in pale pearly tints, spangled with icy sparkles of frosty white. They see art work and their young minds wonder about the artist. Actually, the ornate patterns are the result of complicated atmospheric conditions, but a detailed explanation would bewilder the minds of these young art lovers. So we tell them that the icy pictures are painted by an invisible artist named Jack Frost.
This answer may satisfy the toddlers. But not for long. They were born in the Age of Science and they want the downright scientific answers to what goes on in the world. So we bid adieu to Jack Frost and proceed in search of the more satisfying truth. The truth concerns basic laws that govern the behavior of the airy atmosphere. Temperature and water vapor are two of the causal conditions.
The air can hold so much gaseous water vapor and no more, depending upon its temperature. This rule operates on a set ratio. A cubic meter of air at 43 degrees Fahrenheit can hold 7.27 grams of water vapor and no more. If the same air chills to freezing point, it is saturated with only 4.85 grams of vapor. Below saturation point, the air must change its surplus vapor into liquid water. And if the surrounding temperature is below 32 degrees, this excess moisture freezes in microscopic crystals of solid ice.
Icy artwork forms on the windows when the temperature zig zags around freezing point. The earth grows chillier through the night and the windows are coldest just before dawn. The slightly warmer air touches the glass, chills below its saturation point and deposits a layer of tiny crystals in an icy frosting. Then a draft of warmer air puffs on the windows. Some of the crusty crystals melt and tiny trickles of moisture wander through the frosting. Then a cold breeze arrives and freezes the moist trickles in groovy patterns. Several warm and cool spells may add to the art¬work. The network of trickling grooves grows deeper and the feathery ridges of solid white ice grow thicker.
Frosty patterns form on the outside of the window glass. In very cold weather, if the furnace is turned down, they may begin to form around midnight. The night may be freezing, but the outside surface of the house grows even colder than the air. It steals a few degrees from the air that touches it and the chilled air deposits tiny crystals on the windows. But the inside of the house is not uniformly cold. It pro¬vides a few warm drafts to melt the frosting into delicate patterns of trickling mois¬ture. When the warm draft inside is spent, the feathery patterns are frozen solid again by the cold outdoors.