Imagine that you live in an area where the winters are very cold. Then one morning you look out and see that last night’s fog has left, but the pine trees are now covered with a furry whiteness. Tree branches, clotheslines, icicles, weeds, and birdhouses are also covered with white. The whiteness responsible for this winter wonderland is rime frost.
This beautiful sight didn’t just happen overnight. It began with the clear sunny weather the day before. The sun’s rays caused the earth’s surface to warm. But after the sun set, the ground began to lose that warmth. Without clouds to hold the heat down, the warm air escaped quickly to the upper air levels. The air near the ground cooled until the water vapor in it condensed into small drops of water. These droplets became fog—a cloud that rests above the ground. The droplets of water in the fog or clouds and temperatures that were below freezing combined to form rime frost.
Scientists call the droplets of water in the air supercooled droplets. These are carried by moving air against objects that are very cold such as trees, rocks, weeds, powerlines, or any upright stationary object. The tiny droplets instantly freeze into ice crystals and stick to the edges of anything solid they touch. Then they build on one another in layers. Since the wind moves the droplets to the cooled objects, the layers of ice crystals always form on the side toward the wind.
The most notable rime frost forms on mountaintops that are covered with supercooled clouds. The frost there sometimes becomes so thick and heavy that large pine trees break under the load. Other times the droplets freeze into ice instead of ice crystals. If seen under a microscope, the frozen droplets would look like groups of white pearls. When they form on the wings and body of an airplane, pilots call the phenomenon icing.
How is rime frost different from the frost that forms on the ground and rooftops in fall and early spring? Other kinds of frost have formed when water vapor, a gas which can’t be seen, is changed into ice crystals at freezing temperatures. Rime frost is formed at below freezing temperatures from water droplets in a form that can be seen as fog or mist that is carried by the wind.
Rime frost is sometimes called white frost or hoarfrost. Since it is a part of winter that we don’t see every day, rime is an especially beautiful winter sight.