November 21, 2011
Whenever the air cools, the water vapor condenses, transforming back into droplets of water. Depending on weather conditions, these droplets may then become mist, fog, rain or snow. The water that condenses close to the ground becomes droplets of dew.
Water vapor condenses into dew when the plant leaves or grass blades become colder than the air around them. The temperature at which this occurs, called the dew point, is variable, depending on the air’s humidity. Once the dew point dips below 32 degrees Fahrenheit, water vapor near the ground will become frost rather than dew.
Just like snow, frost is made up of tiny, often microscopic, ice crystals. Frost crystals are usually six-sided and frequently take on needle-like shapes just as snow crystals do. The difference between the two is that snow crystals form in the clouds, while frost crystals form on solid surfaces near the ground.
Weather Conditions And Frost
When conditions are right, frost forms overnight because once the suns goes down, the air temperature quickly drops. A cold, clear and windless night provides ideal conditions for frost formation. In contrast, a night sky containing low clouds essentially creates a blanket over the earth. This keeps warm air on the surface, and thus makes frost formation less likely.
The personification of frost formation is a mischievous elf-like creature named Jack Frost, who visits on crisp, cold nights, leaving behind fanciful designs on window panes and a coating of white on the ground. He's also sometimes depicted with a paintbrush, giving leaves their rich autumn colors. This character may have originated in Norse folklore as Jokul (icicle) Frosti (frost). Russian fairy tales attribute cold and frosty weather to an old man named Father Frost. In German folklore, frost is caused by an old woman who shakes white feathers from her bed.