How Frost is Formed

hoar frost

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° 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 sun 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. Frost is white because the crystals contain air.

Types of Frost

Did you know that some particular kinds of frost have their own names? When frost forms patterns on glass windows, it can resemble lacy plant leaves, so it’s sometimes called fern frost. For window frost to form, the glass must be exposed to freezing temperatures on the outside and humid air on the inside. With the invention of better-insulated double pane windows, window frost is not such a common sight as it was in the past.

Hoar frost is a particularly severe frost deposited when there is a good supply of water vapor that forms into extra large ice crystals. It takes its name from the Old English “har,” (derived from the same root as the German honorific “Herr”) meaning old, venerable, or showing signs of old age. When covered with hoar frost, bushes and whole trees take on a particularly grizzled appearance. On a cold winter morning you have a good chance of seeing hoar frost on exposed plants near an unfrozen lake or stream, according to Kenneth G. Libbrecht, a professor of physics at the California Institute of Technology who specializes in the physics of crystal growth. The water source supplies large amounts of water vapor, capable of condensing into impressively large crystals overnight. The feathery structure of those loose ice crystals, called  hoar frost, reflects light from all their surfaces and appear so white.

Another distinctive form of frost is called rime, which is made of small water droplets carried in mist or fog. When the wind blows these droplets onto objects that are below 32° Fahrenheit, they freeze into ice crystals. Rime looks more like solid ice than hoar frost, which is feathery. Rime frost looks like icing around the edges of petals or leaves and only occur when the temperatures are very low.  Rime contains air pockets that causes the frost to be white in contrast to a glazed frost which is clear, solid ice with little or no trapped air.

So the next time you see those beautiful frosty trees, you will know how that frost was formed.

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