It’s fall and temperatures are dropping, which means frost is descending on much of the U.S. and Canada. Driveways are slippery, windshields need to be scraped, trees and grass will appear ghostly grey, and any garden plants that haven’t been harvested may be lost. There are many different types of frost—here’s a list of the types you might see on these chilly mornings and what they look like.
What Causes Frost?
Frost forms when the temperatures on surfaces, such as cars and trees, drop below freezing, causing water vapor to form into ice crystals. This happens most often on clear, cold nights, when there are no clouds to hold warmth in.
Here in Maine, we’ve already had a hard frost this third week of September 2020. Mid- to late-October is the most common time of the year for first frost dates across the United States. Florida has the latest frost dates of any state, with many areas seeing only a couple of days of frost per year, during the month of January.
There are actually several different kinds of frost. Here’s a look at some common varieties:
Rime frost looks like sugar sprinkled onto the edges of leaves and flower petals. It occurs whenever damp winds are coupled with extremely low temperatures. The word “rime” means “crust.”
Hoar frost is frost that resembles spiky hairs. This type of frost gets its name from the word “hoar,” which means “ancient,” because it resembles an old man’s bushy, white beard. It happens when water vapor freezes instantly after coming into contact with a very cold surface.
Fern frost is a kind of frost that appears on windows when there is very cold air on one side and moist air on the other. This causes tiny water droplets to form on the cold glass and freeze into patterns that resemble leaves or ferns.
Caleb Weatherbee is the official forecaster for the Farmers' Almanac. His name is actually a pseudonym that has been passed down through generations of Almanac prognosticators and has been used to conceal the true identity of the men and women behind our predictions.