On a cold morning, if you can think anything beyond “I’m cold”, you may have noticed that frost doesn’t form very much in areas underneath large trees. Why is that?
First, what is frost? Well, there are a few types, including frozen dew, but the most common is “hoar frost”.
“Depositional frost is also known as white frost or hoar frost. It occurs when the dewpoint (now called the frost point) is below freezing. When this frost forms the water vapor goes directly to the solid state,” meteorologist Jeff Haby explains for Weather.gov. “Depositional frost covers the vegetation, cars, etc. with ice crystal patterns (treelike branching pattern). If the depositional frost is thick enough, it resembles a light snowfall.”
Frozen dew, meanwhile, occurs when dew is formed and then freezes during the night thanks to a cold front or outgoing longwave radiation cooling, as heat is radiated from Earth back out into space.
“At night, heat radiates away from the ground into space. This radiative cooling is more extreme on clear and calm nights because the heat can escape more freely into space,” meteorologist Ben Domensino explains for Weatherzone. “However, if you place an object above the ground at night, this object will reflect some of this heat back towards the ground, as well as emitting its own radiation towards the ground.”
This is why clear nights feel colder than cloudy nights, and why frost is more likely to form when the skies are cloudless.
“You can experience this phenomenon yourself by going outside on a cold, clear and calm night and looking up at the stars. Under clear skies with no wind, you should start to feel your face getting colder as heat radiates away from your skin,” Domensino continues. “However, if you hold a piece of paper above your face, you should feel some of the warmth reflected and emitted by the paper on your skin.”
When clouds are missing, a tree, a car, or any other object will do. The canopy provided by a tree can prevent heat from being radiated away, preventing frost from forming on the ground. This can help protect the plants themselves.
However, though they are very good at handling freezing temperatures, if the water inside their cells freezes, it can be deadly to the tree. Fortunately, trees have a number of mechanisms to cope with the cold, including producing more sugar, which lowers water’s freezing point and helps the plant to survive, as well as dehydrating for the winter.
All “explainer” articles are confirmed by fact checkers to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current.