How does frost form?
I thought it would be a good topic to explore. I do get questions about frost so let’s get to know frost and its complexities.
Frost formation is a complicated process, and conditions have to be right for it to occur. Frost forms on surfaces directly from the vapor state, without condensing as dew. If dew forms, frost formation is unlikely, even if the temperature drops below freezing.
Frost is more likely to form on surfaces above the ground first because the air immediately above the ground is usually a few degrees warmer than air a few feet higher. Some heat transfer occurs from the ground to the air a few centimeters above it.
Frost is more likely to form on surfaces above the ground first because the air immediately above the ground is usually a few degrees warmer than air a few feet higher. Some heat transfer occurs from the ground to the air a few centimeters above it.
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If there is much wind, frost won’t form. (Neither will dew, as both require little or no wind, so the atmosphere will not stay mixed.)
If skies are cloudy, usually dew or frost won’t form because clouds reflect the radiated heat from the ground, which helps keep the lower layers mixed.
The ideal conditions for frost formation is a night with clear skies, light winds, and a temperature forecast to be near or a little below freezing. Standard temperature measurements are taken from about 2 meters above ground. On a calm night the ground temperature can be as much as 5-7 degrees cooler than the standard temperature reading. If there is some wind, the air stays mixed and the temperature difference disappears.
Window frost forms when a pane of glass is exposed to below-freezing temperatures on the outside and moist air on the inside. Water vapor from the air condenses as frost on the inside surface of the window. Window frost was more common in the past because most windows today are double paned and well insulated.
Hoar frost: Frost is to dew as snowflakes are to raindrops. When water vapor condenses into liquid water, you get raindrops and dew. When water vapor condenses directly into ice, then you get snowflakes and frost. When frost forms as minute ice crystals covering the ground, we just call it all frost.
Sometimes the frost grains grow larger and are called hoarfrost crystals. Good hoarfrost isn’t that uncommon. Hoarfrost grows whenever it’s cold outside and there is an ample source of water vapor nearby. One of the best places to find hoarfrost is on exposed plants near unfrozen lakes and streams. It’s a nice thing to look for if you’re out hiking in the woods on a winter morning.
You can make your own hoarfrost crystals if you have a cold, wind-free location — for example, an unheated outdoor shed. Just plug in a hotplate and heat an open pan of water over a low heat for a day or two. You’ll want to heat the water without heating the whole shed, so it helps if it’s good and cold outside. Put a branch or some other artistic surface above the pan for water vapor to condense onto. How the crystals grow will depend on the air temperature, so you may find different types of hoarfrost crystals on different days.
If you see something interesting when you try this, take some pictures. Frost is beautiful. Closely examine the complexity of a frost crystal — it’s pretty amazing.
