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Bill's Weather 101 at Home Edition: Spring Frost

Posted at 8:42 PM, Mar 20, 2020

Frost is a possibility for us this weekend as a push of much colder air, but not unusually so for the latter third of March, arrives. It’s also the subject for today’s Bill’s Weather 101 At Home.

With our warm winter, many plants are well ahead of schedule this year. There are some estimates from horticulture experts that we’re running about 3 weeks ahead of normal for plant development…keep in mind last year we were late after a cold March.

We’re still not far enough along for frost advisories to be issued, so the plants you have that are making it look like spring, and perhaps getting you to fire up the lawn mower, should not be harmed significantly.

So what is frost? First, frost is NOT frozen dew. Although they are both formed by a similar process.

Frozen dew is simply ice. Although frost is certainly ice, the process that forms it creates its own type of beauty. Frost forms directly from water vapor skipping the liquid phase. This process is called DEPOSITION. The surface temperature must be below the frost point. As water vapor comes in contact with the cold surface it turns into that small crystal of ice. As the process continues larger areas can take on that thin mantle of white.

The same process works with a cold frosty mug. Water vapor touches the surface of a mug from the freezer and in a properly moist environment, frost forms on the surface. It’s also the same process that forms a snowflake (which is NOT a frozen raindrop). Frost also forms more readily in low lying areas. Cold air is heavier and denser than warm, so it tends to flow downhill. This is why you won’t find fruit orchards in valleys, but usually on hillsides. It’s also why Monticello in Wayne County is usually our cold spot on a clear calm night. The airport sits in a small bowl and the cold air flows into it.

Frost can be a danger to plants that aren’t tolerant of the cold. Hardy plants like the crocuses, daffodils, and some tulips that have come up are fine under these temperatures. The grass in the front lawn will be ok as well. But things that we plant later in the year, annuals would be in danger from these conditions. This is also why we say wait until Mother’s Day to plant things like that. The frost and cold temperatures can freeze the water inside the plant causing cell damage. However the hardy plants out now can survive this.

Frost advisories are usually issued when the temperature is expected to be between 32 and 36 degrees.

Now you’re asking why 36 degrees for a frost advisory? On a clear calm night the coldest temperatures can be right at ground level. In fact, there can be 3 to 5 degree swing in temperature from the ground to the height where official measurements are taken, which is approximately 5 feet. So you can can have an official temperature of 36 and still see frost on the ground.

In river and creek valleys you can see something called a hoar frost. The high moisture environment near the body of water can lead to dazzling beautiful frosty scenes on the trees.

Check in for the next Bill’s Weather 101 At Home.