Why frost happens even when temperatures don’t reach freezing

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Chilly temperatures are headed back into Iowa soon, which means the likelihood of frost as well. What exactly is frost, though? And why do meteorologists forecast it even when temperatures will be warmer than 32 degrees? Frost formation Simply put, frost is frozen dew. Both dew & frost form during nights when skies are clear & winds are light, which allows the temperature to drop quickly. If the temperature drops below 32 degrees, water vapor in the air will freeze on contact with any cold surface (the ground, car windshields, plants, etc.) and form ice crystals. That’s frost. Frost is dangerous for plants because the water inside a plant’s leaves & stems can freeze. Since ice takes up more space than liquid water, it tears apart the inside of the plant’s cells, damaging or killing them. Can frost form above 32 degrees? Technically no, but there’s a catch. The temperature that appears on TV or weather apps comes from thermometers on weather stations. These weather stations are usually mounted around 5 to 6 feet off the ground. During calm, chilly nights, the coldest air will sink all the way to the ground level. Sometimes this leads to a temperature difference of several degrees between the air around the weather station and on the ground itself. A weather station reporting a 36-degree temperature could have air 32 degrees or colder just a few feet below.Average first frost & freeze October is usually the time for our first frosty mornings of the season in Iowa. On average, Des Moines sees its first 32 degrees of fall around Oct. 16. Northern Iowa’s first freeze averages around the first week of the month, while in far southeast Iowa, the average date isn’t until late October.

Chilly temperatures are headed back into Iowa soon, which means the likelihood of frost as well.

What exactly is frost, though? And why do meteorologists forecast it even when temperatures will be warmer than 32 degrees?

Frost formation

Simply put, frost is frozen dew. Both dew & frost form during nights when skies are clear & winds are light, which allows the temperature to drop quickly.

If the temperature drops below 32 degrees, water vapor in the air will freeze on contact with any cold surface (the ground, car windshields, plants, etc.) and form ice crystals. That’s frost.

Frost is dangerous for plants because the water inside a plant’s leaves & stems can freeze. Since ice takes up more space than liquid water, it tears apart the inside of the plant’s cells, damaging or killing them.

Can frost form above 32 degrees?

Technically no, but there’s a catch.

The temperature that appears on TV or weather apps comes from thermometers on weather stations. These weather stations are usually mounted around 5 to 6 feet off the ground.

weather station vs. ground temperature

During calm, chilly nights, the coldest air will sink all the way to the ground level. Sometimes this leads to a temperature difference of several degrees between the air around the weather station and on the ground itself. A weather station reporting a 36-degree temperature could have air 32 degrees or colder just a few feet below.

Average first frost & freeze

October is usually the time for our first frosty mornings of the season in Iowa.

On average, Des Moines sees its first 32 degrees of fall around Oct. 16.

average freeze dates des moines

Northern Iowa’s first freeze averages around the first week of the month, while in far southeast Iowa, the average date isn’t until late October.

iowa first freeze map

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