Time to get out the scraper because it's that time of year when a thin blanket of ice forms on the windshields. Let's chat about the science behind frost!
The National Weather Service defines frost as the formation of thin ice crystals on the ground or other surfaces in the form of scales, needles, feathers, or fans. Frost forms on cars, patio furniture, and vegetation that have a surface temperature of 32 degrees or below. Water molecules in the atmosphere freeze when the dew point (now known as the frost point) is below freezing (32 degrees F). If the surface temperature and the dew point are the same, relative humidity will be at 100% (the frost point). This signifies that there is enough moisture available to create frost.
Some may think that the air temperature needs to be 32 degrees or below for frost to form... that is not necessarily true. You can have nights or mornings where air temperatures above the surface (5 feet and higher) are slightly above freezing (33-37 degrees), and the ground temperature is be colder, either at or below freezing. The coldest layer of air is located at the surface because it is more dense than the air above it...making it sink to the ground level when winds are calm and relaxed. This is common in the autumn and winter months.
When temperatures drop below the frost point, water vapor in the atmosphere can transition directly to a solid state (ice) without becoming liquid first. This is through a process called "deposition".
Widespread frost is common when we have the following:
1. Clear skies at night in open areas help heat escape into the atmosphere
2. Calm to light winds to prevent the mixing of air, making the air more dense
3. Some moisture in the air and on the ground
4. Cold air at the ground (32 degrees F or below)
(temperature sensors are located about 5+ feet above the surface so the air above the surface temperature may be above freezing)
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