Weather Academy: How does frost form?

Published: Dec. 6, 2021 at 6:11 AM CST
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MONROE, La. (KNOE) - On this week’s Weather Academy, we are diving into a new topic, and something quite common for this time of year, frost. During the winter, we will frequently wake up to see frost coating the grass, leaves, and sometimes even our cars in the morning. Frost can develop at any time of day, but typically we see it develop overnight because that’s when we usually see the necessary conditions for frost to form. There are three key things we need for frost to form.

First, we need temperatures to be at or below freezing. Water turns to ice at 32 degrees Fahrenheit. Next, we need calm winds under five mph. Calm to light winds prevents stirring of the atmosphere, which allows a thin layer of supercooled temperatures to develop at the surface. And last, we need clear to mostly clear skies.

This third step needs a little further explaining. During the day, the sun heats the surface of the earth. Temperatures get warm enough for the air to hold onto water vapor. This is otherwise known as humidity. At night, the heat escapes back up into the atmosphere. When we have clouds in the sky, they act as a blanket and can help trap in that hit, remitting it back down towards the earth’s surface. Nothing stops the heat from escaping on clear nights, and temperatures are generally cooler, especially at the earth’s surface.

Frost can form when the official low temperature is above freezing, as temperatures are measured 2 meters above the ground. But because cold air sinks, ground temperatures can be 5 to 10 degrees cooler than the air just above the surface. Eventually, our temperatures will cool off enough to reach the dewpoint. The dewpoint is the temperatures our air must reach for the water molecules in the air to condense. This is when we get dew on the grass in the summertime. But when temperatures are at or below freezing, the dew will freeze into frost.

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