Behind the Forecast: How frost forms

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Behind the Forecast: How frost forms
On average, Louisville's first fall frost occurs on October 25 while the first fall freeze happens on November 4. (Source: Pexels)

LOUISVILLE, Ky. (WAVE) - As the year wears on and cooler temperatures return to the forecast, frost becomes more of a concern.

The city of Louisville saw its earliest Fall frost (36°) on September 25, 1950, and Fall freeze (32°) on October 3, 1974. On average, Louisville's first fall frost occurs on October 25 while the first fall freeze happens on November 4.

So, what is frost anyway? The National Weather Service (NWS) 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, the thin layer of ice that forms on a solid surface, can only occur on a surface that is below freezing. Once the water vapor touches the sub-freezing surface deposition occurs, turns from water vapor to ice, as the water vapor reaches its freezing point.

The object must be colder than the dewpoint of the air surrounding it. If the atmosphere is too humid, water vapor may condense into a liquid before freezing; this technically results in icing instead of frost. Frost has a feathery and crystal-like appearance. Frozen dew is more transparent, round, and hard.

The following conditions are favorable to frost formation:

  • Clear skies: Clear skies allow more heat to escape through radiational cooling from the planet's surface and into the atmosphere.
  • Freezing temperatures: When super-cooled freezing temperatures can drop to the dewpoint, then frost can form on various objects.
  • Calm winds: Light winds allow a thin layer of super-cooled temperatures to form near the ground. This layer can be up to 10° cooler than air 5 feet above it where observations are usually taken, according to the NWS.
  • Topography: The lay of the land can play a big part in how and where frost forms. Colder air usually settles in valleys since it is heavier than warm air. Valleys can also block strong winds, enhancing frost formation.

Frost is most impactful during the growing season. A frost severe enough to stop or postpone a growing season is called a killing frost. Frost damage occurs when the temperature drops enough to freeze the water inside of a plant's cells.

Here are the different types of frost:

  • Air Frost: Temperatures drop below the freezing point of water, allowing frost to form.
  • Ground Frost: Ice forms on sub-freezing objects or the ground, creating ground frost. If the surface gets colder faster than the air, ground frost can happen without an air frost.
  • Hoar Frost: This occurs when there is an ample supply of moisture in the air like unfrozen streams and lakes. The crystal patterns are larger and more complex.
  • Glaze and Rime: Glaze and Rime are a bit different from typical frost. Rime is a "rough white ice deposit which forms on vertical surfaces exposed to the wind," according to the UK Met Office. It forms when supercooled water droplets of fog freeze when it touches a surface. Glaze forms when supercooled raindrops touch the ground or when a non-supercooled liquid touches sub-freezes surfaces; glaze appears as a clear ice deposit.

Frost can occur on nights where the thermometer doesn't reach 32°F. Official temperatures are typically measured at a particular location (for Louisville, it's the Muhammad Ali International Airport). These temperatures are also measured at a specific height above the ground (most temperature readings are taken around 6 feet or 2 meters above the ground).

A layer of colder air can form closer to the ground because cold air sinks; the ground also transmits heat into the atmosphere efficiently on clear, calm nights. Metal and glass objects, like your car roof and windshield, release heat more quickly than other objects so they can also see frost at temperatures that are slightly above freezing.

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