Behind the Forecast: Frost

Behind the Forecast: Frost
Frost is common this time of year as temperatures begin to drop. (Source: Pixabay)

Listen to Science Behind the Forecast with Meteorologist Tawana Andrew every Friday on 89.3 WFPL at 7:45 a.m.

LOUISVILLE, KY (WAVE) - Frost is common this time of year as temperatures begin to drop. So how does it form?

You need a few very important weather conditions before frost can form. First, clear skies are necessary. This allows the greatest amount of heat to escape from the surface into the atmosphere. Next, you need light or calm winds. A lack of movement in the atmosphere allows a layer of super-cooled temperatures to form at the surface. For example, the air several feet above the ground may be near 35° but closer to the ground temperatures could be closer to 30°. The last thing needed is cold temperatures which promote the formation of ice crystals. If air temperatures can drop to the dew point or frost point in these conditions then the moisture in the atmosphere is forced to condense or depose (when water vapor turns directly to ice).

One study found that patchy frost can occur in temperatures ranging from 38° to 42°, areas of frost in a range from 33° to 37° and widespread frost typically occurs when it is colder than 32°.

The frost that we’re most familiar with is also known as white frost, hoar frost or depositional frost. This is formed when the dew point is below freezing which forces water vapor to turn directly into ice. Due to the deposition, crystal-like patterns are formed.

Frost that forms because of liquid water transitioning to ice is called frozen dew. Dew forms when temperatures and dew points are above freezing. Through the night, radiational cooling, which occurs more quickly under clear skies, helps the temperature drop to or below 32° and the dew freezes.

(Source: WAVE 3 Weather)

When issuing Frost Advisories and Freeze Warnings the National Weather Service (NWS) works with the agricultural community. Their Frost and Freeze Program varies from year to year and depends greatly on the status of crops in the area. Widespread frost will end the growing season and that’s when the NWS will typically end their program, according to NWS Louisville Meteorologist Ryan Sharp. Sharp also explained that there have been years where it did not get cold enough for a frost or freeze to occur well into November. If a frost has not happened by election day, the program will end. If the frost or freeze comes in segments, for example, if it only occurs over a certain part of the coverage area, then the program could be canceled in segments as well (canceled for sections of Kentucky and Indiana but not others). Check out the National Weather Service Louisville’s criteria for Frost Advisories and Freeze Warnings below.

NWS Louisville's Frost Advisory and Freeze Warning criteria
NWS Louisville's Frost Advisory and Freeze Warning criteria (Source: NWS Louisville)

If a frost period is significant enough to end the growing season or delay the start of the growing season it’s called a killing frost. Frost Advisories and Freeze Warnings can be issued during both the Fall and Spring because of the impact on agriculture.

Here are some frost statistics from NWS Louisville for the city of Louisville. Learn more here.

Records:

  • Latest spring frost (36°): May 27, 1961
  • Latest spring freeze (32°): May 10, 1966
  • Latest spring hard freeze (28°): April 23, 1986
  • Earliest final spring frost (36°): March 15, 1884
  • Earliest final spring freeze (32°):March 5, 1927
  • Earliest final spring hard freeze (28°):February 12, 2011
  • Earliest fall frost (36°): September 25, 1950
  • Earliest fall freeze (32°): October 3, 1974
  • Earliest fall hard freeze (28°): October 10, 1964
  • Latest first fall frost (36°): November 23, 1902
  • Latest first fall freeze (32°): December 5, 1885
  • Latest first fall hard freeze (28°): December 13, 1939
  • Longest growing season: 257 days in 1884
  • Shortest growing season: 166 days in 1976

Averages:

  • Last spring frost (36°): April 18
  • Last spring freeze (32°): April 5
  • Last spring hard freeze (28°): March 24
  • First fall frost (36°): October 20
  • First fall freeze (32°): November 1
  • First fall hard freeze (28°): November 12

Normals (1981-2010):

  • Last spring frost (36°): April 14
  • Last spring freeze (32°): April 3
  • Last spring hard freeze (28°): March 23
  • First fall frost (36°): October 25
  • First fall freeze (32°): November 4
  • First fall hard freeze (28°): November 16
Science Behind the Forecast: Frost

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