Winter Handbook: Frost Advisories

Weather

(WOWK) – When temperatures reach the mid 30s down to just above the freezing mark, the National Weather Service will issue a Frost Advisory so people will know they may need to take care of plants they don’t want to suffer from the cold.

The definition of frost from the American Meteorological Society:

  1. The fuzzy layer of ice crystals on a cold object, such as a window or bridge, that forms by direct deposition of water vapor to solid ice.
  2. The condition that exists when the temperature of the earth’s surface and earthbound objects fall below freezing.

What’s important to note here is that frost is a form of freezing water vapor. The catch is, the air temperature may be well above freezing above the ground, but the grass blades or leaf surfaces or tree branches may continue to radiate heat out and the surface temperature can drop to the freezing mark.

Image: NOAA

The National Weather Service says the conditions that help lead to frost include:

  • Clear skies lead to radiational cooling, allowing the greatest amount of heat to exit into the atmosphere.
  • Calm to light winds prevent stirring of the atmosphere, which allows a thin layer of super-cooled temperatures to develop at the surface. These super-cooled temperatures can be up to 10 degrees cooler than 4-5 feet above the surface, where observations are typically taken. For example, if conditions are favorable, air temperatures could be 36 F, but the air in contact with the surface could be 30 degrees or colder.
  • Cool temperatures, with some moisture, that promote ice crystal development. If the super-cooled, freezing temperatures can cool to the dew point (the temperature at which, when cooled to at constant pressure, condensation occurs; moisture will have to come out of the atmosphere as fog, frost, etc) frost could develop on exposed surfaces.
  • Local topography has a large role in determining if and where frost develops. Cold air will settle in the valleys since it is heavier than warm air, therefore frost conditions are more prone in these regions. Valleys also shelter the area from stronger winds, enhancing the potential for frost. 
  • Other local effects, such as soil moisture/temperature and stage of vegetation “greenness” are factors that can affect the possibility of frost forming.
Frost on leaves of plants – Photo: NC State University NC State Climate Office

Here are ways to protect plants from the University of California Division of Agriculture and Natural Resources:

  • Water the soil thoroughly (except around succulents). Wet soil holds heat better than dry soil, protecting roots and warming air near the soil.
  • Bed sheets, drop cloths, blankets and plastic sheets make suitable covers for vulnerable plants. Use stakes to keep material, especially plastic, from touching foliage.
  • Remove the coverings when temperatures rise the next day.
  • For a short cold period, low plantings can be covered with mulch, such as straw or leaf mold. Remove once the danger of frost has passed.
  • Cluster container plants close together and, if possible, in a sheltered spot close to the house.

Any time there is a frost advisory or hard freeze warning for our area you will see those listed here with county by county info from StormTracker 13.

Be sure to stay ahead of winter the WOWK StormTracker 13 App free from the App Store or Google Play.

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