Some Frost/Freeze Knowledge

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By Stephen Gode on September 25, 2012, 11:29am

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Photo by NOAA

In the map above, the 10% probability of 32°F or less on an earlier date for most of interior Connecticut is highlighted for September 15^th to October 1^st. The extreme northeast and northwest portions of Connecticut is highlighted for September 1^st to September 15^th. The shore is highlighted for October 1^st to October 15^th except for the majority of southern Fairfield County (October 15^th to November 1^st).

General NWS terms:

Outlook
Indicates the potential for significant weather events up to 7 days in advance with a forecaster confidence around 30%.

Watch
Indicates that conditions are favorable for the particular weather event in and near the Watch area, and which may pose a risk to life and property. Watches are issued up to 48 hours in advance with forecaster confidence around 50%.

Warning/Advisory
Indicates that a particular weather event is imminent or occurring. Advisories are issued if the weather event will lead to nuisance conditions, while Warnings are issued for significant weather events which will pose a risk to life and property. Warnings and Advisories are issued up to 48 hours in advance with forecaster confidence of at least 80%.

Note: Watches and Warnings issued for Severe Thunderstorms, Tornadoes, and Flash Flooding have much shorter lead times, on the order of hours for Watches or even minutes for Warnings.

From the NWS frost and freeze terms are:

Freeze - A condition occurring over a large area when the surface air temperature remains below 32 degrees Fahrenheit for an extended period of time possibly leading to the damage of certain crops.

Freeze Warning - Issued during the growing season when surface temperatures are expected to drop below freezing over a large area for an extended period of time, regardless if frost develops or not. They are usually issued to highlight the first few freezes of the fall, or unusually late freezes in the spring.

A "hard freeze" is used to imply temperatures that are sufficiently cold, for a long enough period, to seriously damage or kill seasonal vegetation. For example, the Mobile AL forecast office lists criteria for its hard freeze warning as temperatures 26 degrees or lower for at least 5 hours. The purpose of this warning to alert people to the potential for frozen pipes, radiators, livestock and so on, not just damage to sensitive plants.

Frost - A covering of ice on exposed surfaces when the air temperature falls below the frost point.

Frost point - temperature, below 0° C (32° F), at which moisture in the air will condense as a layer of frost on any exposed surface. The frost point is analogous to the dew point, the temperature at which the water condenses in liquid form; both the frost point and the dew point depend upon the relative humidity of the air.

Frost Advisory - Issued during the growing season when widespread frost formation is expected over an extensive area. Surface temperatures are usually 33°F to 36°F Fahrenheit.