As we head out of Summer and into Autumn, temperatures will begin to cool down and leaves will begin to change their color. This is also the period that signals the end of the growing season as temperatures begin to fall below freezing. Below are some facts and figures on the climatological average first frost for Missouri.
The earliest date of first killing frost/freeze in Autumn was September 24, 1989.
The latest date of first killing frost/freeze in Autumn was November 25, 1931.
The average date of first killing frost/freeze in Autumn (1971-2000) is October 15th.
Frost is the formation of ice crystals on the ground or other surfaces in the form of scales, needles, feathers, or fans. Frost develops under conditions similar to dew, except the temperatures of the Earth's surface and earthbound objects fall below 32°F. As with the term "freeze", this condition is primarily significant during the growing season. If a frost period is sufficiently severe to end the growing season or delay its beginning, it is commonly referred to as a "killing frost". Because frost is primarily an event that occurs as the result of radiational cooling, it frequently occurs with a temperature reading in the mid 30's. Frost may also form on aircraft in flight during descent from subfreezing into a warmer, moist layer below. This may cause considerable consternation if the windshield glazes over or the windows frost over.
How can frost form with temperatures above freezing?
The official temperature is typically taken in an instrument shelter 5 feet above the ground to protect against such elements as sunlight and wind. On clear nights when winds are light, the temperature on the ground will typically cool more rapidly than at the level of the thermometer.
This could result in a situation where temperatures at the ground reach the point of frost formation (typically a dew point of 32 F or less) but not at the temperature shelter. Frost will also form on colder surfaces. A light colored car or lawn will absorb less sunlight during the day and therefore be cooler than a driveway or wood deck.
A Frost Advisory is issued during the growing season when widespread frost formation is expected over an extensive area. Surface temperatures are usually in the mid 30s Fahrenheit.