Just as the sprouting of tender green leaves is considered one of the first signs of spring, the first frost of the cool season signals that fall has officially settled in and that winter isn't far behind.
How Frost Forms
Look for frost to form when these atmospheric conditions are present:
- clear nighttime sky conditions,
- at or below freezing air temperatures at the surface, and
- calm winds (speeds less than 5 mph (1.6 km/h)).
Clear skies and calm winds allow for daytime heating to escape the Earth's surface. This heat out into the upper atmosphere and outer space. What's known as a temperature inversion layer forms (temperatures increase rather than decrease as one travels upward in the air), and allows cold air to settle near the ground. As the ground temperatures cool to below freezing, what water vapor resides in the air ices up onto exposed surfaces--thus forming frost.
Frost vs. Freeze
The terms frost and freeze are usually mentioned together, however they describe two very different happenings. A hard freeze usually occurs when a cold air mass moves into an area and brings temperatures of 32°F (0°C) or below. This freezing cold air is often blown by winds, or advected, into an area, and may therefore be associated with light or variable wind speeds. Frost on the other hand has to do with the formation of ice crystals on the ground and on other surfaces. It occurs in the absence of wind, and the freezing temperatures are the result of radiational cooling.
The Dangers of Frost/Freeze Conditions
The majority of individuals don't notice frost, except when it forms on their car windows and delays their morning departure by several minutes' time. However, agriculturists and farmers consider it is a critical weather event. This is because most plants (except a few varieties that actually need a hard freeze to coax seeds into germination) are extremely sensitive to it.
A frost too early, or too late, in the growing season can result in crop failure and a shortage of food supply.
How to Protect Vegetation from Frost/Freeze Damage
There are a number of ways to protect against frost damage, including:
- Cover plants. When plants are covered, frost can settle on a barrier rather than on the vegetation directly. For this reason, plants not in direct contact with the covering material have the highest level of protection. Woven fabrics, such as sheets, work best and can offer 2° to 5°F of added warmth.
- Irrigate the soil and plant leaves before the frost arrives. This might sound strange considering water will freeze when the temperature drops, but rest assured there is a method to this madness. Moist soil is capable of holding up to four times more heat than dry soil. Likewise, if fruit trees have begun their yield, spraying the outside skin with water can actually help keep internal temperatures above freezing by allow the outside to freeze and create an insulating barrier.
Determine Your Frost/Freeze Dates
When can you expect the first frost to occur? To find the average date of the first fall (and last spring) frost for your area, use this frost and freeze data product, courtesy of the National Climatic Data Center. (To use, choose your state, then locate the city nearest you.)