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.
The terms frost and freeze are usually mentioned together, however, they describe two very different happenings.
Freezes Imply Lows Near 32 °F
A freeze means that widespread temperatures are expected to fall to or below the freezing mark (32 °F). A hard freeze implies that widespread temperatures are forecast to fall below freezing (most NWS offices use 28 °F as the threshold criteria) for long enough to seriously damage or kill seasonal vegetation. For this reason, hard freezes have earned the monicker "killing frosts." A hard freeze usually occurs when a cold air mass moves into an area and brings temperatures of 32°F 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.
Frosts Imply Lows Near 32 °F and Moist Ground Air
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. Whereas freezes have to do with air temperature alone, any weather alert having to do with frost not only implies that temperatures are expected to be 33 to 36 °F, but also that the amount of moisture residing in the air at these temperatures is adequate for frost formation near the surface.
Can a Freeze Occur Without Frost Forming?
Yes, freezes can happen even if a frost doesn't. This seems odd since it takes colder temperatures (at least 32 degrees) to get a freeze. It seems like you'd get a frost (which requires temperatures of 33 to 36 degrees) first. It would make sense that moisture would frost before freezing except that frost is less likely to form when the dew point temperature falls below the mid-20s. This is because, at such cold temperatures, there just isn't enough moisture in the air for significant frost formation -- despite the fact that cold enough temperatures are in place to support it.
Frost & Freeze Weather Safety
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.
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. Potted plants should be covered or brought indoors.
- 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.
- Keep plants watered to fight off drying from cold winds.
- Bring pets indoors whenever extreme cold is expected.
- Cover exposed pipes and outdoor faucets to discourage freezing.
When to Expect Your First Frost/Freeze
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.)