If the sprouting of tender green leaves is considered to be a first sign of spring, the first frost of the season must signal the onset of fall, and segue into winter.
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.
Clear skies and calm winds allow for daytime heating to escape 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.
For most of us, frost goes unnoticed, or maybe delays our morning departure by 2-3 minutes. However, for agriculturists, it is a critical weather event. While some plants (including certain varieties of perennials) actually need a hard freeze to coax seeds into germination, most plants 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:
- Covering 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.
- Irrigating soil and plants before the frost. 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.
When to Expect a First Frost
Curious to know when to expect your first frost? This date varies by location. To find the average first fall (and last spring) frost dates for your area, use this (free) National Climatic Data Center frost and freeze data product. (Choose your state, then locate the city nearest you.)