Cold Weather Terms

One of the primary factors in determining the potential and type of damage from cold temperatures is the amount of wind. Radiant Cooling refers to "calm" weather situations where the wind is less than 5 to 10 mph and Advective Cooling usually occurs when an Arctic cold front moves through with winds of 10 mph or greater. If there is little or no wind and the surface air temperature drops, the heat from the ground will "radiate" upwards creating an inversion with cold temperatures on the ground and warmer temperatures above. If sufficient moisture is present in the air a frost will form on plants ("hoarfrost" or "white freeze"). If the air is too dry to form ice crystals, one experiences a "black frost".

Based on this a "Frost" will occur when temperatures dip below the freezing point and the winds do not exceed 10 mph. A "Freeze" will occur if the wind exceeds 10 mph. To be more specific (from the National Weather Service), "[a] freeze is when the surface air temperature is expected to be 32°F or below over a widespread area for a climatologically significant period of time. Use of the term is usually restricted to advective situations or to occasions when wind or other conditions prevent frost. "Killing" may be used during the growing season when the temperature is expected to be low enough for a sufficient duration to kill all but the hardiest herbaceous crops."

Why is the distinction between frost and freeze important to the home gardener? In a frost wrapping plants in some form of fabric or other shielding may be very effective. However, in a freeze the wind reduces the effectiveness of wrapping and the cold temperatures can prove to be more damaging.

Some helpful resources: