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Frost and Freeze

September 24 1999

“Frost” and “freeze” define a condition that exists when the temperature of the air or of an object near the surface drops to the freezing point of water.

Freezes may be advective, radiative, or a combination of both. Advective freezes are common in early spring or late fall, it occurs after a cold front associated with a low pressure system moves through the area.  It maybe accompanied by moderately strong winds that bring down cold arctic air. Under cloudy conditions freezing temperatures may persist throughout the day. Advective freezes are severe, and little can be done to prevent crop damage.  Freeze damage may be prevented by “timely” planting and harvesting, and proper crop selection for the area.

Radiative freezes occur on nights with clear skies and calm winds.  In the absence of clouds heat is lost from the earth’s surface.  Radiative freezes occur in an areas which is under the influence of cool dry air mass associated with a high pressure weather system.  Rapid cooling after sunset causes the thermometer to dip to below-freezing levels.  Since radiative freezes depend on nighttime cooling, they occur in the early morning hours.  Using “active” methods, such as covering plants or adding heat can prevent freeze damage to crops.

Radiative freezes occur in late spring and early autumn, but can also occur during summer in northern areas. Crop damage is usually localized, occurring on specific farms, fields or portions of fields because of temperature differences over short distances.Most crop-damaging freezes that occur in Ontario are a combination of the advective and radiative types.