Although the terms "frost" and "freeze" are mistakenly interchanged, they describe two distinct phenomena. An advective, or wind-borne, freeze occurs when a cold air mass moves into an area, bringing freezing temperatures. Wind speeds are usually above 8 kilometers per hour, and clouds may be present. The thickness of the cold air layer ranges from 150 to 1,500 meters above the surface. The options for orchard protection under these conditions are very lim ited. A radiation frost occurs when a clear sky and calm winds (less than 8 kilometers per hour) allow an inversion to develop, and temperatures near the surface drop below freezing. The thickness of the inversion layer varies from 9 to 60 meters.
Other factors besides wind speed and clouds affect the minimum temperature that occurs. Growers in mountainous, hilly, or rolling terrain are familiar with frost pockets or cold spots. These are formed during radiation frosts by cold air drainage, i.e., cold, dense air flowing by gravity to the lowest areas of an orchard, where it collects. This causes temperatures to differ in relatively small areas, called microclimates.
Soil moisture and compaction can also have an effect on minimum temperature. A moist, compact soil will store more heat during the day than a loose, dry soil. Thus, it will have more heat to transfer to the trees at night. Groundcover reduces the heat stored in the soil below it. However, the frost protection disadvantages of groundcover management must be weighed against the benefits such as erosion control, dust reduction, improved soil aeration, etc.
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