HORTICULTURE Mark Longstroth District Extension Horticultural Agent
Spring frosts can cause significant losses to fruit crops. Classic radiation frosts with clear skies and calm conditions are common in all fruit growing regions. In a radiation frost the ground cools by radiation to the sky. The cold ground cools the air above it. It is this cold air which causes frost or freeze damage. It is a good idea to routinely assess the air drainage on your farm. Plant growth and construction will cause air drainage to change over time and maintaining good air drainage can save a grower a crop in years when frost hits a region.
There are numerous sites where a degree or two could be gained by removing obstructions from air drainage passages under radiation freeze conditions.
Physics of Air Drainage
Air is subject to the laws of thermodynamics and hydraulics. Cold air is denser than warm air. Because of external energy and wind currents the air can have different temperature layers vertically or horizontally. Cold air sinks flowing downhill to lowest available point where it accumulates until dispersed by heat or wind. Even on still nights there is air movement on slopes. Steeper slopes cause air to move faster. It takes energy to move air through or around obstructions. Therefore cold air often accumulates above obstacles. For air drainage under calm conditions we are only concerned with low level air currents from ground level to 200 feet or less.
Diagnosis:
Fall mornings with low lying ground fog (10 to 20 feet) are ideal for assessing air drainage. At dawn go to highest spot on the farm and make note of where fog patches lie. Walk through these areas noting where fog is thickest and where it thins out. Look for obstructions that keep the fog in the area. Watch for moving fog banks, they show the natural airflow if there is no wind. Follow surface water drainage pattern, it will tell you where cold air travels.
Fruit sites should have large air storage areas within a half-mile or at least allow a general airflow to a storage area within one mile. Cold air storage over bodies of open water is larger than it appears. Heat and water vapor from open water warms the air in contact with it. This causes air to rise, making room for more air to enter.
Even on still nights, air movement down slope creates a slight breeze (1 to 3 miles per hour). One good way to find these breezes is with a helium balloon on a string. The balloon will tip in direction of airflow.
Treatment
There are basically three things you can do to help air drainage: Remove obstacles to air drainage, maintain drainage ways in good condition and create more cold air storage off site. Solid obstructions block airflow causing it to stop and pile up higher than the obstacle (often 2 to 3 times higher) before it pours over the obstacle. Colder dense air will pile higher than warmer air. Obstructions tight to ground are more effective air dams than obstructions with openings near the ground. Sixty feet is the minimum width opening for good airflow. The larger the area draining through the opening the wider the opening should be.
Places to clear of obstructions:
Points to Remember:
Maintaining good air drainage is a never-ending task. Analyze every field for potential alternations before replanting. Plant appropriate varieties. Remember that north and east facing slopes get cold slightly faster than south to west facing slopes. Place earlier growing fruits in warmer sites and later fruit in the cooler sites.
Constantly assess air drainage. Any time you have ground fog pay attention to where it is. This usually happens in the fall during harvest as moisture is condensed out of the air. Be aware of air drainage problems off your farm that will affect you. Work with your neighbors to open air drainage even if you have to cover all expenses. Lastly, Don’t let the county or state fill a road cut across your air drainage area without compensation.
Frost pockets are sometimes easy to find.
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Posted: April 27, 2002
Updated: November 10, 2009