HORTICULTURE

 

Radiation Frosts

Most of the Freezes that cause problems in the spring in Michigan are radiation freezes.  These freezes occur after the passage of a cold front, preceeding a mass of cool dry air.  Usually there is a stormy period as the cold front moves through followed by clearing and light winds.  During the night the ground cools by radition to the sky. The cold ground chills the air above it so cold air collects close to the ground.  This layer of cold air becomes thicker and thicker so that a temperature gradient occurs between the  cold air close to the ground and warmer air above it.  Normally warmer air is located closed to the ground and the air temperature falls as you rise upward in the atmosphere.  In a radiation freeze this is reversed and cold air is located close to the ground with a warm layer above it.  This warm layer is called an inversion.  Wind machines work by mixing the warm air above with the cool air close to the ground.  The effectiveness of the wind machine depends on whether the warm air layer is close enough to the ground for the machine to reach.  Often you will hear that the inversion is high or low describing the relative height of the inversion.  Long ago when fuel was cheap orchard heaters were used to heat fruit plantings.  These were very effective when the inversion was low.  Fuel prices are now prohibitive for heater but wind machines remain effective and work best when the inversion is low.

Radiation freezes are chararterised by clear skys and dry air.  Often the day is warm and pleasnt when the sun is out.  After sundown since there is no mixing of the air the the temperatures fall rapidly.  The temperatures will fall to the dewpoint and then the drop in temperture will slow as water is condensed out of the air as dew or frost.  If the dewpoint is near or below the critical temperature of fruit buds damage is likely.  Even the passage of a few clouds will raise the temperture and reduce damage. 

Air Drainage:

Air drainage describes the movement of the cold air layer down slope because cold air is heavier than warm air.  The movement of cold air down slope is slow and often a large mass of cold air is necessary to begin the flow.  In 2006, large areas that were located high up on the tops of hills were damaged because there was little slope on the site so that the cold air piled up until it was thick enough and cold enough to cause damage.