Nice explanation of what is about to occur in my backyard…
Radiation Frosts A radiation frost is probably the most common during “cold sensitive” times in
Washington and is the easiest to protect against. Almost all frost protection systems/methods
available today are designed to protect against radiant-type frost/freezes. Radiant frosts occur when
large amounts of clear, dry air move into an area. There is almost no cloud cover at night. During
these times, the plants, soil, and other objects which are warmer than the very cold sky will “radiate”
their own heat back to space and become colder and colder. In fact, the plants cool (radiate)
themselves to the point that they can cause their own damage! The plant tissues which are directly
exposed to the sky become the coldest.
These radiation losses (as much as 1-1.5 million BTU/ac/hr) can cause the buds, blossoms, twigs,
leaves, etc. to become 2-4°F colder than the surrounding air which radiates very little of its heat.
The warmer air then tries to warm the cold plant parts and it also becomes colder. The cold air
settles toward the ground and begins a laminar flow towards lower elevations. This heavier, colder
air moves slowly (“drifts”) down the slope under the influence of gravity (technically called
“katabatic wind”), and collects in low areas or “cold pockets”. This drift can also carry a
considerable amount of heat out of or into (from higher elevation heating activities) an orchard or
vineyard.
The general rate of temperature decrease due to radiative losses can be fairly rapid until the air
approaches the dew point temperature when atmospheric water begins to condense on the colder
plant tissues (which reach atmospheric dew point temperature first because they are colder). The
heat of condensation is directly released at the point of condensation, averting further temperature
decreases (at least temporaily). Thus, the exposed plant parts will generally equal air temperature
when the air generally reaches dew point. At the dew point, the heat released from condensation
replaces the radiative heat losses and further air temperature decreases will be small and occur over
longer time periods. A small fraction of the air will continue to cool well below the general dew
point temperature and drift down slope.
Thus, having a general dew point near or above critical plant temperatures to govern air temperature
drop is important for successful, economical frost protection programs (which is, fortunately, often
the case). Economically and practically, most cold temperature modification systems must rely on
the heat of condensation from the air. This huge latent heat reservoir in the air can provide great
quantities of free heat to an orchard or vineyard. Severe plant damage often occurs when dew points
are below critical plant temperatures because this large, natural heat input is lacking and our other heating sources are unable to compensate. There is little anyone can do to raise dew points of large,
local air masses.
Concurrent with the radiative processes and in the absence of winds, a thermal inversion condition
will develop where the temperature 50 to several hundred feet above the ground may be as much as
a 10-12°F warmer than air in the orchard. Springtime temperature inversions in central Washington
will often have a 3-5°F temperature difference (moderate inversion strength) as measured between
6 and 60 feet above the surface. Many frost protection systems such as wind machines, heaters and
undertree sprinkling rely on this temperature inversion to be effective.
41F with a 14F dew point here… not good. …winds have come way down/skies are clear.