Live foliage does not radiate heat into a cloudless sky as readily as do
metals and minerals.
By the way, because of dissolved substances (e.g., sugars) in the
moisture that is internal to plant tissues, that moisture has a lower
freezing point than 32F.
The confusing bit is,
"Thus, frost might form on plants until (32F, 0C)."
It seems to imply that once "the air temperature is actually at or below
freezing", frost formation will stop. I doubt that is what you were
trying to say but that is what you said.
"For the first time in the history of the world, every human being is
In terms of the "official" air temperature, no.
In terms of the very immediate area of the plant, yes.
Beware of frost in open areas when the nights are clear, the overnight
temperatures are expected to be lower than 38 deg F, the dewpoint is
near (or lower than) 32 deg F and the winds are calm.
Heat lost to radiation to the open sky will drop the temperature near the
The dewpoint limits the amount of radiational cooling. When the dewpoint
is at or below freezing, frost will form.
This goes doubly so in lower lying areas, as cold air will flow downhill to
accumulate there. The bottom end of my vegetable garden is very slightly
downhill from everywhere at this end of the block. It may be the only
place that frosts on some days. The slope is very subtle, but it is enough
of a slope to create a frost pocket.
(A good meteorologist will tell you the dewpoint. Relative humidity is
no where near as useful, in my experience.)
I agree most of what you're saying.
What I do disagree about it airflow in lowlying areas. Your final
perception is correct, low spot is colder. You have to understand that heat
is what's being lost, not cold gained. Cold is the LACK of heat energy.
The heat is moving uphill, rather than cold moving downhill
If "cold" was moving downhill, it would have point of origin. Which it
doesn't if the ambient air temperature is warmer that the cold pocket in the
low spot you indicate.
Kind of reminds me of "hole flow:" vs. "electron flow" in electronic
circuitry theory. Both theories work, but the former ignores what is
actually moving. But, that's another story.
"Cold" doesn't move, but cold air most certainly does. And cold air drainage
to low lying areas makes them more prone to frost.
Cold air drainage
On calm nights, the shallow layer of cold dense air that forms near the
ground surface will tend to flow downslope if the ground is not completely
level. This phenomenon is known as cold air drainage. It has a major
influence on the distribution of minimum temperature, rendering some
areas much more frost-prone than others nearby. The coldest air settles
in depressions commonly called frost hollows or frost pockets, or it
collects in other areas where cold air drainage is obstructed by some
kind of barrier.
On a small scale, the effects of cold air drainage may be visible in the
widely variable frost damage observed within a medium sized garden.
The lowest areas are hardest hit while higher portions may escape all
frost damage. This underlines the importance of considering topography
when choosing garden sites. On a larger scale, entire fields may be
affected. Even if the slopes involved are very gradual, some fields are
much more suited to cold-sensitive and long-season crops than others
Cold air does move down in elevation. Heat moves up. Heat is lost pretty
uniformly on cool, non-windy nights/early morning at the ground surface.
And the coolest air settles in low pockets. Got it, thanks.
Most of the responses are for the most part correct, just not entirely
accurate in every detail because it would require an entire library to
describe the frost phenomena with exquisite detail, whereas most folks on
newsgroups do not tend towards verbosity. Ommission and condensation do not
necessarilly equal erroneous information, which is why discussion is ongoing
and no one gets the final word, not ever. One should always be wary of self
proclaimed experts; self praise is no recommendation. Those who enter
discussion claiming expertise typically know the least... belittling other's
knowledge and abilities is a mechanism whereby one attempts to disguise
their own insecurities regarding their lack of ability and knowledge. The
farblondzhet when questioned react with funfeh.
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