Notice the mention of dew point.
Bill who thinks the frost acts like a magnifying glass hence the plants
burn. Not sure if this is correct only know that late frost about here
has me out spraying water on tender new plants.
On Sat, 28 Mar 2009 15:33:28 -0500, "Denis Mitchel"
For moisture or water to freeze the temperature must be no higher than
32 degrees F, and then additional heat must be removed for the water
to phase-change to a crystal lattice structure (ice). No ice will
form above 32 degrees, but certainly there can be supercooled (or
contaminated) liquid water below 32 degrees, which is added frost
protection for the plant.
In article ,
Below freezing, because the plant(s) will carry over some heat from the
day and the ground, if dry, will be reflecting some heat as well. This
heat must be removed before freezing can occur. As Bill alluded to, as
long as your making ice (there is water to freeze) the ice will remain
at 32F (0C).
We often get frost on roofs when the overnight air temperature stays
above 32F. I think this is a case of the roofing materials radiating
enough heat into a clear, cloudless sky that they might actually be
colder than the air.
For plants, however, I don't think they can lose enough heat to drop
their temperatures below that of the surrounding air. Thus, frost might
form on plants until the air temperature is actually at or below
freezing (32F, 0C).
Liquid water from a hose is above freezing. In the process of turning
into ice, such water must first give up heat just to be chilled to 32F
(0C); then it must give up significantly more heat just to become ice
even if the temperature remains at 32F. That's why farmers in my area
spray water on citrus, avocados, strawberries, etc if there is a late
freeze. For "radiation frost" with air temperatures above freezing
(described in the Wikipedia article you cite), they do nothing since
those crops are generally hardy enough to withstand the chill.
No. The dew on plants can freeze without the air temperature dropping to
below 32ºF... this often occurs with lawns and other low growing plants
because even though the air near the ground is heated by the radiant heat to
above 32ºF yet the fine droplets will freeze... fog can freeze and settle on
plants (like snow). But just because the dew freezes on the plant doesn't
mean the plant freezes, many plants contain chemicals that act as
antifreeze, and many plants will suffer 'frost' damage above freezing,
especially young tender seedlings. There's good reason why weather
forcasters mention "dew point", has to do with barametric
Why do you ask?
In article ,
Because . . . . ?
Fascinating, can we have a cite?
Uh huh, I think we already covered the heat of fusion.
And that reason is . . . ?
Whatever the true identity of brooklyn1, brooklyn1 certainly is as
scatter brained as Shelly, who has a thing for forest pansies, heh, heh
(as Shelly would say).
Still, one last wrinkle. Wind chill could allow standing water, as small
as a dew drop even, by evaporative cooling in conjunction with low
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.
I think you have this backwards, the air near the ground is cooled by
radiant loss of heat in frosty conditions. Under a clear sky frost can form
at or near the ground even though the "official" temperature is not below
freezing. This is because the recorded temperature is taken above ground (I
forget the distance but it's about 4-5ft) while the temperature on the
ground can be quite a few degrees lower due to radiant heat loss.
There's good reason why weather forcasters mention "dew
Dew point is another way of saying humidity, it is the temperature at which,
with the current moisture content, the air would reach saturation. If the
dew point is near the "official" air temperature (above the ground) then on
the ground the temperature may below precipitation point. I cannot see why
you relate it to barometric pressure or elevation
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.)
In article ,
Evaporative cooling could suck a lot of heat out of wet ground, not so
much with dry soil.
Barometric pressure (air pressure) changes with elevation. In the heart
of Jupiter (a gas giant), H2 is a solid because of the immense pressure
of its' atmosphere. Contrarily, water will boil at room temperature in a
vacuum. Some where between these two extremes, air pressure will have
some effect on the formation of dew or the deposition of frost (phase
changes), as will the humidity of the air.
In article ,
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.
Exactly. Often the frost one sees on lawns during early morning is actually
precipitation... the water in the ground evaporates, rises to a colder air
layer (albiet a low altitude, perhaps just a few feet), where those small
droplets freeze and fall... what one sees on plants that's called frost is
actually a fine snow. As soon as the sun appears the frozen droplets will
melt and evaporate, if it stays cloudy the radiant heat from the ground will
eventually melt the frozen droplets, they will condence into larger
droplets, rain down and be reabsorbed into the ground. The frost one sees
on plants protects them through insulation, it acts as a buffering layer
(same as layered clothing, same way igloos insulate, ice contains a lot of
traped air, air is an excellent insulator) just in case more fridgid air
comes barreling in. The occurance of frost is very random (can't be
predicted with any degree of accuracy) and is dependant on a delicate
balance between many meteorological factors. Don't confuse frost with a
soft and hard freeze. I've often seen heavy snow squalls pass through here
in late April early May when temperatures are above 60ºF... could be like a
half inch accumulation. It quickly melts as the dark clouds scud past,
caused no plant damage unless accompanied by high winds... nature's way of
pruning weak wood.