"Billy" wrote in message
...
In article ,
"David Hare-Scott" wrote:

brooklyn1 wrote:
"Denis Mitchel" wrote:

Does the temperature have to be at or below freezing for frost to
form on plants?

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).

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.

Evaporative cooling could suck a lot of heat out of wet ground, not so
much with dry soil.

But just
because the dew freezes on the plant doesn't mean the plant freezes,
many plants contain chemicals that act as antifreeze, a

True

and many plants
will suffer 'frost' damage above freezing, especially young tender
seedlings.

yes

There's good reason why weather forcasters mention "dew
point", has to do with barametric pressure/elevation.


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

David

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.

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.