Re: Frost

In article <gqn787$nrk$1@xxxxxxxxxxxxxxxxx>,
"David Hare-Scott" <secret@xxxxxxxxxx> wrote:

brooklyn1 wrote:
"Denis Mitchel" <DLM@xxxxxxxxx> 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.
--

- Billy
"For the first time in the history of the world, every human being is
now subjected to contact with dangerous chemicals, from the moment of
conception until death." - Rachel Carson

http://www.youtube.com/watch?v=WI29wVQN8Go

http://www.haaretz.com/hasen/spages/1072040.html
.

Relevant Pages

  • Re: Frost
    ... Does the temperature have to be at or below freezing for frost to ... The dew on plants can freeze without the air temperature ...
    (rec.gardens)
  • Re: Frost
    ... Does the temperature have to be at or below freezing for frost to ... The dew on plants can freeze without the air temperature ...
    (rec.gardens)
  • Re: Inadvertant IMC
    ... We have learned when the dew point and the temperature are getting ... cold saturated clear air to opaque cloud. ...
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  • Re: Drying an unheated space: how do you measure moisture?
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    (sci.electronics.design)
  • Re: Frost
    ... growing plants because even though the air near the ground is heated ... I think you have this backwards, the air near the ground is cooled by radiant loss of heat in frosty conditions. ... This is because the recorded temperature is taken above ground while the temperature on the ground can be quite a few degrees lower due to radiant heat loss. ... Dew point is another way of saying humidity, it is the temperature at which, with the current moisture content, the air would reach saturation. ...
    (rec.gardens)