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.

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