In article ,
Frank wrote:

David E. Ross wrote:
On 3/28/2009 12:33 PM, Denis Mitchel wrote:
Does the temperature have to be at or below freezing for frost to form on
plants?

Thanks



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).


You are correct. It is more widely discussed in Wiki:

http://en.wikipedia.org/wiki/Frost

"Thus, frost might form on plants until the air temperature is
actually at or below freezing (32F, 0C)."

Are you saying that once the temperature of freezing is reached,
freezing stops?
--

- 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

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.
--

- 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

In article ,
"David E. Ross" wrote:

On 3/28/2009 6:37 PM, Billy wrote:
In article ,
"David E. Ross" wrote:

On 3/28/2009 12:33 PM, Denis Mitchel wrote:
Does the temperature have to be at or below freezing for frost to form on
plants?

Thanks


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).

Huh?

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.
--

- 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

On 3/29/2009 8:49 AM, Billy wrote:
In article ,
"David E. Ross" wrote:

On 3/28/2009 6:37 PM, Billy wrote:
In article ,
"David E. Ross" wrote:

On 3/28/2009 12:33 PM, Denis Mitchel wrote:
Does the temperature have to be at or below freezing for frost to form on
plants?

Thanks


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).
Huh?
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.

Oh! I left "not" out. It should read:
"Thus, frost might not form on plants until (32F, 0C)."

--
David E. Ross
Climate: California Mediterranean
Sunset Zone: 21 -- interior Santa Monica Mountains with some ocean
influence (USDA 10a, very close to Sunset Zone 19)
Gardening diary at http://www.rossde.com/garden/diary

"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.

In article ,
"brooklyn1" wrote:

"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.

If those are dark scud clouds, I wouldn't sweat the garden. Shelly never
could read well.
--

- 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

"Pat Kiewicz" wrote in message
...
Denis Mitchel said:


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

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
ground.

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.)


--
Pat in Plymouth MI

"So, it was all a dream."
"No dear, this is the dream, you're still in the cell."

email valid but not regularly monitored



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.
--
Dave
Confront and fight Obama zombieism

Billy wrote:
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.

Yes

Some where between these two extremes, air
pressure will have some effect on the formation of dew or the
deposition of frost (phase changes)

Maybe, but how much? I am not saying you are wrong but what effect will it
have and is it worth worrying about? In what situation would this effect
make a practical difference?

, as will the humidity of the air.

I think we all agree on that.

David

Jangchub wrote:
On Sun, 29 Mar 2009 01:29:42 GMT, "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). 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 pressure/elevation.

Why do you ask?


First of all, fog creates a blanket and prevents frost more than it
causes it.

Yes because it reduces radiant heat loss like cloud cover

I won't say anything else. I'm just dazzled by some of
the inaccuracies in this whole thread.

I can understand that you may not want to dispute with people, do you assume
that a dispute will necessarily follow? If we never find out what is wrong
how will we learn?

David

Jangchub wrote:
On Sat, 28 Mar 2009 15:33:28 -0500, "Denis Mitchel"
wrote:

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

Thanks


Denis, I have seen so many wrong answers that it would behoove you to
do a search and avoid Wikapedia at all cost. Some of my plants are
valued at over 1500 dollars. I have Brugmansia's in my greenhouse
which are 13 years old and over ten feet tall. If I listened to some
of these explanations I'd certainly lose some of my mother plants.
Just because you hear it on the Internet, doesn't make it true. Best
to get technical information directly from a University website or
some other professional grower.
Victoria


I find this odd that you tell us you are an expert and that much of this
discusion is wrong but you won't say what is right. I am not trying to have
an argument but to learn.

David

Dioclese said:

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.

"Cold" doesn't move, but cold air most certainly does. And cold air drainage
to low lying areas makes them more prone to frost.

http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/agdex9

quote

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
nearby.

end quote


--
Pat in Plymouth MI

"So, it was all a dream."
"No dear, this is the dream, you're still in the cell."

email valid but not regularly monitored

"David Hare-Scott" wrote:
Jangchub wrote:
On Sat, 28 Mar 2009 15:33:28 -0500, "Denis Mitchel"
wrote:

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

Thanks


Denis, I have seen so many wrong answers that it would behoove you to
do a search and avoid Wikapedia at all cost. Some of my plants are
valued at over 1500 dollars. I have Brugmansia's in my greenhouse
which are 13 years old and over ten feet tall. If I listened to some
of these explanations I'd certainly lose some of my mother plants.
Just because you hear it on the Internet, doesn't make it true. Best
to get technical information directly from a University website or
some other professional grower.
Victoria


I find this odd that you tell us you are an expert and that much of this
discusion is wrong but you won't say what is right. I am not trying to
have an argument but to learn.

David
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.

"Pat Kiewicz" wrote in message
...
Dioclese said:

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.

"Cold" doesn't move, but cold air most certainly does. And cold air
drainage
to low lying areas makes them more prone to frost.

http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/agdex9

quote

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
nearby.

end quote


--
Pat in Plymouth MI

"So, it was all a dream."
"No dear, this is the dream, you're still in the cell."

email valid but not regularly monitored



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.
--
Dave
Confront and fight Obama zombieism

"David Hare-Scott" wrote in message
...
Jangchub wrote:
On Sun, 29 Mar 2009 01:29:42 GMT, "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). 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 pressure/elevation.

Why do you ask?


First of all, fog creates a blanket and prevents frost more than it
causes it.

Yes because it reduces radiant heat loss like cloud cover

I won't say anything else. I'm just dazzled by some of
the inaccuracies in this whole thread.

I can understand that you may not want to dispute with people, do you
assume that a dispute will necessarily follow? If we never find out what
is wrong how will we learn?

David

Am guessing here, mind you. In the area of TX where Elizabeth and I live,
foggy and misty mornings are quite normal.

Fog mist is the result of water saturated air being cooled enough to wrench
a small amount of moisture from the air and make visibility a problem. It
creates more air temperature uniformity at the ground surface by slowing
heat loss at the surface. So, frost is an exception here. Such foggy/misty
mess in the dead of winter here are rare exceptions, does this foggy mist
freeze. Typically, the fog is dissipated by the morning sun later in the
day. And all is dry as a bone by the heat of the afternoon. This occurs
regularly here.

The rare times we get a frost, its from a high pressure area rolling
overhead, but hasn't leached all the moisture from the air to create such
frost. Usually, its very light.

Something I didn't see pointed out was thickness of brush, trees, and
ground-hugging plants is also a deterrent to frost on a typical no-wind
frost day. Another sort of blanket, it you will.
--
Dave
Confront and fight Obama zombieism

"Jangchub" wrote:

fog doesn't freeze or it would be called snow.

When fog (visible water vapor near the ground) freezes it's called frost...
snow is different, snow is the inverse.