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

Thanks

In article ,
"Denis Mitchel" wrote:

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

Thanks

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

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.

--
Garden in shade zone 5 S Jersey USA

We have 5% of the world's population; we have 25% of the world's
known prison population.

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



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 ,
"Denis Mitchel" wrote:

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

Thanks

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).
--

- 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 3/28/2009 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).

--
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

On 3/28/2009 12:53 PM, Bill wrote:
In article ,
"Denis Mitchel" wrote:

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

Thanks

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

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.


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.

--
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

"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?

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?
--

- 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 ,
"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...
Because . . . . ?
fog can freeze and settle on
plants (like snow).
Fascinating, can we have a cite?
But just because the dew freezes on the plant doesn't
mean the plant freezes,
Uh huh, I think we already covered the heat of fusion.
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.
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
temperatures.
--

- 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 3/28/2009 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.

--

David E. Ross
http://www.rossde.com/.

Don't ask "Why is there road rage?" Instead, ask
"Why NOT Road Rage?" or "Why Is There No Such
Thing as Fast Enough?"
http://www.rossde.com/roadrage.html

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

"Denis Mitchel" wrote in message
...
Does the temperature have to be at or below freezing for frost to form on
plants?

Thanks


The air temperature AT the exact location of said plants has to be below 32F
for frost to form. The common misleading factor seems to be read air
temperature at some other location.
--
Dave
Confront and fight Obama zombieism

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

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

In article ,
Jangchub wrote:

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

Presumably, a university website is on the internet. So you must
be exhorting people to check the authority of their sources of
information.
--

- 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