Frost

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Does the temperature have to be at or below freezing for frost to form on plants?
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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.
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On 3/28/2009 12:53 PM, Bill wrote:

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.
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wrote:

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.
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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).
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No. The dew on plants can freeze without the air temperature dropping to below 32F... 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 32F 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?
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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.
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brooklyn1 wrote:

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

True
yes
There's good reason why weather forcasters mention "dew

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
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much with dry soil.

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.
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wrote:

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 60F... 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.
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If those are dark scud clouds, I wouldn't sweat the garden. Shelly never could read well.
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Billy wrote:

Yes
Some where between these two extremes, air

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?

I think we all agree on that.
David
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Jangchub wrote:

Yes because it reduces radiant heat loss like cloud cover
I won't say anything else. I'm just dazzled by some of

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
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wrote:

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.
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When fog (visible water vapor near the ground) freezes it's called frost... snow is different, snow is the inverse.
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brooklyn1 said:

One of the most amazing things that can happen on a cold morning is a freezing fog leading to rime frost on trees. I've most often seen this in the river vally just north of my house.
It's staggeringly beautiful, but usually quite ephemeral, when the sun is shining; millions of rainbow crystals glittering in the morning light.
Some photos:
http://i1.trekearth.com/photos/73582/hdr_ticino.jpg
http://i1.trekearth.com/photos/73582/copia_di_2009_gennaio_053.jpg
Wiki articles (with illustrations): http://en.wikipedia.org/wiki/Soft_rime http://en.wikipedia.org/wiki/Hard_rime
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"Pat Kiewicz" wrote:

Your pictures remeinded me that I have some similarly depicting the frost phenomena, not nearly as artistic:
http://i44.tinypic.com/30ttp8g.jpg
http://i41.tinypic.com/23lecrd.jpg
My "Fat Albert" Colorado blue spruce's first Christmas (2008):
http://i42.tinypic.com/sm64bc.jpg
Here it is at night... at first I was suprised, until I realized the flash made the otherwise invisable water vapor appear, eerie:
http://i40.tinypic.com/s4ayb9.jpg
My new spruce is now covered with buds that weren't there last fall, albiet still tightly closed, very exciting waiting to see the progress.
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brooklyn1 said:

You are most certainly welcome.
I wish I had done more than found them on the web. I did pick them out of thousands, though.

Nice bit of land you've got there.

Ah, I think you posted this one before! I like the misty effect.

I finally found a crocus blooming near the SE corner of the garage. April already, and the snowdrops are still in bloom. The buds are finally showing some sign of swelling on the amelanchier.
We had one warmish day last month. Today is supposed to be warm, but I have commitments all day. And this weekend it will be turn COLD again. So the outside gardening is running behind.
Meanwhile, inside I'll be gearing up to start the tomatoes, eggplants, and peppers (and here's hoping we don't have a repeat of the late May hard frost this year).
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Pat in Plymouth MI

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On 3/28/2009 12:33 PM, Denis Mitchel wrote:

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).
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David E. Ross
Climate: California Mediterranean
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Huh?
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