Frost

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On 3/28/2009 6:37 PM, Billy wrote:

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

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

You are correct. It is more widely discussed in Wiki:
http://en.wikipedia.org/wiki/Frost
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"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?
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- Billy
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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.
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Dave
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Denis Mitchel said:

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.)
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Pat in Plymouth MI

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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.
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Dioclese said:

"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>
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Pat in Plymouth MI

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

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
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"David Hare-Scott" wrote:

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.
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