Frost

[Denis Mitchel]

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

Thanks

[Bill]

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.

[Phisherman]

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.

[Billy]

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).

[David E. Ross]

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]

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.

[brooklyn1]

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?

[Billy]

Huh?

[Billy]

Because . . . . ?

Fascinating, can we have a cite?

Uh huh, I think we already covered the heat of fusion.

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.

[David E. Ross]

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 Hare-Scott]

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

[Dioclese]

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.

[Pat Kiewicz]

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.)

[Frank]

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

[Billy]

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

[Billy]

actually at or below freezing (32F, 0C)."

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

[Billy]

Evaporative cooling could suck a lot of heat out of wet ground, not so 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.

[Billy]

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.

[David E. Ross]

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

[brooklyn1]

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.