Frost and Freezing

By: Lee Grenci , 8:38 PM GMT on December 18, 2012

georgevandenberghe broached the issue of frost in his reply to my blog yesterday (many thanks, George). Any mention of frost, as it relates to gardening, is almost always perceived in a pejorative context. Indeed, there are routinely a spate of frost advisories in spring and autumn around the beginning and end of the growing season (respectively). Portraying frost as a killer of tender plants always bothers me because frost is actually Nature's way of trying to protect them. Yes, frost gets an undeserved rap, in my opinion.

By definition, frost typically forms on clear nights with light winds and relatively low dew points as ice crystals deposit on the ground and other objects (tender plants, car wind shields, etc.). Frost can form when two-meter temperatures are as high as the mid 30's. That's because temperatures at ground level on such nights are sufficiently far below the melting point of ice (in other words, there's a nocturnal inversion extending from the ground upward).

In case my reference to the "melting point of ice" (instead of "freezing") caught you off guard, I confess that I stubbornly refuse to call 32 degrees Fahrenheit (0 degrees Celsius) "freezing." For the record, water almost always freezes at temperatures lower than 32 degrees Fahrenheit because impurities in the water introduce a "disorder" that interferes with the formation of the highly ordered lattice of ice. Only pure water freezes at 32 degrees Fahrenheit, and distilled water doesn't freely exist in Nature. Forgive me, but I regularly shake my head when I hear or read 32 degrees Fahrenheit being described as "freezing." I'm afraid it's hopelessly ingrained in our culture (more on freezing and my disdain in just a moment).

Let's get back to the issue of frost. When water vapor deposits as frost on a tender plant, latent heat of deposition acts to keep the plant warmer (a release of 680 calories per gram; see the energy staircase below). As long as deposition occurs, the temperature of the plant will not precipitously fall into the 20's. At such readings, ice crystals can form inside plants...if plant cells freeze, they can rupture and then there's real damage.


The energy staircase for water. When water vapor deposits onto objects as ice, 680 calories per gram are released (latent heat of deposition). Larger image Courtesy of A World of Weather: Fundamentals of Meteorology.

Are the owners of orchards crazy to spray their tender plants in order to avoid such damage? Facing the loss of their crop, would they willingly exacerbate matters? Orchard owners don't have to know squat about science. All they need to know is that spraying water on plants offers them some protection against dangerously low temperatures. And so it is with frost, which is Nature's way of trying to protect plants (via the release of latent heat of deposition). Killing plants is just not even in frost's purview.

Granted, frost advisories can alert gardeners that temperatures might become threateningly low (I have no problem with this kind of interpretation). Unfortunately, I suspect that most people interpret frost advisories literally...it's the frost that will kill. If gardeners perceive frost as a killer of plants (the appearance of ice crystals on the exterior of plants), please rid your brain of this notion.

Which leads me back to the issue of 32 degrees Fahrenheit being called "freezing." Honestly, meteorologists should know better. Almost all precipitation that falls over the middle latitudes begins as snow at cold, high altitudes. Here, ice crystals grow at the expense of surrounding tiny water drops. Yes, tiny water drops can resist freezing down to minus 40 degrees Fahrenheit (which equals minus 40 degrees Celsius). So I take issue with meteorologists who, on one hand, accept the Bergeron-Findeisen process (and its underlying assumption that tiny water drops resist freezing down to extraordinarily low temperatures), but who, on the other hand, describe 32 degrees Fahrenheit as "freezing." Like it or not, 32 degrees Fahrenheit is the melting point of ice. Indeed, all ice begins to melt at 32 degrees (actually, ever so slightly above 32 degrees), but water does not freeze at 32 degrees Fahrenheit unless it's pure (not present in Nature).

There, I feel a lot better.

Lee Grenci

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13. WunderAlertBot (Admin)
5:41 PM GMT on December 19, 2012
24hourprof has created a new entry.
12. Lee Grenci , Retired Senior Lecturer and Forecaster
3:33 PM GMT on December 19, 2012
Quoting georgevandenberghe:

7. Lee Grenci, Retired Senior Lecturer and Forecaster
12:54 PM GMT on December 19, 2012 0

Quoting Balwanz:
Pertaining to the freezing point, some of us must think differently. In my thoughts are two freezing points, one is the national standard triple point and the other is the interface between such as tap water in contact with the atmosphere and its frozen form. I use this latter, with crushed ice, as the reference junction for thermocouple calibration. Thermocouples are too inaccurate for its small error to matter.

At the same time I'm aware that ice can begin to form at much lower temperatures. This doesn't matter because once it begins to for, its solid-liquid interface is back to the same 0 C.
Perhaps I'm wrong in this latter assumption, but I'm not concerned, I don't work with this situation.



I understand what you're saying, and thanks for your comments, but I've seen presentations during winter where the 32-degree isotherm is drawn to represent the rain-snow line, which sends a very bad message, in my opinion. During spring, I've seen it snow at 44 degrees. I've seen it rain at 7 degrees Fahrenheit during winter (the rain froze on objects, of course). So, while you make valid points, my concern is that the general public uses 32 degrees Fahrenheit as some magical threshold where all water freezes.

As I pointed out, tiny water drops in high, cold clouds, bereft of freezing nuclei, resist freezing down to minus 40 degrees Fahrenheit, rendering 32 degrees as the "freezing mark" somewhat laughable to me.

But I guess folks walking around touting 32 degrees as the "melting point of ice" will never, ever happen. Just one of my pet peeves (that's what happens when you get old :-)

Thanks again.



************************************************* **********


How do you feel about the 5400m 50-100hpa thickness contour being the rain snow line, a trap which forecasters who should know better often fall into. I've seen rain at very low thicknesses and sometimes snow at higher thicknesses depending primarily on low level temperature distribution and airmass stratification. Convective situation critical thicknesses are MUCH lower than stratiform ones.

There are a lot of ways to get 540 thickness. THe well mixed dry adiabatic way produces surface temperatures near 70F.

In practice it's difficult to maintain liquid clouds below about -20C. Almost every impurity acts as a freezing nucleus at temperatures that cold. When it's that cold at 850mb, every fragment of stratocumulus that passes will produce a very light flurry.

This is another thing I noticed by observation (before learning) in January 1977 in Blacksburg VA (as a freshman at VPI) That month was extremely cold but it was also remarkable how many hours of light snow we got from broken or scattered cloud cover that month.


When I forecast precipitation type in winter, I look at ensemble forecasts...forecast skew-T's, 1000-500-mb thickness, 1000-850-mb thickness, 850-700-mb forecasts, 850-mb zero-degree isotherm...

More importantly, I look at the big picture...the overall synoptic-scale pattern within which the aforementioned tools are weighed.

I do not look at any tool unless I have a firm grasp of the big picture, which I tried my best to instill in my students.

Keep in mind that 5400 meters is not a universal critical thickness. For example, the critical thickness at Seattle, Washington, and Portland, Oregon, is in the neighborhood of 5220 meters (owing to the presence of the marine layer). So, in addition to the big picture, knowing climatology is next in the line of priority of any good forecaster.

I'll try to talk about issues like these in greater detail as winter evolves. Thanks for your patience.
Member Since: October 24, 2012 Posts: 128 Comments: 1063
11. Lee Grenci , Retired Senior Lecturer and Forecaster
3:23 PM GMT on December 19, 2012
Quoting georgevandenberghe:
How do you feel about Alistair Fraser's pet peeves, in particular almost everyone talking about "cold air not being able to hold as much water hence condensation [ which is wrong ] )? He's got a bunch of others
on his site.

http://www.ems.psu.edu/~fraser/BadScience.html


George,

Alistair was one of my mentors. He instilled in me the discipline to question everything. In my opinion, everything on Alistair's Web site is doctrine.

Popular notions such as "clouds act as a blanket at night," "warm air holds more water vapor," "clouds trap heat,"...the list goes on and on...are merely shortcuts aimed at circumventing the science in an attempt to make science more palatable to lay readers and consumers. These shortcuts are not scientific truths, however (see my blog about the blue sky).

I hold Alistair Fraser's science in very high esteem.
Member Since: October 24, 2012 Posts: 128 Comments: 1063
10. georgevandenberghe
3:01 PM GMT on December 19, 2012
How do you feel about Alistair Fraser's pet peeves, in particular almost everyone talking about "cold air not being able to hold as much water hence condensation [ which is wrong ] )? He's got a bunch of others
on his site.

http://www.ems.psu.edu/~fraser/BadScience.html
Member Since: February 1, 2012 Posts: 19 Comments: 4867
9. georgevandenberghe
2:44 PM GMT on December 19, 2012

7. Lee Grenci, Retired Senior Lecturer and Forecaster
12:54 PM GMT on December 19, 2012 +0

Quoting Balwanz:
Pertaining to the freezing point, some of us must think differently. In my thoughts are two freezing points, one is the national standard triple point and the other is the interface between such as tap water in contact with the atmosphere and its frozen form. I use this latter, with crushed ice, as the reference junction for thermocouple calibration. Thermocouples are too inaccurate for its small error to matter.

At the same time I'm aware that ice can begin to form at much lower temperatures. This doesn't matter because once it begins to for, its solid-liquid interface is back to the same 0 C.
Perhaps I'm wrong in this latter assumption, but I'm not concerned, I don't work with this situation.



I understand what you're saying, and thanks for your comments, but I've seen presentations during winter where the 32-degree isotherm is drawn to represent the rain-snow line, which sends a very bad message, in my opinion. During spring, I've seen it snow at 44 degrees. I've seen it rain at 7 degrees Fahrenheit during winter (the rain froze on objects, of course). So, while you make valid points, my concern is that the general public uses 32 degrees Fahrenheit as some magical threshold where all water freezes.

As I pointed out, tiny water drops in high, cold clouds, bereft of freezing nuclei, resist freezing down to minus 40 degrees Fahrenheit, rendering 32 degrees as the "freezing mark" somewhat laughable to me.

But I guess folks walking around touting 32 degrees as the "melting point of ice" will never, ever happen. Just one of my pet peeves (that's what happens when you get old :-)

Thanks again.



************************************************* **********


How do you feel about the 5400m 50-100hpa thickness contour being the rain snow line, a trap which forecasters who should know better often fall into. I've seen rain at very low thicknesses and sometimes snow at higher thicknesses depending primarily on low level temperature distribution and airmass stratification. Convective situation critical thicknesses are MUCH lower than stratiform ones.

There are a lot of ways to get 540 thickness. THe well mixed dry adiabatic way produces surface temperatures near 70F.

In practice it's difficult to maintain liquid clouds below about -20C. Almost every impurity acts as a freezing nucleus at temperatures that cold. When it's that cold at 850mb, every fragment of stratocumulus that passes will produce a very light flurry.

This is another thing I noticed by observation (before learning) in January 1977 in Blacksburg VA (as a freshman at VPI) That month was extremely cold but it was also remarkable how many hours of light snow we got from broken or scattered cloud cover that month.
Member Since: February 1, 2012 Posts: 19 Comments: 4867
8. Lee Grenci , Retired Senior Lecturer and Forecaster
1:05 PM GMT on December 19, 2012
Quoting georgevandenberghe:
I said I'd summarize my frost protection methods.
Here's what I currently do.

My house gets too much shade so I have a rental garden plot for vegetables. This is in a broad shallow stream valley and is frosty in spring and fall. I've seen light frost there as late as May 22 in 2002 and as early as October 4. Otherwise it's a good site.

To avoid frost I start stuff as transplants near my house with exposure to only a portion of the sky. This works in cool temperatures when respiration requirements are not high. The last freezing temperatures in these spots occur sometime in March
and I don't worry much about it because before late March daily mean temperatures are too low for good growth anyway.

In the garden (which also can't be frequently attended because of remoteness) I avoid mulch in spring. It makes for more weeding but I can plant a week to ten days earlier without mulch because of the frost risk over mulch. This is esp. an issue for young potatoes which I really like but which require cool temperatures AND are as sensitive to frost as tomatoes. It isn't an issue for tomatoes and most warm season crops because the days are usually too cool for good growth prior to early May (2012 excepted). The latest I've had damaging spring frost even over mulch is May 11 in 2010, an otherwise very warm year. Potted transplants put on top of a mulch pile (oops) got nipped.

The other spring protection is to cover the soil with clear plastic. Clear plastic passes most radiation but blocks convection
( by the way, the pictures in textbooks showing IR being blocked by greenhouse glass panes are incorrect or at least misleading. Most of the warming in a greenhouse is due to blocked convection. Air warmed in the greenhouse can't rise through the glass or otherwise exchange with cooler outside air)
Covering PLANTS with clear plastic is very risky. In spring sunshine temperatures can be 30-40 degrees F warmer under clear unventilated plastic and experience has been that I'll eventually bust a sun forecast or be otherwise occupied and cook the covered plants. Clear plastic provides only a few degrees of frost protection at night. I find warm soil under clear plastic also provides several degrees of temperature boost OVER the plastic at night. That's very helpful in spring but harmful in summer. Weeds are also a problem under clear plastic
so I only use it in spring.

When I've needed to protect a few plants, the combination of clear plastic soil mulch and a cloth blanket has worked well.

In March 2012 the U.S. east of the Rockies had an exceptional warm spell. I decided to take a huge risk and plant one of my tomato plants in the already warm mellow soil March 15 2012. It got another ten days of early May conditions and set a few fruit. Then the frost protection trials started.

On March 27 a shallow arctic outbreak drove temperatures to near freezing (underdone by the models until 36H prior) The combination of cold with wind, and low dewpoints and dessication risk warranted piling leaves around the plant and a blanket. Radiation freeze conditions on March 30 and then again several days in the first half of April were an annoyance and warranted using the blanket again but not the leaves. The last risk was April 13. The plant had a clear plastic soil warming mulch. Potatoes that did not have the plastic mulch froze lethally Monday night April 2 when I failed to protect them. I plant most of my potatoes later because of frost risk and these were just a risky experiment. Warming soil for potatoes reduces yields a lot. They set tubers best in cool soil.

Subsequent mild and then warmer than normal weather allowed me to get my first tomato June 3, three weeks earlier than customary for this area.


I do not use sprinklers or water for frost protection. At my scale it's more trouble than it's worth.

I grow potted florist gardenias for my wife. These stay outside until early December and then move to an unheated garage or crawl space. They will tolerate a lot of chilling but not deep freezing. I keep them on the front porch in fall and early spring where they both look good and get a lot of IR radiation on calm clear nights. Like tree canopies my porch overhang provides a good temperature boost on calm clear nights. There is little benefit with advective freezes but these are rare before late November or after late March in DC. Spring seedlings get a similar treatment.

That's pretty much it. With three kids I don't
have time for more elaborate methods. I have a lot of defenses against cold. I don't have any against heat esp. the high dewpoint heat typical of the eastern half of the U.S.





Thanks. When I was in graduate school (good grief...a long, long time ago), I produced a feature on public television that delved into research at Penn State designed to protect strawberries on clear, cold nights during spring. I found this pdf file which might interest you.

Thanks again for your interest.
Member Since: October 24, 2012 Posts: 128 Comments: 1063
7. Lee Grenci , Retired Senior Lecturer and Forecaster
12:54 PM GMT on December 19, 2012
Quoting Balwanz:
Pertaining to the freezing point, some of us must think differently. In my thoughts are two freezing points, one is the national standard triple point and the other is the interface between such as tap water in contact with the atmosphere and its frozen form. I use this latter, with crushed ice, as the reference junction for thermocouple calibration. Thermocouples are too inaccurate for its small error to matter.

At the same time I'm aware that ice can begin to form at much lower temperatures. This doesn't matter because once it begins to for, its solid-liquid interface is back to the same 0 C.
Perhaps I'm wrong in this latter assumption, but I'm not concerned, I don't work with this situation.


I understand what you're saying, and thanks for your comments, but I've seen presentations during winter where the 32-degree isotherm is drawn to represent the rain-snow line, which sends a very bad message, in my opinion. During spring, I've seen it snow at 44 degrees. I've seen it rain at 7 degrees Fahrenheit during winter (the rain froze on objects, of course). So, while you make valid points, my concern is that the general public uses 32 degrees Fahrenheit as some magical threshold where all water freezes.

As I pointed out, tiny water drops in high, cold clouds, bereft of freezing nuclei, resist freezing down to minus 40 degrees Fahrenheit, rendering 32 degrees as the "freezing mark" somewhat laughable to me.

But I guess folks walking around touting 32 degrees as the "melting point of ice" will never, ever happen. Just one of my pet peeves (that's what happens when you get old :-)

Thanks again.
Member Since: October 24, 2012 Posts: 128 Comments: 1063
6. georgevandenberghe
5:48 AM GMT on December 19, 2012
I just can't get everything right today. I forgot to add that "Frost" in my previous post refers to an instance of temperatures at or below freezing in the plant canopy, NOT formation of hoar frost.
Member Since: February 1, 2012 Posts: 19 Comments: 4867
5. georgevandenberghe
5:46 AM GMT on December 19, 2012
I said I'd summarize my frost protection methods.
Here's what I currently do.

My house gets too much shade so I have a rental garden plot for vegetables. This is in a broad shallow stream valley and is frosty in spring and fall. I've seen light frost there as late as May 22 in 2002 and as early as October 4. Otherwise it's a good site.

To avoid frost I start stuff as transplants near my house with exposure to only a portion of the sky. This works in cool temperatures when respiration requirements are not high. The last freezing temperatures in these spots occur sometime in March
and I don't worry much about it because before late March daily mean temperatures are too low for good growth anyway.

In the garden (which also can't be frequently attended because of remoteness) I avoid mulch in spring. It makes for more weeding but I can plant a week to ten days earlier without mulch because of the frost risk over mulch. This is esp. an issue for young potatoes which I really like but which require cool temperatures AND are as sensitive to frost as tomatoes. It isn't an issue for tomatoes and most warm season crops because the days are usually too cool for good growth prior to early May (2012 excepted). The latest I've had damaging spring frost even over mulch is May 11 in 2010, an otherwise very warm year. Potted transplants put on top of a mulch pile (oops) got nipped.

The other spring protection is to cover the soil with clear plastic. Clear plastic passes most radiation but blocks convection
( by the way, the pictures in textbooks showing IR being blocked by greenhouse glass panes are incorrect or at least misleading. Most of the warming in a greenhouse is due to blocked convection. Air warmed in the greenhouse can't rise through the glass or otherwise exchange with cooler outside air)
Covering PLANTS with clear plastic is very risky. In spring sunshine temperatures can be 30-40 degrees F warmer under clear unventilated plastic and experience has been that I'll eventually bust a sun forecast or be otherwise occupied and cook the covered plants. Clear plastic provides only a few degrees of frost protection at night. I find warm soil under clear plastic also provides several degrees of temperature boost OVER the plastic at night. That's very helpful in spring but harmful in summer. Weeds are also a problem under clear plastic
so I only use it in spring.

When I've needed to protect a few plants, the combination of clear plastic soil mulch and a cloth blanket has worked well.

In March 2012 the U.S. east of the Rockies had an exceptional warm spell. I decided to take a huge risk and plant one of my tomato plants in the already warm mellow soil March 15 2012. It got another ten days of early May conditions and set a few fruit. Then the frost protection trials started.

On March 27 a shallow arctic outbreak drove temperatures to near freezing (underdone by the models until 36H prior) The combination of cold with wind, and low dewpoints and dessication risk warranted piling leaves around the plant and a blanket. Radiation freeze conditions on March 30 and then again several days in the first half of April were an annoyance and warranted using the blanket again but not the leaves. The last risk was April 13. The plant had a clear plastic soil warming mulch. Potatoes that did not have the plastic mulch froze lethally Monday night April 2 when I failed to protect them. I plant most of my potatoes later because of frost risk and these were just a risky experiment. Warming soil for potatoes reduces yields a lot. They set tubers best in cool soil.

Subsequent mild and then warmer than normal weather allowed me to get my first tomato June 3, three weeks earlier than customary for this area.


I do not use sprinklers or water for frost protection. At my scale it's more trouble than it's worth.

I grow potted florist gardenias for my wife. These stay outside until early December and then move to an unheated garage or crawl space. They will tolerate a lot of chilling but not deep freezing. I keep them on the front porch in fall and early spring where they both look good and get a lot of IR radiation on calm clear nights. Like tree canopies my porch overhang provides a good temperature boost on calm clear nights. There is little benefit with advective freezes but these are rare before late November or after late March in DC. Spring seedlings get a similar treatment.

That's pretty much it. With three kids I don't
have time for more elaborate methods. I have a lot of defenses against cold. I don't have any against heat esp. the high dewpoint heat typical of the eastern half of the U.S.



Member Since: February 1, 2012 Posts: 19 Comments: 4867
4. Balwanz
4:13 AM GMT on December 19, 2012
Pertaining to the freezing point, some of us must think differently. In my thoughts are two freezing points, one is the national standard triple point and the other is the interface between such as tap water in contact with the atmosphere and its frozen form. I use this latter, with crushed ice, as the reference junction for thermocouple calibration. Thermocouples are too inaccurate for its small error to matter.

At the same time I'm aware that ice can begin to form at much lower temperatures. This doesn't matter because once it begins to for, its solid-liquid interface is back to the same 0 C.
Perhaps I'm wrong in this latter assumption, but I'm not concerned, I don't work with this situation.
Member Since: November 23, 2011 Posts: 0 Comments: 9
3. georgevandenberghe
10:45 PM GMT on December 18, 2012
Need to review my posts. First Prof. Grenci made the point first about frost being protective from phase change energy releases.

I've observed frost formation on exposed surfaces with 2M temperatures as high as 40F and it probably happens higher than that if the air is dry enough. 40F on dry not breezy nights is my rule of thumb for worrying about submelting temperatures at ground level esp. over mulch (which I don't do in spring for this reason)
Member Since: February 1, 2012 Posts: 19 Comments: 4867
2. georgevandenberghe
10:12 PM GMT on December 18, 2012
Good point about "freezing point". I'm among the 99% who slip and use it
often.

It would be interesting and simple to do a probability distribution of nighttime minimum temperatures and see if there is at least a small mode just below freezing in spring and fall due to the phase change energy effects. I've never done it.
Member Since: February 1, 2012 Posts: 19 Comments: 4867
1. georgevandenberghe
9:59 PM GMT on December 18, 2012
There is a second older definition of frost as the occurrence of subfreezing temperatures. I think it is more common in Europe where a "Frost" is an incidence of freezing temperatures, sometimes well below freezing. It does not refer to the formation of frost. But that's hairsplitting.

Many people think frost is what kills the plants. It is not. Frost actually provides some protection by releasing latent heat of evaporation and then latent heat of fusion. It's insulating effect otherwise is minimal.

Most "Frost tender" plant tissue freezes at between 30 and 31F. Plant fluids have sufficient solutes within them that they don't freeze at 32F. But for many species cooling below 30F results in certain freezing. On frosty nights whether they cool to that temperature depends enormously on sky exposure and a tree canopy can provide 5-10 degrees of frost protection compared with an open exposure. This happens because the canopy is a source of IR radiation countering radiative loss from the plant and conduction to the air.

Freeze protection by spraying with water sometimes works and sometimes is disastrous. It is most effective on calm nights where the dewpoint is close to the air temperature. On windy cold nights with an advective freeze the dewpoint can be well below the air temperature. Plant tissues will then stay near 32F while water continues to freeze on them as long as the sprinklers stay on. If they
fail or are turned off, tissues cool to the WET BULB temperature which can be a lot lower than the air temperature. If you use sprinklers they must stay on until the wet bulb temperature reaches freezing or the sun is well out countering evaporative cooling. The first time I read of this was of a description of a 1962 December Citurus Freeze in central FL. Trees that were sprinkled and had the sprinklers shut off at dawn were severely damaged by chilling well into the teens F. Trees that were not protected had moderate damage. Some trees were also damaged by limb breakage as large amounts of ice accumulated and broke them. However sprinkling is probably the cheapest way to bail you out of a radiative frost situation with very tender plants.

More later. I'll also post what I do to protect tender plants. Some very simple practices make a 5F difference in spring.


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About 24hourprof

Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.

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