From the Lee Side
Frost Gets a Bad Rap
Posted by: Lee Grenci, 1:40 PM GMT on May 12, 2013 +5
During the growing season, the National Weather Service routinely issues frost advisories when "widespread frost formation is expected over an extensive area. Surface temperatures are usually in the mid 30s Fahrenheit." For this blog, I define frost as the formation of ice crystals on the ground and other surfaces (see photograph below).

The reference above to temperatures in the mid-30s might give you pause. Keep in mind that official air temperatures are measured at roughly five feet above the ground. On clear nights with light winds, the lowest air temperature occurs in a thin layer in contact with the earth's surface. In this lowermost layer, the air is cooled dramatically by conduction with the chilly ground. So it's quite plausible for the official air temperature to be in the mid 30s, while the air temperature right next to the ground is much lower. Meteorologists refer to this vertical temperature profile as a temperature inversion. In such situations, the coldest air is in contact with the ground and increasingly "warmer" air (more accurately, air that's not as cold) lies above the ground.


A photograph of frost. No doubt that low temperatures and frost go hand in hand, so plant damage can occur on clear nights with light winds, even though the formation of frost releases latent heat of deposition (details to follow). Courtesy of Emmanuel Boutet.

When it comes to the formation of frost, the temperature of the ground is most important. If the ground becomes sufficiently cold, frost will form as invisible water vapor in the air next to the ground deposits onto grass, plants, etc.

I confess that I still throw my bad-call brick at my television when I hear forecasters cast frost in a bad light. "Frost kills plants," they'll say. And, of course, there are frequent references to a "killing frost." As a result of this kind of propaganda, the mere mention of a frost advisory during the growing season has farmers and gardeners scrambling. If you read the frost advisory (below) issued by the National Weather Service in State College early this morning (May 10, 2013), wouldn't you get the impression that frost was the arch enemy of farmers and gardeners?



A frost advisory issued by the National Weather Service in State College, PA, at 3:34 A.M. EDT on May 12, 2013.

Contrary to popular perception, frost is Nature's last line of defense of tender plants. Yes, you read the last sentence correctly. Allow me to elaborate.

It takes energy to cause ice to transition directly to water vapor (an invisible gas). For example, on cold, sunny days during winter, snow on the ground can decrease as ice sublimates (the formal meteorological term) directly to water vapor. When the reverse process occurs (water vapor directly deposits on cold objects as frost), energy must be released in order to maintain a balanced energy budget. This release is formally called latent heat of deposition (in this context, "latent" means "hidden").

The "energy staircase" for water is shown below (larger image) (courtesy of, and copyright by, Penn State's online certificate program). Ice is the lowest energy phase of water. It takes 80 calories to melt one gram of ice (a calorie is the amount of energy required to increase the temperature of one gram of water by 1 degree Celsius). On the flip side, when one gram of water freezes, 80 calories of energy are released (latent heat of fusion). It takes 600 calories to evaporate one gram of water. Clearly, water vapor is the highest energy phase of water. When one gram of water vapor condenses, 600 calories are released as latent heat of condensation. When water vapor deposits onto the ground as frost, 680 calories are released as latent heat of deposition.

Orchard growers apply a similar principle when they spray their tender crops with water, anticipating that latent heat of fusion (freezing) will keep their plants warm enough to survive the night. Indeed, it takes energy to melt ice, so, in order to maintain a balanced energy budget, energy, in the form of latent heat of fusion, must be released when water freezes (revisit the energy staircase below).



The "energy staircase" for water. Larger image. Courtesy of, and copyright by, Penn State's online certificate program.

Of course, one must continue to spray in order to take advantage of latent heat of fusion, increasing the risk that ice will eventually become too heavy and damage the crop. When I was a member of the Weather World team (Weather World is a daily, 15-minute weather magazine show produced at Penn State), I produced a feature on the research of a graduate student in the Department of Agriculture (back in the late 1980s, if my memory serves me). His research dealt with the protection of tender strawberry plants during cold, spring nights.
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Essentially, the temperature of the plants was monitored by a computer, which sent electronic signals to turn on a fine spray when the temperature of the plants approached the danger zone, which is the temperature at which ice crystals start to form inside plants (ice crystals forming inside plants cannot be good for their well being). When plant temperatures were out of the danger zone (via the release of latent heat of fusion), the computer would turn off the spray. The computer continued to monitor the plants throughout the night and would repeat spraying if needed. In this way, the weight of the accumulating ice was kept to a minimum, thereby reducing the potential damage to plants by the sheer weight of the accumulating ice.

Are orchard owners crazy? Facing the loss of their crop, would they intentionally make matters worse? I don't think so. All orchard growers need to know is that spraying water on plants at dangerously low temperatures gives them some protection.

Not surprisingly, I believe that frost gets a bad rap. To the contrary, frost delays the formation of ice crystals inside the plant via the release of latent heat of deposition (if plant cells freeze, they typically rupture). Of course, frost has its limitations. If plants are exposed to dangerously low temperatures for too long, not even frost can stave off the inevitable (plant cells freeze and rupture).

Once ice crystals form inside plants, the damage is done. In these cases, all we can see is frost on the outside of the plants. And so people jump to the conclusion that frost is a killer (a "killing frost" is common language). Frost is not a killer of plants. It is a protector. It's prolonged low temperatures at night and the freezing of plant cells that kills plants.

Lesson learned: Frost is Nature's last line of defense (via the release of latent heat of deposition).

Yes, in my view, frost always gets a bad rap.

Lee
Categories:Thermodynamics Synoptic Meteorology
Updated: 10:48 AM GMT on May 13, 2013   Permalink | A A A
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Sloppy Language Can Breed Sloppy Science
Posted by: Lee Grenci, 5:35 PM GMT on May 07, 2013 +17
Perhaps I'm getting more eccentric in my old age, but I still cringe at some of the sloppy language I hear on television. In this world of shorthand texting, sound bites, and headlines that border on hyperbole, I sometimes feel like a dinosaur when I complain about sloppy language. So be it. I have bigger fish to fry today.

For whatever reason, the phrase "close proximity" has become popular lately, and it's driving me nuts. According to Merriam-Webster, ...
Categories:Synoptic Meteorology Mini-Blog
Updated: 2:20 PM GMT on May 11, 2013   Permalink | A A A
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Dramatic Dryness in Reno, Nevada
Posted by: Lee Grenci, 2:33 PM GMT on May 04, 2013 +4
The fires, winds, and low relative humidity in California made national news this week. I've been following the weather pattern in the West, and I sat up and took notice while reading the Area Forecast Discussion (AFD) issued by the National Weather Service at Reno, Nevada, on Wednesday afternoon (May 1, 2013):

VERY DRY AIR HAS SETTLED OVER THE REGION TODAY. IN FACT, THIS MORNING`S SOUNDING FROM RENO/KREV INDICATED THAT WE MAY HAVE HAD ONE OF THE DRIEST A...
Categories:Fire Weather Mini-Blog
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Tornadoes and Anafronts
Posted by: Lee Grenci, 7:19 PM GMT on April 30, 2013 +2
In the comments below my last blog (snow occurring at temperatures in the 40s), Thrawst asked me an interesting question about anafrontal cold fronts and tornadoes. I thought the question was worthy of a blog, so here goes.

By definition, a synoptic-scale cold front qualifies as an anafront when there's upward motion, clouds, and precipitation relatively far back into the cold air mass behind the front. For example, the cold front stretching from the Grea...
Categories:Mesoscale Forecasting Synoptic Meteorology Severe Weather
Updated: 1:24 PM GMT on May 03, 2013   Permalink | A A A
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Snowing with Temperatures in the 40s
Posted by: Lee Grenci, 1:19 PM GMT on April 24, 2013 +8
When it comes to predicting snow or rain showers during spring, weather forecasters need make an adjustment from their winter mindsets. For starters, check out the meteogram for Syracuse, New York, on Friday, April 20 (below). Note the 20Z observation (4 P.M. EDT). Light snow was falling at a temperature of 40 degrees, exposing the fallibility of the 32-degree surface isotherm as the sole indicator of precipitation type, especially during spring. For the record, I'v...
Categories:Weather Forecasting Synoptic Meteorology
Updated: 4:07 PM GMT on April 24, 2013   Permalink | A A A
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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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