The MOF&G Online
Do I Need to Worry about a Frost Tonight?
by Eric Sideman, Ph.D.
Those of us who raise strawberries are nervous wrecks during the last weeks of May and early June. Those of us who try to cheat on Maine’s short growing season and put out corn, cucumber, tomatoes, eggplants, etc. in late May are a wreck too. Then, come fall, those of us who cannot let summer harvests slip away may leave the Common Ground Country Fair early to get home and cover our plants. More years than not, the most common phrase I hear at the Fair (besides "The food is great!") is, "Do you think it’s goin’ to frost tonight?" The common answer is often, "Nah, the full moon was last week," or "I sure think so, 'cause the full moon is tonight."
What's up with the moon? Why worry about frost? And why can't someone just say for sure whether we’ll have a frost?
Some people attribute the weather to spirits or cosmic effects, but physical factors actually can explain events such as frost.
Chilling vs. Freezing
Cold damage to plants, like most physical or chemical stresses, is many sided. Tropical plants usually are susceptible to chilling: They can really suffer or even die, even before the temperature approaches 32 degrees. Last season proved that, with a spring that never warmed up, and some growers got caught sneaking cucumbers out when the nights were dropping into the low 40s. Chilling damage is less well understood than freezing damage, but it may be due to changes in some sensitive proteins. We do know that the tiny young roots of cukes die, then the plants cannot get enough water when the sun comes out; they wilt and dry even when the soil is wet.
Freezing is much more dramatic. The plant’s internal water freezes and consequent formation of internal ice roughs them up. Dehydrated plant materials, such as seeds, usually are not damaged by freezing, while actively growing tissue is the most sensitive. The actual damage that kills the plant is two-fold: Ice crystals may cause mechanical damage and break membranes, disrupting the cell organization; and the tissue may actually dry up because it contains no water, just ice. For reasons beyond the scope of this column, water tends to leave the cell and collect and freeze in the intercellular spaces. As this water leaves the cell, the water left inside the cell becomes a more concentrated mix of the salts and sugars in the plant, and thus is more and more resistant to freezing. In plants that resist freezing, water moves back into cells as the plant thaws. In nonhardy plants, the water tends to remain in the intercellular spaces outside the cells, so the plant tissue dies.
Will We Have a Frost Tonight?
Many factors contribute to frost. First and foremost is the air mass. Regardless of the phase of the moon or other factors, if a warm air mass is on top of us, we won’t get a frost. When the air mass is cold, other factors simply raise or lower the possibility of a frost.
The dew point temperature (the temperature at which dew begins to form) measured in the evening often indicates how low the temperature will drop on a clear night. Most local weathermen can provide you with the dew point. For example, if the temperature is 50 when you go to bed and the dew point is 40, I would not worry, but if the dew point were 33, I would be concerned.
Radiational cooling is another factor. Heat escapes from all materials, even air. If the sun did not continually warm the atmosphere, the atmosphere would just get colder and colder as it lost its heat to space. Plants lose their heat to the air, and the air loses its heat to space by radiational cooling. Some factors stop or reduce this loss. Wind is beneficial if the air temperature stays above freezing, because as plants lose their heat to the air by radiation, they can actually get colder than the air, just like a car that has frost on it in the morning even though the air temperature never went down to 32 degrees. On a windy night, the mixing of air against the plant tends to equilibrate the air and plant temperatures. Sometimes, windy nights prevent frost just because the warm and cold air mix, so the normally cold pockets never drop below 32 degrees. But if the temperature is below 32, then wind will not save plants; in fact, wind can be really bad. If conditions are wet and windy, the wind evaporates water in and on plants, and evaporating water takes heat with it (evaporative cooling). This is the same effect as having wind blowing on you and cooling you after you swim.
Cloud cover plays a big role in the probability of frost by reflecting back to earth the heat that would have been lost from the air to space on a clear night. Just as a blanket traps our body heat near us when we sleep, clouds trap heat and keep it from radiating back into space.
The moon, as everyone knows, plays a role too. This is not a cosmic effect but results when the heat of the sun is reflected by the moon. The phenomenon seems backwards at first, but because this heat is not enough to warm the earth, it does not prevent a frost. Actually, frosts are more likely the closer we are to a full moon, because the heat reflected by a full or nearly full moon is enough to evaporate the water in the upper atmosphere that otherwise would have reflected the sun’s heat back to earth. When the water evaporates, heat escapes and the air gets colder.
Covering plants is the most common way to protect them from frosts. Sheets, blankets, floating row covers or other materials will reduce heat loss mostly by reducing evaporative cooling and radiation loss. Eventually the temperature under the cover will be the same as the air, but it will take longer to drop to that point, and we hope that morning comes and the temperature warms up before too much heat is lost.
Large-scale growers use overhead irrigation instead of blankets. The warmth of water sprayed on the plants plays some role in preventing freezing, but that is a small part of the story. If the air temperature drops below 32, the water will freeze, and that freezing is what actually protects the plants. In the morning after a cold night, when I have been up most of the night running the irrigation, the beauty of my strawberry field covered by nearly an inch of ice helps keep me from being really cranky.
Ice formation prevents damage to my strawberry blossoms because open blossoms can tolerate temperatures down to about 27 degrees (earlier stages of blossoming can tolerate even lower temperatures), and the ice that forms on them from irrigation water never goes below 32 degrees--as long as liquid water is continually applied and continually freezes. Molecules of liquid water have more energy than molecules of solid water, and when the liquid transforms into a solid, this heat is give off. That heat given off keeps the ice at 32 degrees.
Of course, the occurs when ice becomes liquid: Heat from the environment is required to make that change, even though the air temperature may not change from 32 degrees. Melting ice can actually chill the plant below 32 degrees, so you have to keep the irrigation running until all of the ice is melted.
Is there going to be a frost tonight? Check the dew point, cloud cover, wind and moon.
About the author: Eric is MOFGA’s director of technical services. You can address your questions about farm and garden crops to him at firstname.lastname@example.org or by calling 946-4402.
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