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A touch of frost

September 22, 2010
Adirondack Daily Enterprise

Cooler weather has arrived. I saw proof of that recently, when I stopped to visit with friends who live a few miles outside of Saranac Lake. Early that morning, temperatures at their home had fallen to about 32 or 33 degrees F for a few hours, and even though, by and large, their vegetable plants and flowers seemed to be okay, it was plain to see that a light frost had hurt their most frost-sensitive plants; their squash vines for example. Unfortunately, for several others that I know living in the Adirondacks, it was the end of the season for all but the heartiest of their frost tolerant vegetables, greens, herbs and flowers.

Where I live, just outside of Malone, the morning dews have been heavy at times, but I have yet to see even a single patch of frost on a nearby garden, lawn or in a hay field. Nonetheless, I know that it's only a matter of time now. As fall arrives, so does frost. It will strike even the mildest locations before long.

According to the Farmers' Almanac, the average first frost date for Lake Placid is Sept. 11. Cornell Gardening Resources show that the average first frost for this region can be expected before Sept. 10 at the highest elevations and between Sept. 10 and Sept. 30th for most of us. For the Champlain Valley region of Clinton County and northeastern Essex County however, the first frost shouldn't arrive until sometime between Sept. 30 and Oct. 10.

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One of the questions I am often asked this time of year is "Exactly what is frost and what causes it?" Well, I'm not a scientist and the answer isn't nearly as simple as the question. But I am willing to give it a try.

Let me start by asking, "Why does it warm up during the day?" or "Why does it cool down at night?" You would think that the answer would be, "Because the sun warms the air during the day." Right?

No. It's the ground surface that heats up during the day. That is, of course, unless the ground is snow covered. The earth absorbs the sun's radiant warmth and the heat that is absorbed into the ground then radiates into the air. Have you ever sat on a rock ledge that has been exposed to the sun for several hours? It feels warm, doesn't it?

As night falls, heat that has been stored in the ground continues to be released into the air. As the ground surface cools, air that is in contact with the ground cools too. On still nights, when skies are clear, heat can easily escape into the upper layers of the atmosphere causing temperatures to fall rapidly. But, when it's cloudy, that cloud cover works as a barrier that absorbs the rising heat and then radiates it back into the air below, slowing and reducing energy and temperature loss. And since air is an effective insulator, as you get away from the earth's surface, the cooling effect is lessened. Wind, however, can help keep the surface temperature warmer; by stirring up the air up, thereby creating more uniform temperatures.

The temperatures we hear about as daily highs and lows, the temperature readings we see on our thermometers; these are not surface temperatures. Even if the air temperature just 5 feet above ground remains above freezing, the temperature at the ground surface will likely be several degrees colder. Cold air is heavier than warm air and collects near the ground. If you have ever experienced the loss of frost sensitive garden vegetable plants or annual flowers even though the air temperature was well above 32 degrees F, this is why.

Dew point temperature is even less understood. The dew point is the temperature at which moisture in the air turns to liquid. Take a cold beverage out of the refrigerator and set it down for a few minutes and moisture will form on the bottle or can. That's because moisture in the air is condensing directly onto the surface. In other words, the dew point temperature in the room is higher than the temperature of the bottle. Fog is made of tiny water droplets suspended in air. Dew is made of larger droplets that occur on a surface.

Frost forms when a surface's temperature falls to freezing and the droplets of water that are in direct contact with that surface turn to ice. Ground frost occurs when this takes place at the earth's surface. The process is stepped up as water vapor that comes into contact with already frozen droplets also freezes.

Frost tends to form on surfaces that lose heat more quickly, first. For that reason frost will form on the windshield of your car before it forms on vegetation.

Frost protection measures can be taken and will depend on how cold it gets and for how long. Even so, whatever measures you take, your growing season will soon be coming to an end.



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