Whitesburg KY

First frost is sign of winter’s approach

The change of season is definitely underway and as mid September rolls quickly toward October, you know that winter is not far off. One of the first indicators of winter’s approach is the first frost.

There are two requirements for the formation of frost. There must be sufficient water vapor in the air, and the temperature must fall below freezing. The dewpoint, or more precisely the dewpoint temperature, is the temperature at which the water vapor in the atmosphere condenses – changes from an invisible gas into liquid water – on exposed surfaces. Warm air can hold more water vapor than cold air. As the temperature falls through the night-time hours, the air is able to hold less and less moisture. When the dewpoint temperature is reached through the cooling process, condensation occurs. When the air is below freezing, then instead of condensation into a liquid, the water vapor freezes, and becomes a layer of ice crystals. Sometimes, very beautiful patterns are laid down on windows as the frost forms.

Very often on calm, clear nights, the temperature near the ground can be three or four degrees cooler than temperatures several feet above the ground. The thermometer might read 36 degrees on top of a fence or near a roof, but near ground level, it might read 32 degrees. This is because the ground loses heat quickly. So water vapor in the air, because it is so cool, condenses as frost. When the air temperature is just near the freezing mark, frost tends to form on glass, such as car windshields or windows, metal or rock surfaces first because these tend to lose more heat quicker. So usually, a car windshield will frost over before vegetation does.

Cold air is dense and it tends to sink. On a calm night, cold air will tend to sink into low spots. That’s why valleys can be much cooler than the surrounding area because the cold air from the hills sinks into the lower areas. So when the cold air starts sinking, frost will form quicker in the low spots than the high ones. It’s not uncommon for some valleys to be cooler by 10 degrees or more, thus more frost tends to form in these low areas. Also, some soil types, such as sand, retain less heat than others, and frost tends to form quicker on these.

Typically, latitude is the primary determiner of the first freeze. Areas farther to the north reach 32 F sooner than areas south. Our area’s geography is complex and the length of the “growing season” also varies with terrain. Valley locations can receive their first freeze before the ridges. More often in the fall than any other season, our area experiences a split in temperatures between the ridge tops and the valleys at night. This occurs on clear nights with calm winds. The earth’s surface is able to radiate heat faster. Since cold air sinks and warm air rises, the cold air “drains” into the valleys. On some nights there can be a difference of almost 20 degrees.

For the Jackson National Weather Service (NWS) office, with an elevation of 1,381 feet, the median date (meaning half of the dates are earlier and half are later) for a temperature of 32 F is Oct. 26. For West Liberty, with an elevation of 934 feet, the median date for 32 F is Oct. 6, nearly three weeks before Jackson. For a station south of Jackson, London, with an elevation of 1,212 feet, has a median date for 32 F of Oct. 13, still almost two weeks earlier than Jackson.

At the end of the growing season when the first frost is expected, the National Weather Service will issue a frost advisory to alert the public to take protective actions to protect tender vegetation. Once a killing frost or freeze has occurred, no further frost advisories will be issued until spring when early warmth may trigger plants that can be damaged by frost to begin growing.

For more weather information, visit the National Weather Service in Jackson at http:// www.weather.gov/jacksonky.

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