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A Warm-Air Sandwich
Written By: Donna Rhodes | Issue: 2015/04 - April
Gardeners in this part of the country know not to plant before Mother’s Day. Even though April showers might bring warm days and May flowers, there is always a chance of a hard freeze this time of year. And so it was April 28, 1858, when the area experienced not only a hard freeze, but a monstrously disastrous one.
It was that very freeze that affected Silas McDowell (famous for his naming of Highlands) in two powerful ways. One, his apple crop at Sugartown Valley was decimated, and he was forced to plant a hardier crop, grapes. Two, he noted in subsequent frosts that mountain land above the valley 350 feet or more was warmer and dryer and didn’t suffer from the cold like lowlands did. That went against all logic. Everyone knew the higher the elevation, the cooler the temperature.
What was going on here?
He discovered what he called a “vast green ribbon” of land stretching across and above a perfectly horizontal frost line through the entire length of the mountains. He dubbed it a “verdant zone” or “thermal belt.” In late fall, trees within the belt were the last to turn. In the spring they were the first to green. Fruit grown within this zone were usually a guaranteed crop. Grazing animals fared well in these areas year round.
What causes a thermal belt? Valley land absorbs heat during the day. Hot air rises up the mountain. Cool air from the mountain circulates downward. Cold air above the mountain acts as a kind of lid to keep this exchange going. The air above and below the belt is cooler. Warm air is sandwiched in between, creating a semi-frost-free zone.
McDowell was so exuberant about his discovery that he wrote a poem to explain this unusual phenomenon, some of which is excerpted here:
The reason why a thing is so Is sometimes very hard to know.
…A case in point in April last
When the frost killed all our fruit and mast.
…Yet still the mountain tops and covers,
Preserved the verdure of their groves.
…And yet the books have taught most clear
That as you reach the atmosphere
The cold’s increased by calculation neat
As is one degree to three-hundred feet:
That these facts are so, our senses shocks, and we’d have you explain the paradox.
And make plain, & clear, the perplexing case,
Why a mountain’s top is warmer than its base.
For those living in this belt, perhaps there are exceptions to the Mother’s Day planting rule. Still, I will submit this disclaimer: plant at your own risk! I couldn’t bear it if my or Silas McDowell’s advice caused your aspidistra’s demise.
