Ever notice in winter that the temperature is sometimes warmer on Mount Sexton than in Medford, even though Sexton is at higher elevation?
Or perhaps you've been up on Roxy Ann, looked down at the valley and noticed that smoke or steam rises to a certain height and then levels out.
These are signs of a temperature inversion. The inversion traps cold air near the ground, so it's actually warmer above the inversion layer.
For more on inversions, see Page 140.
This phenomenon also helps explain why wind machines can keep fruit from freezing in local orchards.
"These inversions tend to have a ceiling, and above that point — 20 to 50 feet in the air — it's actually warmer," explains Jon Meadors, superintendent of orchard and vineyard operations at Hillcrest Orchard in east Medford.
The wind machine, with a tower that is 40 feet tall, reaches above the layer of trapped cold air. At 18 feet in length, its blade stirs up both the trapped cold air and the warmer air above and mixes them together. The result is that the air gets just enough warmer to keep fruit from freezing.
"They are pretty effective," Meadors says.
One machine can cover 8 to 12 acres of orchard. It will move air laterally up to 700 feet.
These machines work best on nights of mild frost, says Meadors. If it is really cold, orchardists need to supplement the wind machines with another frost protection method. These other methods include under-tree sprinklers and heaters fueled by propane or diesel.
But even then the wind machines play a positive role.
At Hillcrest they've found that when using heaters and wind machines simultaneously, fuel consumption by the heaters drops by about 75 percent.
Under-tree sprinkling works with wind machines because it puts ice along the ground and on lower branches, and the freezing action gives off enough heat that the wind machines can then effectively circulate. And the under-tree system uses about 50 percent less water than over-tree sprinklers.
Hillcrest has 12 wind machines. Two are electric and the rest are run by propane or diesel engines.
It also uses diesel-fueled heaters and under-tree sprinkling systems as supplemental heat — no smoky smudge pots anymore.
Wind machines have actually been around on the local scene for half a century.
The first one was introduced here in 1956. It's powered by electricity, and it still works.