An unexpected run of freezing overnight temperatures in California may have damaged nearly a billion dollars' worth of unharvested citrus fruit. Farmers rushed to pick what they could before the cold snap hit, and have been staying up all night in an effort to keep their orchards warm. How do you keep fruit from freezing?
With irrigation systems, wind machines, and, er, helicopters. Citrus fruit that's exposed to freezing temperatures for more than four or five hours will become slushy on the inside and can no longer be sold. Oranges and lemons tend to freeze when the temperature drops below 28 degrees—the exact freezing point depends on the fruit's sugar content—but temperatures at or below 20 degrees may even damage the trees themselves. Since a few degrees can make a million dollars' worth of difference, farmers will go to great lengths to warm their groves, spraying them with warm water all night long, blowing warm air down onto orchards with towering fans, or even burning piles of peach pits.
Warm water dispensed through irrigation systems can help raise the temperature of a particular grove: As the water cools, heat is released. The wind machines—which look like 40-foot-tall windmills—take advantage of a common weather phenomenon in California's citrus valleys called an inversion layer. This results in a canopy of warm air high in the atmosphere that traps colder air closer to the ground. (The inversion layer is also responsible, in part, for keeping Los Angeles swimming in a miasma of smog.) By blowing the higher air down toward the surface, farmers can sometimes warm their trees enough to avert a freeze. But in the absence of inversion conditions, all the wind in the world would do no good.
Farmers without wind machines can turn to helicopters, which also push warm air down toward the earth. Their high cost per hour (as much as $1,650) and the limited area they're able to warm makes them a choice of last resort. Farmers who never installed wind machines will occasionally send out choppers to save a small area of a grove that might otherwise perish.
Eighty percent of citrus farmers use wind machines to warm their groves, and the California Farm Bureau Federation estimates that the statewide cost of protecting trees and crops this winter is already nearing $95 million. Is it worth it? A freeze of only three days in December 1998 destroyed 85 percent of California's citrus crop, a loss valued at $700 million.
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Explainer thanks Shirley Batchman of California Citrus Mutual and Dave Kranz of the California Farm Bureau Federation.