Can Rain Start a Forest Fire?

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Gardening Web sites tell you not to water during the day for fear of scorching your plants. Some have speculated that raindrops might even be able to act like a magnifying glass and focus sunlight to set a leaf on fire. Are they right?

A group of scientists in Hungary and Germany set out to discover the truth (their findings appear in New Phytologist). They started by placing small glass spheres on maple leaves and exposing them to sunlight. The leaves were quickly sunburned. However, when the glass spheres were replaced with water droplets on both maple and ginkgo leaves, there was no visible burn. Water drops are usually ellipsoidal in shape and are less able than a sphere to concentrate light. In addition, the ellipsoidal shape is able to intensify sunlight only when the sun is low in the sky—when the light is not so strong—and the water itself provides cooling.

There was an exception, though, with plants that have small waxy hairs covering their leaves, like floating ferns. The hairs are hydrophilic and water is held in spheres above the leaf's surface. Like the glass spheres, these water droplets can intensify sunlight enough the burn a leaf. The scientists say that if water droplets accumulated on a parched plant, sunlight could theoretically spark a fire. They write, "however, the likelihood of this is considerably reduced by fact that after rain the originally dry vegetation becomes wet, and as it dries water drops also evaporate. Thus, claims of fires induced by sunlit water drops on vegetation treated with a grain of salt."

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