Can Rain Start a Forest Fire?

At Smithsonian, Sarah Zielinski notes the proverbial wisdom among some gardeners that watering plants on scorching hot days can start fires because droplets of water can focus light like a magnifying glass. Scientists decided to test this idea:

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 should…be treated with a grain of salt.”

Link | Photo: US Department of Health and Human Services

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I'm not so sure. The shape matters- how tightly the sunlight is being focused.

The total amount of sunlight hitting the leaf is of course constant.

If a small droplet and a large droplet focus light onto the same area, the large one will produce higher temperatures, yes. But it the small droplet focuses the light more tightly, it might generate higher temperatures in a smaller area, and thus start a fire that can spread on its own to the rest of the leaf.

Also, smallerdroplets are more spherical than larger ones (surface tensions vs. gravity), so the lensing properties definitely vary with size.
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What really matters, surely, is the diameter of the droplet. To collect enough sunlight to bring something to ignition temperature would take a fairly large area presented to the sun - I don't think that's likely with a droplet. Sometimes large droplets will form, pooled in waxy leaves, but they're not lenticular and won't focus the light into a spot.
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