Transcript:

The Recipe for RainAmbience: thunder, rainWhenever it rains, the water in clouds must first freeze. But it turns out that cloud water is able to freeze at a higher temperature than pure water. I'm Jim Metzner and this is the Pulse of the Planet.Vinatzer: The recipe for rain is that we need high humidity in clouds; so we need water. Then we need impurities around which the water can freeze, and then we need cold temperature.That's Boris Vinatzer, an Associate Professor in the Department of Plant pathology, Physiology & Weed science at Virgina Tech. Vinatzer: For a long time, atmospheric scientists wondered what are the impurities in clouds that allow water to freeze at the temperatures that we find in clouds? And there was a scientist in the 1970s, who thought these impurities may come from soil. He found that soil that was rich in decaying material allowed water to freeze at relatively high temps. Soil without decaying plant materials did not. That as the first step in realizing maybe plants could be the source of some of the impurities in clouds that allow water to freeze there.OK, but then how do these soil impurities get up into the clouds?Vinatzer: When water evaporates from the ground, from a street, from plants, it takes bacteria with it. That's big area of research that still needs to be explored.We'll continue to explore the mysteries of rain in future programs, finding out what exactly in clouds gets water to freeze and how it works. Pulse of the Planet is made possible in part by the National Science Foundation; I'm Jim Metzner.