Transcript:

To Freeze or Not to Freeze Ambience: Thunder, rainThis week we'll be learning some of the secrets of rain, for example, for it to rain, water in clouds first has to freeze! I'm Jim Metzner and this is the Pulse of the Planet.Vinatzer: When it rains, each raindrop starts with an ice crystal. In the clouds, it must get very cold for rain to form. That's Boris Vinatzer, an Associate Professor in the Department of plant pathology, Physiology & Weed science at Virginia Tech. I asked him if water has to freeze before it can precipitate out of a cloud, how come it rains instead of falling as snow or sleet?Vinatzer: In the summer, it does not snow, although rain drops start as an ice crystal, because when the ice crystal starts falling, it falls through the atmosphere that is warmer and it will melt.The recipe for rain is 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.So a cold temperature isn't enough to get water to freeze, you need something in the water to get it to start to crystalize.Vinatzer: Pure water does not freeze at 32 degrees Fahrenheit. To start, water crystals need a nucleus some kind of impurity around which water molecules start attaching and then start growing into the ice crystal. If they cannot attach to any solid support, the crystal cannot start. Now it turns out that the temperature inside of clouds may not be cold enough to freeze water even with an impurity, and yet the water still freezes. We'll find out why in future programs.Pulse of the Planet is made possible in part by the National Science Foundation. I'm Jim Metzner.