Listener Question: Supercool Soft Drinks
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Hi I'm Dave Thurlow for the Mount Washington Observatory and this is The Weather Notebook. Today we hear from a listener in nearby Madison, New Hampshire:

"My name is Richard Laurie and I have a question for you. I notice on several occasions I happen to leave a, say, Diet Coke in my vehicle when I park it at night. When I get back in the vehicle in the morning, I look at the Diet Coke and it's still liquid. Yet, when I opened the top up, it almost instantly forms ice. I don't know what causes this. Is it the carbonation in it? Is it the cold air suddenly hitting the liquid or what? I listen to you on WMOU just about every morning on my way to work. Thank you."

Here's the deal Richard. Soft drinks are bottled under pressure to keep in the carbonation. This forced pressurization forces the freezing point of the soft drink inside to go way down, way below 32°F. So after a long night in the car the diet coke is what's called supercooled. Its temperature could be 20 degrees but it's still liquid. Now when the bottle is opened the air pressure, and the freezing point, reverts to normal. So in an instant you have ice.

The same process makes skates glide on ice. The skates put pressure on the ice, therefore lowering the freezing point under the blades, the ice melts, the skates glide on the film of liquid water, and then the water refreezes when the pressure is released, as the skate goes gliding by.

Thanks for the question. If you have a weather question you'd like us to answer, give us a call at 1-888-RAIN-001. Thanks to Subaru and the National Science Foundation.