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40 Below
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Hi, I'm Dave Thurlow from the Mount Washington Observatory and this is The Weather Notebook. Today I'm going to talk about the significance of 40 degrees below zero. Now you're probably saying to yourself "Oh boy that sounds fascinating", but curious things happen at forty below zero, give or take a few degrees. Scientifically, 40 below is a temperature worthy of note.

First of all, am I talking about 40 below fahrenheit or forty below celsius? Well that's a bit of a trick question because the answer is both. Forty below is the only point where fahrenheit and celsius are the same, where the two scales cross.

It's also near the point at which mercury freezes. Mercury is the most commonly used fluid in thermometers and it's fine for temperatures above minus forty. For colder temperatures a different fluid is used, usually alcohol. Though mercury is more precise, alcohol doesn't freeze until minus 200 fahrenheit.

Interestingly enough, if you figure the average of the record lows for each state in the country, it comes to minus 40.

Minus 40 is also the point where it's no longer safe to expose any skin to the cold. At this temperature, or windchill temperature, skin can freeze almost instantly. This was determined, believe it or not, by a bunch of scientists who stood around in Antarctica until their skin froze.

Forty below also represents the ultimate freezing point of water. Microscopic drops of water can stay unfrozen down to around minus forty, seventy-two degrees colder than what we all know to be freezing for big water. This means that even clouds and fog are made of tiny ice crystals at this frigid temperature, but can actually be made up of drops of water at temperatures well below freezing. Forty below -- you might never feel it but at least now you.

The Weather Notebook is underwritten by Subaru with major support provided by the National Science Foundation.