An over-glorified thermos. A dewar is a bottle used to hold liquid nitrogen or liquid helium. It's generally made of silvered glass with a vacuum insulating the interior surface from the exterior.

When you fill a dewar with liquid nitrogen, the nitrogen boils and steams until the dewar cools down. Then, you can hold your breath and look down to see the liquid nitrogen at the bottom. But whenever there is a mist blocking the view, I always make the mistake of trying to blow it away. Bad plan! It's not smoke, it's steam.

Update: Thanks, hands, but I know it'a not really steam. See Webster 1913's second definition: "The mist formed by condensed vapor; visible vapor; -- so called in popular usage." I think it sounds best this way.

it's not steam, it's water vapor. the difference is that steam is produced by raising the temperature of water above its boiling point, or lowering the pressure until the water boils. this steam is usually invisible. the "steam" that appears around liquid nitrogen (or anything else that is cold in a warm environment, i.e. when you open your freezer) is visible water-vapor, which is created because the air near the object is cooled. cooler air has a smaller capacity to hold gaseous water, so all the water that the now-cooler air used to contain condenses into water vapor. once the air warms back up, the water vapor is re-absorbed into the air and the "steam" magically disappears.

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