Phase+Changes

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= Phase Changes  = =Curiosities: How does snow sublimation work?= **Q** It seems like whenever it’s really cold and dry, there’s less snow on the ground, but it’s not melting. Is it evaporating? **A** Not exactly, but close. The snow is “sublimating,” through a process that is similar to evaporation. “Whenever there is an interface of air and water, either liquid or solid, you have molecules trying to leave the water,” said Steve Ackerman, a professor of atmospheric and oceanic sciences at the UW-Madison and one of the “Weather Guys” in the Wisconsin State Journal and Wisconsin Public Radio. The constantly vibrating molecules in a liquid or solid are restrained by other forces, but a water molecule will escape the water and enter the air when it moves violently enough. “We call that evaporation from a liquid, and sublimation from a solid,” Ackerman said. Sublimation is obvious in your freezer, where the ice cubes tend to shrink with time. It’s also visible in snowfields that eventually waste away without melting. “You notice there’s no liquid water, none of the hard ice that results from melting and refreezing, but the snow still shrinks,” Ackerman said. On humid days, water vapor in the atmosphere may be captured by snow, adding to its mass, although not to its volume. Low relative humidity, as often found on cold days, speeds sublimation, and so does strong sunlight. Snow absorbs much of the near-infrared portion of sunlight, and that energy accelerates its water molecules, speeding sublimation. “This helps explain why a shaded snowfield stays around longer,” Ackerman said. //— Produced in cooperation with University Communications//

This recipe uses snow and salt to freeze the ice cream (an example of [|freezing point depressions] ), but this recipe doesn't involve actually eating the snow. This is a great recipe if you want to play with the snow, but don't consider it clean enough to eat.
 * Snow Ice Cream without Eating Snow **
 * Fill a gallon freezer bag halfway full with snow or crushed ice.
 * Add ~6 tablespoons of salt to the snow. This will lower the freezing point of the snow so you can freeze your ice cream.
 * In a quart ziploc bag, mix:
 * 1/2 cup half and half
 * 1 tablespoon sugar
 * 1/2 teaspoon vanilla extract
 * Zip up the quart bag, squeezing out any excees air, and put it inside the gallon bag.
 * Close up the gallon bag, again removing any excess air since it makes mixing difficult.
 * Wear gloves or else put a dry kitchen towel between your hands and the snow/salt bag. Squish the bags with your hands until the ice cream is frozen.
 * Remove the smaller bag and enjoy your frozen treat!