Liquid nitrogen is really amazing. It has a boiling point of -196 degrees Celsius (-321 degrees Fahrenheit), so it must be kept incredibly cold in order to exist as a liquid. In a vacuum chamber, the reduced pressure allows the nitrogen to boil more quickly. However, the evaporation process cools down the remaining nitrogen, allowing it to exist as a liquid. When the temperature is brought down to -210 degrees Celsius (−346 degrees Fahrenheit), nitrogen will freeze and become a solid that looks like glass.
The solid nitrogen then tries to force itself into a more tightly-packed configuration, and as the atoms begin the chain reaction of rearranging themselves, it creates flakes of snow that get shed off in the process.
Seeing solid nitrogen is really rare, and seeing nitrogen snow is incredibly amazing. Check out this video by ChefSteps: