Why does snow crunch when it gets stepped on?

As you know snow is made of ice crystals and each ice crystal can contain up to six points.  Many snowflakes are comprised of multiple ice crystals.  In between each one of those ice crystals is a pocket of air.  When you step on snow you are compressing it below your feet.  As the snow is compressed that layer of air in between the crystals is forced out and we hear the sound of the ice crystals breaking. 

Another ingredient for squeaky snow is cold temperatures.  Snow contains less moisture when the air is cold, thus allowing for more air to get trapped between each ice crystal.  Even though temperature is a factor, it is the structure of the snow that plays the largest role when it comes to “crunch” time.