I think you have two choices for the best way to cast footwear impressions in the snow. They are:
Sulfur casting. Sulfur casting is achieved through the melting of powdered or prill sulfur to a liquid state. The liquid is removed from the heat and as crystallization occurs, the liquid is rapidly poured into a pour channel leading to the impression.
Dry casting. Dry casting is the process of applying alternating layers of powdered dental stone and misted water into an impression. Dental stone is sifted into the impression in shallow layers, typically not more than 1/8" in depth. The water is then misted on the dental stone to absorb into the casting material. This process is repeated until a firm base is created over the impression. Once this base has cured to a sufficient depth, a normal mixture of dental stone can be poured over the base to strengthen it.
Two good articles on casting in the snow are (the first one addresses the different properties that can occur in snowpacks):
Hey thanks i was also looking for this answer and the way you have explained it by differentiating into different kind of castings is great. Thanks again. Can you also the temperature limits of Sulfur and Dry castings ?