Manipulation of spheres of ice and observations of ice crystals colliding with a fixed crystal under conditions of controlled temperature and vapor pressure have been employed to determine the limiting conditions for the aggregation of ice crystals to form snow flakes. It is shown that the amount of aggregation is strongly dependent upon environmental vapor pressure and temperature. At ice saturation, no aggregation occurs at temperatures below −25C and aggregation increases and becomes a maximum as 0C is approached. At vapor pressure less-than-ice saturation no aggregation occurs at temperatures below −4C and aggregation increases rapidly as 0C is approached. Under conditions of supersaturation with respect to ice, aggregation occurs at all temperatures. These results are best explained by the existence of a liquid film on the surface of ice at temperatures below 0C where the thickness of the film is a function of temperature and vapor presuure.