Saturation vapor pressure is the amount of pressure exerted by water vapor in the air when the air is saturated. Air with a warmer temperature can hold more water vapor, so the water vapor will exert more pressure. The amount of pressure exerted , however, is different over ice than it is water for temperatures below freezing. The following two graphs show how saturation vapor pressure over ice and liquid water change with temperature.
This first figure shows the saturation vapor pressure over liquid in red and over ice in blue.
This next figure shows the difference in vapor pressure over ice and over liquid. This biggest difference occurs near -13 degrees Celsius.
This property of water vapor, saturation vapor pressure over ice and liquid are different, plays a major role in snowflake formation. Water vapor will travel from areas of high vapor pressure to low areas, thus moving from liquid droplets to ice crystals. The water vapor then deposits on the ice crystals causing the ice crystal to grow in size. When the saturation vapor pressure gradients are greatest, around -13 degrees Celsius, snow crystals grow the quickest.
The type of snowflake that forms depends on the saturation vapor pressure and temperature. The following image shows the types of flake that generally form with different conditions.