Several mechanisms cause air parcels to rise in elevation, thus promoting cloud formation and precipitation.

Credit: Pearson Prentice Hall, Inc.

Precipitation requires two key ingredients: adequate moisture and atmospheric lift.  If an air parcel with a given amount of moisture is forced to rise, the air will expand and cool as the parcel gains elevation.  Cooling temperatures increase the relative humidity and cause water vapor to form clouds and precipitation.  There are a number of mechanisms that promote atmospheric lift.  The distribution of snowfall during a storm will vary depending on what mechanism is driving the lift.  The orographic effect accounts for the majority of winter snowfall in the U.S., and thus snowfall is typically deepest in mountain ranges that see favored orographics, which depends on wind direction.  Frontal passage wedges air masses upwards, which distributes snowfall in bands that coincide with the front. Diverging air aloft and converging air at the surface, also called dynamic forcing, is also a major source of atmospheric lift.  This can be caused by low pressure systems or jet stream behavior.  Dynamic lift and frontal passage typically enhance snowfall across a region with less influence from mountain topography.  Lastly, convection produces a patchwork of rising parcels with a hit-or-miss pattern of snowfall that depends on where surface warming triggers convective cells.  Read more to explore the various sources of atmospheric lift.