Snow depth is the total depth of snow and ice on the ground at the time of observation, including older snow that fell days or weeks earlier. It differs from snowfall, which counts only the new snow since the last observation.
How it is measured
National Weather Service (NWS) cooperative observers take snow depth once a day at their scheduled observation time, pushing a measuring stick down to the ground. They report it to the nearest whole inch and record a trace when less than half of the exposed ground is covered.
Community Collaborative Rain, Hail and Snow Network (CoCoRaHS) volunteers use the same idea: depth is the total snow on the ground at the scheduled observation time, ideally near 7 a.m. Where snow has drifted, observers average several readings taken in the least drifted spots.
Why depth and snowfall disagree
Depth reflects everything that has happened to the snow since it fell. CoCoRaHS gives an example of 4.2 inches of new snow that settles and melts to 2.6 inches by the next morning. The snowfall report stays 4.2 inches, while depth reflects only what remains.
During a long storm, new snow also presses down the layers beneath it, so depth often climbs more slowly than the running snowfall total.
Mapping depth between reports
Point reports leave wide gaps. The National Operational Hydrologic Remote Sensing Center (NOHRSC) combines daily ground-based, airborne and satellite snow observations with a snow model run at 1 square kilometer resolution to produce daily National Snow Analyses for the lower 48 states, including snow depth and snow water equivalent.
What depth does not tell you
Depth alone says little about how much water is locked in the snowpack, which matters for melt and flooding. Snow water equivalent, the liquid the snowpack would yield if melted, fills that gap, and a dense old snowpack can hold more water than a deeper layer of fresh powder.