Snow totals reported after a storm come from a defined procedure rather than from sticking a ruler in the yard. The method exists because snow changes after it lands.
Snow compacts under its own weight
Fresh snow is mostly air, and the layers beneath a growing snowpack compress steadily as more falls on top of them.
A measurement taken at the end of a long storm therefore reports less depth than the total that actually fell during it.
Melting at the base against warmer ground adds to the loss, particularly early in the season when soil has not yet cooled.
The snow board standardizes the surface
Observers use a flat board placed on the ground in a representative open location, providing a consistent surface for every measurement.
Snow is measured on the board, then the board is cleared and returned to the surface so the next interval starts from zero.
Adding those cleared intervals produces the storm total, capturing snow that would otherwise have compacted away before the final reading.
Timing intervals are specified
Clearing is done at defined intervals, commonly every six hours, rather than whenever the observer happens to check.
Clearing too often inflates the total, because each fresh measurement catches snow before any settling has occurred at all.
Clearing too rarely understates it, so the fixed interval is a compromise that keeps reports from different observers comparable.
Wind is the hardest problem
Blowing snow scours some areas to bare ground while depositing drifts elsewhere, so depth can vary greatly within a single yard.
Observers are instructed to average several measurements across a representative area and to avoid obvious drifts and scoured patches.
In severe blowing conditions a reliable measurement may be impossible, and the report acknowledges that rather than offering a false number.
Site selection matters as much as technique, since a board placed near a building or a fence line will collect drifted snow rather than what actually fell.
Liquid equivalent is measured separately
The water content of snow is measured by melting a core of the snowpack, and it matters more than depth for hydrology and flood outlook.
The ratio between depth and water content varies widely with temperature and crystal structure, so equal depths can hold very different amounts of water.
Reporting both figures lets the same observation serve road crews concerned with depth and water managers concerned with what will eventually run off.
Volunteer observers supply most of these measurements across the country, which is why standardized instructions matter more than equipment in producing a usable national picture.