Snowfall rate is how fast new snow accumulates, usually given in inches of snow per hour. Forecasters also work with a liquid-equivalent rate, the water content of the falling snow, and the snow-to-liquid ratio links the two.
How fast snow can fall
Rates vary widely between storm types. The National Weather Service (NWS) notes that lake-effect bands can produce 2 to 3 inches of snow per hour or more, and NWS Albany notes that nor'easters can reach 4 to 6 inches per hour, sometimes for several hours.
At rates like those, a few hours can deliver most of a storm's total, so the timing of the heaviest burst matters for travel.
How intensity gets reported
Surface weather observations classify snow intensity by visibility rather than by measured accumulation. Heavy snow is reported only when visibility is 1/4 mile or less.
Visibility is an unreliable stand-in for rate. NASA aircraft icing guidance notes that research shows temperature and day or night lighting also have to be considered to judge snowfall rate from visibility.
How radar, satellites and models estimate it
Radar converts reflectivity to a snowfall rate with empirical relations, and published relations differ by roughly an order of magnitude because snow size and density vary so much.
A passive microwave snowfall rate product from NOAA's satellite service (NESDIS) is displayed in liquid-equivalent inches per hour.
Many model snowfall maps are built by applying a ratio to forecast liquid precipitation, so the hourly snowfall they show depends on that ratio.
What it does not tell you
A liquid rate of 0.1 inch per hour could mean 1 inch of snow per hour at 10:1 or 2 inches at 20:1. Any rate quoted in liquid terms needs a ratio before it describes the ground, and the ratio itself is uncertain.