How snowfall is measured, and why your driveway never matches the airport

How does the National Weather Service measure snow?

With a board, a ruler and a set of rules that every observer follows. Under the NWS snow measurement guidelines:

• The surface is a snow measurement board: a 2-foot square of plywood painted flat white, set in an open spot away from buildings and trees, where drifting is minimal.
• Snowfall is measured to the nearest 0.1 inch, as soon as possible after the snow ends, before it melts or settles.
• Where wind moves the snow around, observers take several measurements, typically three or more, and average them, leaving out drifts and scoured spots.
• Readings are not taken on pavement, rooftops or other surfaces that run warmer than natural ground.

What is the difference between snowfall and snow depth?

Snowfall is the new snow since the last observation. Snow depth is the total on the ground, old and new, reported to the nearest whole inch.

The difference matters. The NWS guidelines give the example of snow that builds to 4.2 inches by mid-afternoon, then settles and melts to 2.6 inches by the next morning. The snowfall for that day is 4.2 inches, because that is what fell.

Trace has two uses. Snowfall is a trace when flakes fell but never added up to 0.1 inch. Snow depth is a trace when less than half the ground is covered.

Why measure only every 6 hours?

Snow compacts under its own weight. Clearing the board every hour and adding up the readings would catch each layer before it settles and inflate the total. So observers who measure through a storm, often at major airports, clear the board no more often than every 6 hours, add those readings for the daily total, and never add more than four of them. Most volunteer observers measure once a day.

Who measures snow in and around Macomb County?

Detroit Metro Airport, in Romulus in Wayne County, is the official climate station for southeast Michigan, and the Detroit normals the NWS publishes come from its record.

Much of the local detail comes from volunteers. CoCoRaHS, the Community Collaborative Rain, Hail and Snow Network, trains people to measure precipitation at home with standard gauges and snowboards and report every morning, ideally around 7 a.m. NOAA's station list includes CoCoRaHS sites all over Macomb County, named for their distance and direction from a nearby town: SHELBY 0.4 SSE, ROSEVILLE 1.7 ENE and NEW BALTIMORE 0.4 SSE among them.

Reports from inside the county show how much one storm can vary, a pattern Macomb County snowfall averages describes for the lakeshore and the north end. On November 9 and 10, 2025, NWS Detroit/Pontiac's snowfall reports ran from 1.0 inch in Shelby to 5.2 inches in New Baltimore, under a lake-effect band off Lake Huron that set up overnight.

Why doesn't my driveway match the forecast or the airport?

How do snow companies decide the trigger has been reached?

By depth on the ground where the work is, not by the airport total or the forecast. Most contracts dispatch at a set accumulation, explained in how much snow before a plow comes. Our trigger is over 2 inches for homes and 1 inch for commercial sites, and we make dispatch decisions from observed conditions across the route. That matters on storms like November 2025, when one side of the county had five inches and the other had one.

How do you measure snow at home?

1. Set out a board before the season. Plywood painted white works; the NWS uses a 2-foot square and CoCoRaHS suggests about 16 inches. Put it on level ground away from the house, trees and the driveway, and mark it with a stake.
2. Use a ruler marked in tenths if you have one.
3. Measure when the snow stops, before it settles. Push the ruler straight down to the board.
4. Take several readings and average them. Skip drifts and bare patches.
5. During a long storm, clear the board no more than every 6 hours and add the readings.
6. On grass, measure to the bottom of the snow, not the soil. Snow sits on top of the blades, sometimes by an inch or more.
7. Never measure on the driveway or walk. Pavement melts and compacts snow from below.

For the bigger picture, see how many plowable snows Macomb County gets in a winter.