How Snowfall Is Measured: Why Totals Disagree by Inches

Snowfall is measured by pushing a ruler into fresh snow on a flat, open surface, usually a white board that is cleared every six hours, and then adding up the readings to get a storm total. Two people a mile apart can report totals that differ by several inches because wind, melting, compaction and timing change what the ruler reads. Official numbers from the National Weather Service follow a set routine, while the totals in your neighborhood app or social feed often do not.

How do weather observers measure new snowfall?

Observers use a snowboard: a flat piece of light-colored wood or similar material, roughly 16 inches (40 cm) square, set on top of the existing snow in a spot that is open and sheltered from drifting. The National Weather Service asks cooperative observers to push a ruler straight down to the board, read the depth to the nearest tenth of an inch, then wipe the board clean and set it back at the surface for the next reading.

Readings are taken about every six hours during a storm, not continuously. Snow settles under its own weight as it falls, so a measurement taken after a whole day of snow would be lower than the sum of four shorter measurements. Adding the six-hour readings gives the storm total. Adding the day’s readings gives the daily snowfall.

Observers take several readings in different spots and average them, because snow is rarely the same depth across even a small yard.

What is the difference between snowfall and snow depth?

Snowfall is the amount that newly fell during a set period, while snow depth is the total layer of snow on the ground at one moment, including older snow. A place can have 2 inches (5 cm) of snowfall on a day when depth is 14 inches (36 cm).

Depth goes up with each storm and down with melting, compaction and sublimation (snow turning directly to vapor). Snowfall can never be negative. That is why a reported storm total and the depth you see on the ground often do not match, even when the measurement is done perfectly.

What is snow water equivalent?

Snow water equivalent is the depth of water you would get if a column of snow were melted. It matters because the same 10 inches (25 cm) of snow can hold very different amounts of water.

A common rule of thumb is a 10 to 1 ratio, meaning 10 inches of snow melt down to about 1 inch (25 mm) of water. Real ratios range widely. Wet, heavy snow near freezing can be closer to 5 to 1, while cold, light powder can reach 20 to 1 or higher.

Weather stations measure it with a precipitation gauge. In winter the funnel and inner tube are removed from a standard gauge so snow collects in the outer cylinder, which is then melted or weighed. Automated weighing gauges record the weight of what falls. In the western mountains, the Natural Resources Conservation Service runs SNOTEL stations that use snow pillows to weigh the snowpack and track its water content through the season. Water managers use those numbers to estimate spring runoff.

Why do snowfall totals disagree between nearby places?

Totals disagree because snow does not fall or settle evenly. Several causes stack up in a single storm:

• Wind blows snow off open ground and piles it into drifts, so one spot can read 3 inches while a lee side reads 12.
• Elevation changes the temperature. A few hundred feet of height can turn rain or slush into snow, and cold air tends to produce a lighter, fluffier snow that stacks deeper.
• Storm bands, especially lake-effect snow, can dump heavy snow on one town and leave the next one nearly dry.
• Melting during the storm, particularly on warm pavement or when temperatures hover near 32°F (0°C), shrinks what a ruler can find.
• Compaction lowers a reading the longer the snow sits before anyone measures it.
• Different observers use different surfaces: grass, a deck, a car roof or a proper snowboard.

Sleet and freezing rain add confusion. They are counted differently from snow and are measured as liquid, not as snow depth.

Why does the official airport total differ from what fell in my yard?

An official total is a single point measurement at a specific site, and your yard is a different point. Many long-term records come from airports because the sites are flat and open, which also makes them windy. Wind scours snow there, and a reading may look low compared with a sheltered suburb a few miles away.

Timing matters too. A storm that ends at 2 a.m. may be split across two calendar days in a record, and a total taken at midnight can miss snow that fell afterward. Forecast offices often publish storm totals from trained spotters and cooperative observers across a county. Those reports are gathered, checked and sometimes thrown out if they look like drifts or measurements from a roof.

The same idea applies to other measured quantities in science. Sea level has to be corrected for tides, land motion and instrument differences, as described in how tide gauges, satellites and GPS measure sea level, and an earthquake’s size and its local shaking are reported separately, as in magnitude versus intensity. A snow total is similar: where and how you measure shapes the number.

How can you measure snow at home accurately?

Choose a spot away from buildings, trees and fences where wind does not pile snow up. Then follow these steps:

1. Place a flat, light-colored board on the snow surface before the storm, or clear a patch of ground and mark it.
2. Push a ruler or yardstick straight down until it touches the board, and read the depth in inches or centimeters.
3. Repeat in at least three spots nearby and take the average.
4. Wipe the board clean and set it back flush with the surface.
5. Repeat about every six hours and add the readings together.
6. Skip measurements from drifts, roofs, car hoods and areas that were shoveled or plowed.

Home measurements are most useful when you write down the time of each reading, so they can be compared with official numbers. When shoveling heavy snow, take care with your back and heart, and keep clear of power lines and roofs with large snow loads.

Quick answers

How much snow is a trace?

A trace is snowfall of less than 0.1 inch (about 2.5 mm), too little to measure with a ruler. It is recorded as a trace and adds nothing to the total.

Can radar tell how much snow fell?

Radar can show where snow is falling and how intense it is, but turning that into inches depends on snow type and the water content, so ground measurements are used to check and correct estimates.

Is 10 inches of snow always one inch of rain?

No. The 10 to 1 ratio is an average. Wet snow holds more water per inch and light, cold snow holds less, so the same liquid amount can produce very different depths.