Weather 101: The Art & Science of Measuring Snowfall

Now that snow has begun falling in earnest across many parts of the U.S., it seemed a good time to talk about how to measure the white stuff. Although it might seem that just sticking a ruler (or a meter stick or yardstick) into the snow to measure snow depth would suffice, snowfall measuring is actually much more complicated. And the importance of accurate measurements can't be stressed enough. That's because snowfall and/or its melted (i.e. liquid water) equivalent is used for determining if an area qualifies for a disaster declaration, state snow budget planning, school closures, legal cases (especially if there are roof collapses), water supply estimations (especially in the western states), flood forecasting (snowmelt with or without rainfall), ski resort marketing ("champagne" powder versus snowball packing snow) and lots more.

Of course, everyone wants to know if the snowfall sets a record (day, month or season). It's hard to do this without quality snowfall measurements.

There are official National Weather Service and cooperative observer guidelines for measuring snowfall. You can read about these in detail, if you wish, at the COCORAHS web site. However, I suspect that most readers would prefer a "Cliff's Notes" approach.

First and foremost, you cannot measure the snow depth in the snow pile within a parking lot or at a street corner (Fig. 1). The same applies for the deepest snowdrift you can locate in your yard. That's cheating!

Instead, you need to find a representative snow depth location. To do this, it's best to take several depth measurements, preferably in places where snow has not been affected by wind. Once you get an average, find a location that approximates that depth (Fig. 2).

Next you will need to measure the liquid water equivalent of the snow.

Take a cylindrical canister (e.g., coffee can, tennis ball container, 4 inch circular rain gage) to the location. Invert the container and take a core sample of the snow (Fig. 3). After you push the container into the snow as far as it can go, clear snow away from the sides of the container and ensure you have collected all the snow you can. Slide something flat and sturdy underneath the opening of the container to ensure that no snow can fall out as you lift and upright the container (Fig. 4).

Bring the container inside and allow the snow to melt.

If your container is see-through and has a flat bottom, you can just measure the liquid water inside. If it isn't, you need to make some mathematical adjustments. The key here is that you have collected a volume of water.

To compute the liquid water volume, use the formula for the volume of a right circular cylinder: πr²h, where r is the radius of the container, h is the height of water and π is 3.14. Note that "h" remains unknown.

You can then pour the water into a see-through circular cylinder and set the initial volume (with the unknown "h") equal to the new volume (computed using the same formula but with r and h both easily obtained). Use algebra to solve for the original "h."

If you used a tennis ball container, you have an additional problem - the container's bottom isn't flat. But there is an easy work-around.

Carefully pour off the snowmelt water into a holding container. Fill the tennis ball container with water to a depth that covers any irregularities in the bottom of the container. Measure the depth of the water. Then pour the snow melt water back into the container. Remeasure water depth. The difference between the two values is the liquid water equivalent of the snow.

You can then compute the equivalent snow:liquid ratio.

Typically, the ratio used is 10:1, meaning that 10 inches of snow would melt down to one inch of liquid water. And, although the NWS has prepared a table that estimates the ratio based on air temperature (and, hence, the available atmospheric moisture content), there are too many variables at work to blindly apply such a table.

For example, if the snow is heavy, new snowfall may compact snow already fallen, yielding much larger ratios. The same net effect happens if sleet or rain falls onto the snow. Sometimes, due to warmer ground temperatures, the snow may start to melt from below and/or water from nearby melting snow may flow into the snow sampling area.

Of course, all of the above presumes that there is just newly fallen snow. If there is already snow and/or ice on the ground, the process is trickier. It's here that snowboards make life easier.

Finally, it's time to compare what you measure and compute to nearby official readings. The National Weather Service and/or your local TV meteorologist will be telling these to you. But, you can also compare by looking at reports from COCORAHS observers.

Consider these reports from the Champaign, IL area following the Alberta Clipper storm event of December 4, 2010 (Fig. 5).

New ... melted ... snow:liquid
snow ...................... ratio
1.5 ........ 0.19 ........ 7.9:1

4.0 ........ 0.24 ....... 16.7:1

6.0 ........ 0.53 ....... 11/3:1

8.8 ........ 0.55 ....... 16.0:1

Finally, the snow:liquid water within a snowpack will change over time as snow melts, water runs off and other factors come into play.

Still, you can easily start to get a better handle on all the factors involved in snowfall. As you can see, it's a lot more than just measuring directly what falls.

© H. Michael Mogil, 2010