How do we measure snowfall? A meteorologist explains how volunteers count winter storms


Bill Syret, Penn State

Blue Hill Observatory, a few miles south of Boston, recorded the deepest snow cover in its 130-year history, an incredible 46 inches, in February 2015. That same month, Bangor, Maine , tied its deepest snow record at 53 inches. . Mountainous locations will sometimes see triple digit snow depths.

Impressive numbers, sure, but assuming you have a measuring stick long enough to reach the ground under all the white, how hard can that be to measure? You stick a ruler or tape measure in the snow and get a number, right? Well, not so fast. It’s a little trickier than you might think to get those all-important snow totals for winter storms.

Since 1890, the US National Weather Service has relied on a network of volunteer observers, all strictly adhering to NWS guidelines, to provide snow measurement figures over an area. There are over 8,700 cooperative observers across the country who send their weather data to the NWS daily, some have been doing so for over 75 years!

Measuring snow is inherently inaccurate, but paying attention to your surroundings and sticking to guidelines helps those of us who do it regularly stay consistent – and with long-term weather records, consistency is arguably just as important. than precision.

First, you need to understand the difference between snow depth and snowfall.

The author measures the meager snow accumulation at his observation location in Pennsylvania. Note the instrument shelters in the background.
Marisa FergerCC BY-NC-ND

How do you measure snow depth?

Snow depth should be a measure of the average depth at a given location and its immediate surroundings. It is usually rounded to the nearest whole number. To get a representative number, you need a site with minimal drift (not always easy to find), and multiple measurements need to be averaged to get a final number. I like 10 because it makes the calculations easier.

Careful measurement is essential in order to make reasonable estimates of the amount of liquid contained in the snowpack. As difficult as it may seem at times, the snow will eventually melt, and a rapid melt could cause flooding issues. In addition, hydrologists’ models that are used to predict water levels are critically dependent on good initial data collection, although improved satellite data has helped reduce their reliance on any individual measurement.

Snow depth is like the sum of individual snowfalls, assuming there is no sublimation – snow turning into water vapor – or melting since the first snowfall. snow so far. This assumption would almost always be wrong, of course. But if you suspend reality for a moment, the depth will still never exceed the sum of all snowfall because snow is compressible. Thus, two snowfalls of 10.5 inches (27 centimeters) can accumulate to a depth of only 17 inches (43 cm).

It is the compressibility of snow that causes the greatest consternation with snowfall measurement.

Snowboard and snow measuring stick from the National Weather Service.
Famartin/Wikimedia Commons, CC BY-SA

How are snow totals measured?

Snowfall is the amount of snow that accumulates over a given time, usually a 24-hour period. In a perfect world, that 24-hour period would end at midnight, but the vast majority of cooperative National Weather Service observers take their daily observation in the morning.

To properly measure snowfall, you need a flat, level surface. As with snow depth, you want to avoid drift zones when measuring snowfall. The National Weather Service suggests using a snowboard, which is a white surface that will absorb very little sunlight and stay close to ambient air temperature. However, any cold surface will do.

Bearing in mind that consistency is key, the goal here is to make an accurate measurement that is representative of the surrounding area and consistent with others making snowfall measurements.

More complicated measurement cases

So let’s consider three common scenarios. Here in the eastern United States, we often have to deal with snow that turns to rain during a storm. Imagine 6.0 inches (15 cm) of snow falling, then one inch (2.5 cm) of sleet compacting the snow to a depth of 4 inches (10 cm). Then, on top of that, an inch of freezing rain fell which further compacted the snow to 2.6 inches (6.6 cm) deep at the end of the observation period. What should be indicated as daily snowfall? Snow depth?

In this situation, when the snow turns to sleet, the snowboard should be cleared and the maximum snow depth recorded – 6 inches (15 cm) in this case. Do the same with sleet and it will add an inch (2.5cm) of snowfall – technically, “solid precipitation” – in total. Freezing rain is never added to the daily snowfall total because it is in liquid form when it reaches the ground. Thus, the daily snowfall is 7.0 inches (measured to the nearest tenth of an inch) (17.8 cm), while the observed snow depth is rounded off to 2.6 inches (6 .6 cm) to 3 inches (7.6 cm).

Imagine a case where three heavy snow showers occur one afternoon, two dumping 1.5 inches (3.8 cm) before completely melting, then a third of 1.8 inches (4.6 cm). The daily snowfall would be reported as the greatest depth reached during this period, 1.8 inches (4.6 cm). This assumes that an observer is there to measure each short-lived accumulation. But because most observers are volunteers, unfortunately this is not always the case.

That sounds like a lot of snow – but how do you quantify that?

Finally, we come to the controversy. Assume a steady snowfall for the entire 24 hour period at a rate of 2.0 inches (5.1 cm) per hour. If the snowboard were clear every hour, the daily snowfall would be 48.0 inches (122 cm).

What if the observer could only be there during the daily observation? By this time the snow will have compacted a portion, depending on temperature and the amount of liquid in the snow, which can vary between about a quarter inch and 3 inches (0.6 to 7.6 cm) of water liquid for every 10 inches (25 cm) of snow. This liquid-to-solid ratio depends on many factors, including the temperature in the cloud where the snow was produced, which in turn helps determine the size of the snowflakes. Large, airy snowflakes produce less dense snow and lower liquid-to-solid ratios, in general.

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This daily observer would record significantly less snowfall – for the exact same event – ​​than the person who cleaned the board every hour. It’s a problem. The National Weather Service responds to this by requiring that no more than four snowfall measurements be taken in any 24-hour period. Ideally, then, a spotter who would go snowboarding every six hours and clear 10.5 inches (26 cm), 9.3 inches (23.5 cm), 11.5 inches (29 cm), and 10.8 inches (27cm) would get the officially correct snowfall measurement. 42.1 inches (107 cm).

Several years ago, an observer in New York measured 77.0 inches (196 cm) of snowfall over a 24-hour period, which would have broken the record for the heaviest single-day snowfall in history. the United States. However, the National Weather Service determined that he cleared the snowboard far too frequently, inflating the daily total and invalidating the record.

Intrigue, suspense, mystery, controversy. Much more than just sticking a ruler in the snow. If all that sounds like a daily dose of weather fun to you, you might have what it takes to volunteer for the NWS’ Cooperative Observer program, where it’s not all about the white stuff. Daily high and low temperatures and precipitation of all forms are also measured, to strict NWS standards, of course.

For those who want to stick to only measuring what falls from the sky, check out the Community Collaborative Rain, Hail and Snow Network. Anyone can participate, but those who live “in the middle of nowhere” can provide an invaluable service by helping to fill in the data gaps that limit the effectiveness of hydrological models.

This is an updated version of an article originally published on February 19, 2015.

Bill Syrett, associate professor of meteorology and atmospheric sciences and director of the Joel N. Myers Weather Center, Penn State

This article is republished from The Conversation under a Creative Commons license. Read the original article.


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