Windswept snow not easy to measure!
Meet the instrument that is used to measure snowfall..
Published Friday, February 17, 2017 12:01PM AST
Last Updated Friday, February 17, 2017 12:08PM AST
Three storms in 7 days... that’s impressive; almost as impressive as the snowbanks out there! . Yesterday’s storm now sits off the tip of Anticosti Island and with the exception of pockets of onshore snow, it’s pretty much over. All that’s left is.. the measuring!
Here in the Maritimes, we rarely get snow without wind and that makes measuring very difficult. It’s not uncommon to find bare ground on one side of the house and drifts over the top of your boots on the other.
After each snowfall I put out a snowfall map and I always get calls from people who have different snowfall amounts to share with me. (By the way, I appreciate those numbers! There are far too few measuring stations in our region).
But that brings up the question of how the snow is measured when the wind is blowing. The answer is, the same way it’s measured when the wind is calm.
Snowfall measurement is very basic. It consists of a few boards, a broom or shovel and a ruler. It starts with pre snowfall preparation. Official observers are set up with at least three, ideally 5 sheets of plywood nailed to a brace on the ground. These are called snowboards and are almost always painted white; the boards are placed several metres apart. The board placement is important: they should be out in the open and as much as possible, away from trees or buildings that could affect the flow of the snow as it falls to the ground. Once the snow has started to fall, an observer will go outside and measure the depth of snow on each board. Typically, that takes place every 6 hours. It's as simple as sticking a metre stick through the snow until it hits the surface of the board. The boards are swept or shoveled clean after each reading.
After the measurements have been recorded, they are averaged out et voila, you have the amount of snow that fell in the last 6 hours.
But there’s more. Most airports use a Nipher gauge; that’s a bell-shaped device that captures the falling snow. It’s mounted on a sliding metal pipe so as snow accumulates over the winter, it can be raised to keep the top about 1.5 m above the surface of the snow. The bell shape was chosen to reduce the turbulence over the top of the gauge. Every 6 hours, the gauge is taken indoors, the snow is melted and the water is poured in a graduated cylinder and measured. On average, the ratio is 10:1 - it takes 10 cm of snow to make 10 mm of water, but that ratio changes with the temperature. The ratio for wet snow will be less than 10:1. For example, a 5:1 ratio is possible. The ratio for dry snow will be greater than 10:1. In extreme cases it can be 30:1.
One last note about airport measurements using a Nipher gauge: when the contents are melted down and converted into a snowfall total, it can be less than what you're seeing in your yard because often times, the wind will blow some of the snow out of the gauge.
So know you know... and hopefully can appreciate how tricky snowfall measurement can be!