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What does the snow-to-water ratio need to be for a roof to collapse?

Science of Snow: Buffalo may be covered in dry, fluffy powder but it can still add up to tons of trouble

Scott Sutherland
Meteorologist/Science Writer

Monday, February 9, 2015, 5:08 PM - Measuring the amount of snow that's fallen in your area can be as easy as finding an undisturbed patch of ground and sticking a ruler into what has accumulated there, or the case of Buffalo this week, finding someone very tall to measure it against. When you're trying to figure out exactly how much of that snow can accumulate on your roof before it collapses, though, or gauge the potential for flooding when all that snow melts over a few days time, a little more science goes into the process.

(Editor's Note: This story originally ran on November 21, 2014, in the wake of the lake effect snowstorm that buried Buffalo, NY.)

The snow that fell over Buffalo on Tuesday, as it streamed off of Lake Erie in a record-breaking snow squall, was actually a fairly dry snow. This was the kind of powdery snow that's praised by skiers, sometimes called "The Greatest Snow on Earth" if you happen to live in Utah. This means that, although it piled up very deep, there was actually a relatively small amount of water locked up in that snow, at least when you compare it to the snow that falls from bigger, wide-spread snowstorms.

The reason for this is the height at which the snowflakes form, and consequently, how much water the flakes had access to on their trip to the ground.

RELATED: Roofs collapsing under the weight of heavy snow in Buffalo

Unique, snow flake by Pen Waggener

In a fairly typical snowstorm, snowflakes can form fairly high up in the air - well over 10,000 feet above the ground. Too heavy to remain aloft, they fall, passing through the lower layers of the clouds, which are packed with supercooled water droplets. Because these snowflakes - called dendrites (pictured to the right) - have a lot of air space between the crystals, they act a bit like a net as they fall, sweeping up these droplets as they pass through the cloud. As this happens, the snow becomes heavier and wetter. Because warmer air can hold more moisture than colder air, even at sub-zero temperatures, the closer the temperature in the cloud is to zero degrees C - without going over - the more supercooled droplets that will be available to be swept up, while the snowflake remains frozen.

This is why snow storms that happen when the temperature near the ground is just at, or even just slightly above freezing, are typically followed by snowball fights, snow-people popping up around the neighborhood and construction projects to produce snow forts and snow tunnels. The snow is so wet and heavy that it holds together very easily.

RELATED: How to supercool water, and an experiment you can do at home

However, for the lake effect snow that fell in the vicinity of Buffalo, the snowflakes formed under conditions similar to those that produce the 'typical' ones described above, but it all happened much closer to the ground, and at colder temperatures, overall.

With the snowflakes forming over the lake only a fraction of the height they would inside a big snowstorm, and temperatures well below freezing all the way to the ground, there were less supercooled water droplets available for the snowflakes than there would be closer to freezing, and there was far less fall-time for the snowflakes to sweep up what droplets were available. With less water added to its weight, this resulted in fluffy, powdery snow on the ground, but it still accumulated over hours to bury some areas around Buffalo to a depth of over 6 feet.

With all that dry, powdery snow, it may seem strange that some roofs have been collapsing under the weight, and that there's a threat of flooding due to a brief shot of warmer weather that's in the forecast. After all, there's not a lot of water locked up in what's on the ground (at least compared to your average snowstorm).

That's all true. The ratio of snow to water in what fell could be up to 30 to 1 - meaning that you'll have about 3 centimetres of snow for each millimetre of water that went into making that snow - or possibly higher. Your average snowstorm has a ratio of more like 10 to 1.

However, the nearly 2 metres of snow that's on the ground in some places around Buffalo probably doesn't reflect the true height of what's fallen there. Simply due to gravity, the lowest layers of what fell would have been compacted down under the weight of what's above it. So, that means the overall ratio of snow to water in those snowbanks might be closer to 20 to 1. At that point, it just comes down to exactly how much weight that represents, and what the roof is rated to hold.

If the snow piled onto the roofs of homes in the area does have an overall snow to water ratio of 20 to 1, ~2 m of snow reduces down to about 100 mm of water. For each square metre of area the snow covers, that's roughly 100 kilograms of water mass. Take into account the area of the roof, say 70 square metres for your basic two-bedroom townhouse, and you're looking at around 7 metric tons - or about the equivalent of a pair of average-sized Asian elephants - resting above your head.

RELATED: Rain, freezing temperatures to complicate clean-up efforts in Buffalo

The shot of warm weather that's expected this weekend is going to make the situation even worse. The rain that will be accompanying this warmer weather will soak into the accumulated snow, increasing its weight just like when the snowflakes were falling through the cloud (although the water drops will end up between the snowflakes, rather than between the snowflake crystals). The fortunate part is that the rain will also be melting the snow at the same time, so the increase in strain on rooftops will likely be short-lived as the water (both rain and melted snow) runs off onto the ground.

However, that will just being up another problem, due to flooding.

"We are preparing now for more flooding than we have seen in a long, long time," New York Governor Andrew Cuomo said Friday, according to CBC News. "We've never had this much snow and this much melting of this much snow in a short period of time."

When it comes right down to it, however you add this much water into storm drains, streams and rivers - through melting snow or straight from rainfall - the result is roughly the same. That much water over three days time can swell streams and rivers to the point where they become dangerous - with fast moving currents and coming close to cresting their banks. In towns and cities, storm drains clogged with snow and ice can cause water to pool up and flood streets, and water accumulating along building foundations can flood basements.

RELATED: One-two punch: Rain could spell more trouble for Buffalo

(Tip-of-the-hat to Dr. Doug Gillham for his invaluable input)

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