Tis’ the season to be Merry, so let us not underestimate the power of snow.
During a heavy snow storm, many buildings, particularly greenhouses and boat houses collapsed due to the snow’s weight, and several people can suffer heart attacks trying to clear walks, doorways and driveways — and likely many more sustained back injuries and muscle pains.
Meteorologists developed a rule of thumb years ago that said ten inches (25 cm) of snow would melt down to 1 inch (2.5 cm) of water (or be the equivalent amount of rain if it fell as liquid), and that proportion was often used to determine the amount of equivalent precipitation from a snowfall. But that 10:1 ratio, we now know, is far from exact, and generally wrong across the breadth of conditions under which snow falls, even within an individual storm.
When snow falls at air temperatures around freezing (32 oF / 0 oC), the ratio of snow to water equivalent comes close to the 10:1 ratio. However the full range is from around 100:1 to 3:1, depending on other weather factors such as air temperature, snow crystal structure and wind speed. In the United States, the ratio typically falls in the 8:1-20:1 range: the standard value used by the US National Weather Service for converting snowfall into liquid water equivalent when observers cannot melt down the gage-caught snow is 13:1.
We know that pure water density is around 1000 kg (or one tonne) per cubic metre or 62.4 lb per cubic foot. Pure ice is just slightly less dense than pure water at about 917 kg per cubic metre (that is why ice floats on a pond or in our drink). Snow on the other hand has less density because it contains more air in a given volume than the ice.
Heavily compacted snow, called firn, can be nearly as dense as ice (around 910 kg per cubic metre) but newly fallen snow more typically weights in at 70 to 150 kg per cubic metre (4.4 to 9.4 lb/cubic foot), but this increases rapidly once snow is on the ground and it begins to compact due to wind, the addition of liquid water and its own weight.
Typically winter snowpacks have a density of 200-300 kg per cubic metre (12.5-18.7 lb per cubic foot), but this can vary with the moisture content of the snow and that can vary across climatic regions.
So how heavy is that snow covering your driveway or sidewalk that demands to be shovelled? You can estimate that rather quickly. First, 1 mm of water spread over a square metre weighs one kilogram. We’ll assume here a 10:1 snow–water ratio, so 1 cm of snow on a square metre weighs a kilogram.
Next calculate the area of your driveway or walk in square metres. For this example, let’s assume a driveway of 15 x 4 metres which gives 60 square metres. Then multiply that by the average depth of snow on the surface, I took 5 cm. Finally multiply that number by 1 kg. That gives 60 x 5 x 1 kilograms or 300 kg. If the drive is covered with heavy, wet snow, you could increase the weight by a factor of two to four, giving 0.6 to 1.2 tonnes!
For Imperial units and the same snow–water ratio, one inch of snow over a square foot of surface weighs about 0.52 pounds. So here, multiply the surface area to be shovelled, say a 45 ft by 10 ft driveway, which is 450 square feet by the depth of snow, say 2 inches, and then multiply that by 0.52 lb gives 468 lb of “normal” snow to move, and 900 to 1800 lb of wet snow.
Weighing Down the Roof
A driveway can be a heavy load of snow to shovel off, but it has Mother Earth to hold up the weight. A roof or other structural component may need to hold that weight and more. My roof is basically flat and has an area of around 100 square metres, so if 5 centimetres of “average” snow accumulated on it, my home must supports weight of 500 kg (1100 lb), wet snow could triple or quadruple the amount.
Human Health…A Weighty Matter
The weight of snow also has human health consequences, both good and bad. First, the good news. Snow shovelling can comprise a good workout for those in proper condition. The US Surgeon General’s Report on Physical Activity and Health for 1996 reported 15 minutes of snow shovelling can count as moderate physical exercise, the same level as running at 15 km/h.
But the bad news is that shovelling a moderate or large amount of snow puts a great stress on the body. Both the cardiovascular system and the muscles come under stress. An average shovel-full of snow can weight 10-30 kg (22 to 66 lb). Shovelling for around fifteen minutes can have you lifting a couple tonnes of weight.
Working this hard in cold weather can increase heart rate and blood pressure and cause shortness of breath. Heart attacks are often the leading cause of death during winter storms, as people unaccustomed to heavy physical activity feel compelled to clean their driveways and sidewalks. Often the anxiety created by the snowfall — fears of being late for work, driving in such conditions, etc. — has the heart rate elevated even before the shovelling starts. Then with the strain of shovelling, heart rates can rise above levels considered safe for hard aerobic exercising.
The second risk of snow shovelling, and perhaps the most common, is injury, mostly pulled muscles and back and neck strain. Most shovellers, out in the cold weather, do not properly warm up their muscles which increases the risk of pulls and strains. Most shovellers try to lift and throw shovel loads of weight that they are unaccustomed to moving. Physiotherapists suggest warming up and stretching beforehand and taking smaller loads in the shovel. And whenever possible, push or pull the snow rather than lift it. Even those who work out with weights can feel the strain as most often the shovelling act includes a twist of the spine with the lift rather than a straight lift.
Even Home Inspectors have to shovel their roofs. ha. ha.
It is important that when shoveling the roof, plastic tools be used so as not to damage the roofing material.
The most important is to make sure one is comfortable on the roof and to leave at least 3" of snow on the shingles to prevent damage and maintain a better foothold for the person on the roof.
These pictures are of a 4" in 12" pitch roof and any steeper would be considered dangerous.
On my behalf, I wish to hope that** all, **this Winter, will be careful in Inspecting Homes, attempting to walk snow covered roofs, and above else shoveling it when needed.