I’m keeping myself updated with news from back home in Sweden. There have been extreme amounts of snow in the country; several roofs have been crushed because of the weather. SvD, one of the major and respected Swedish newspapers, urges house owners to take care:
Ett problem är dock att många fastighetsägare reagerar för sent – många inser inte att snön blir ett större problem när temperaturerna sjunker. Nyfallen torrare snö väger 30-100 kilo per kubikmeter medan packad snö kan väga upp till 300 kilo [sic]. När det töar och snön blir blötare kan den väga upp till 400 kilo per kubikmeter och då ökar risken för skador på tak och byggnader markant.
My own translation:
One problem is that many house owners react too late – many don’t realize that snow becomes a bigger problem with lower temperatures. Fresh, dry snow weighs 30-100 kilograms per cubic meter while packed snow can weight up to 300 kilograms [sic]. When it thaws and the snow becomes wet it can weigh up to 400 kilograms per cubic meter, and then the risk of damage to buildings increases dramatically.
(emphasis mine)
Yet once again we have the issue of snow magically becoming heavier. The article doesn’t even seem to be fully clear about whether it is talking about weight or density. Say that 1000 kg of (dry) snow falls on a roof. Now, with everything else constant, the temperature goes up and it begins to thaw. The question is now: How much mass does the snow have now? (assuming that no snow yet left the roof in liquid form).
Anybody who has shoveled snow knows that it’s much harder to lift a shovel full of wet snow compared to when it’s still cold. Therefore, they assume that the snow is “heavier” when warmer, and assume the same applies to the snow they have on the roof. While the observation is correct, the conclusion is wrong. Because it is just the density that has increased, the molecules are more tightly packed and the weight per cubic meter has gone up. There’s still the same mass of snow lying in the driveway. With higher density, a larger mass (and therefore with a larger weight) of snow can fit in the same shovel, increasing the force you must use to lift it.
Back to our question: Of course it still remains at 1000kg. That’s not even basic physics, its just plain obvious. The mass of snow never increases without snowfall, and while density changes with temperature, the total weight stays the same. The weight of any object depends solely on the number of molecules in it, and a different temperature doesn’t magically make new molecules appear. I’ve seen the myth of increasing weight repeat all over the place, especially during this weather conditions. I was hoping for the editor to have some common sense, especially with these kind of newspapers. Come on! (The article briefly speaks about lower temperatures too, but the above reasoning also applies to when it becomes colder.)
So, if your roof was crushed under snow during thawing temperatures, it didn’t have as much to do with temperature as it had to do with the amount of snow to start with. That it happened at that temperature was just a coincidence. Okay, it could have been that snow melted in one part of the roof, slid down to another part and joined forces with the snow already there, but that’s not what we’re talking about here.
Now, there are real dangers that arise with melting snow, but they are of a completely different character. Falling ice and snow blocks is a real danger to people underneath, and many have become hurt this winter. Stay clear of high buildings, especially downtown, and avoid getting crushed.
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