One of our readers, John Frewen-Lord, has been housebound for four days by the recent heavy snow falls – about 60 cm deep in his area. This has prompted him to provide an illustration of the comparative simplicity of calculating snow loads in metric units.
“Two winter calculations. Which is easier?
Like many parts of the UK in the beginning of December 2010, we in North East Lincolnshire have had a lot of snow. Up to now we’ve had around 60 cm. On the roofs of our cars it is about 50 cm deep. Like all good motorists, and having just spent three days digging one car out, we do not drive off with snow like this on the roof. Apart from it being dangerous to vehicles behind (and possibly illegal as well), it is a huge amount of extra weight to carry around! Carrying all that extra weight not only increases fuel consumption unnecessarily, but it is more likely to get you stuck in deep snow – a lighter vehicle will not bog down in snow as much as a heavier one.
So how much extra weight does this 50 cm of snow amount to? In metric it is a very easy calculation. The only things we need to know are that snow has about 1/10th the density of liquid water, 1 litre (L) is 1000th of a cubic metre, and that 1 L of water weighs 1 kg. Now from this it can be seen that 1 L = 1 m² x 1 mm deep.
Now I measured the pile of snow on the roof of my car, and it was approximately 2 m long x 1.5 m wide, or 3 m². Using the above information, 3 m² of snow x 50 cm deep is equivalent to 3 m² of water x 50 mm deep. Therefore the snow on the roof of my car will have an equivalent water volume of 3 x 50 = 150 L. Which will weigh 150 kg.
As the average adult person weighs 75 kg, that is equivalent to carrying an extra two people around. I think you will agree that those calculations are pretty simple, and could be easily done in your head. Could you do the same in imperial units? I very much doubt it.
Let’s say the snow on the roof of my car is 6ft 6 in long by 5 ft wide, and that it is 20 in deep. We need to convert the feet and inches to feet only, so the area becomes 6.5 x 5 = 32.5 ft². The 20 in of snow is equivalent to 32.5 ft² x 2 in deep of water. Where we do we go from here? The easiest way is to calculate the volume of water in cubic feet, by converting the 2 in to decimals of a foot. This then becomes 32.5 x 0.167 = 5.43 ft³. Now 1 ft³ = 6.22883288 imperial gallons (let’s round that to 6.23 gallons). Our water therefore has a volume of 5.43 x 6.23 = 33.8 gallons. How much does 1 gallon of water weigh? The imperial gallon (but not the US gallon) happens to weigh 10 lbs, so the weight of the snow on the roof of my car would be 33.8 x 10 = 338 lbs.
We’re not finished yet. That weight of 338 lbs, representing the weight of two people of 169 lbs each, needs to be converted to those stones that we British seem to love. There are 14 lbs in a stone, therefore 169 lbs = 12.07 stones = 12 stone 1 lb.
While you may not want to calculate the weight of snow on the roof of your car (though it is an eye-opener as to just how much it does weigh), this sort of calculation is surely typical of the kind that most of have to make continually in our lives. Whereas the metric calculations in this example involved just 4 calculations which were easily done in your head, and involved NO conversion factors, the imperial calculations involved no fewer than 9 calculations, of which 6 involved a conversion factor of one sort or another. And unless you have lots of time on your hands and like doing calculations manually with some awkward numbers, a calculator is necessary.
I am still amazed that we in the UK still want to torture ourselves with calculations like this.”