Tips for Recording Snowfall

Posted in About Weather | General Interest | Tips and Tricks.

Firstly, to cover off measuring the actual depth of snow. By far the simplest method is to use a ruler! Remember that its just an indication of how much snow is on the ground, not how much snow has actually fallen, much may have melted or sublimated (evaporated, which can occur in very dry, sunny or windy conditions). Find a flat area (such as your lawn) away from drifts or any obstacle likely to have caused an obstruction to falling snow; even trees and bushes 2-3 metres away can create areas of uneven depth, especially when it has been windy. We measure to the nearest cm and report the snow depth at least once a day, at 09Z, if possible.

The amount of snow can also be measured by what is known as its ‘water equivalent’. The problem is that unless you’ve got a heated rain gauge (and the UK Met Office does use these), any snow will not drip through into the tipping bucket mechanism either at all or at the time corresponding to that in which in fell in, unless it is very wet. If too much falls then it will soon fill the gauge top up, and blow or fall over the sides, especially with the smaller automatic gauges such as those sold in the less expensive weather kits. Those using the larger funnels, on the Davis weather stations for instance, or a manual gauge will have a little more space available. One tip that can be used is to add a known quantity of antifreeze (if manual) tipping it in to cover right around the inside of the top of the receiving container initially, as soon as snow starts falling or do the same with an auto recorder top and simply subtract any liquid amount that enters the bucket mechanism from your total. Then every so often if possible add further similar amounts of antifreeze to the gauge. As long as you use the same amount each time (and so hence know what volume it equates to in terms of precipitable liquid) then you can continue to take this amount off the total recorded. Of course you will also need to ignore the time aspect here, being clear to mark on the record why such a large amount was suddenly recorded at once.

The increase in the amount of liquid in the gauge indicates the water equivalent of the snow and it varies widely with different kinds of snow. Whilst the water equivalent of wet snow may be as low as 6 to 1 (where 6cm of snow equates to a fall of 10mm of rain) a dry snow may have a ratio as low as 30 to 1, more especially in very cold arctic regions, and a large amount of air is contained in the snow. Sublimation and settling will generally mean that even if the snow doesn’t melt it will become less deep as the days go on, this is particularly notably in mountainous areas, where the ski resorts are situated.

The water equivalent is very important as it determines snowmelt and water runoff in the spring and forecasts of river and stream flooding during the Spring and rapid thaw situations use the snow water equivalent data as the main way of knowing whether there are likely to be problems with this.

Other problems that can occur with measuring snowfall accurately can occur when it is very windy; if the snow is blowing horizontally, as it can easily do in even winds averaging just 15-20 knots, then very little of it will reach down into the gauge lip. It’s not uncommon then to see a large amount of snow piled up on the side of the gauge opposite to the wind direction if its dry snow, and on the same side if it’s wet snow. In reality there’s little we can do in these situations. When the snow event has finished you can try adding the same diameter of snow that would have fallen into the gauge- had it fallen horizontally- using what’s on level ground, mixed with a known amount of warm water, (tip it very gently into a tipping bucket recorder to allow the tip mechanism to function accurately) and see what figure the gauge records. It isn’t a perfect solution but we have to accept in recording the weather that the perfect set up probably doesn’t exist!