Snowfall is much more difficult to measure than rainfall because the crystals are deflected by the wind to a greater extent due to their lightness and size, worsening the aerodynamic problems. Above a windspeed of a few metres per second, the use of raingauges for measuring snow is not feasible without some form of screen. Pit gauges are obviously not practicable for anything but very small falls, although a possible alternative is the use of aerodynamically shaped collectors (Fig. 8.10). After entering the funnel, internal eddies can also lift out dry snow that has been successfully caught (Fig. 8.11). This can be reduced by putting a cross-shaped vertical divider in the funnel, although this increases evaporative loss when the gauge is measuring rain.

As the windspeed increases so does the angle of fall of the snowflakes, becoming increasingly more horizontal, approaching the gauge at such a shallow angle that the orifice presents too thin an ellipse for them to enter.

After falling, dry snow can also be carried elsewhere by the wind as spindrift, which if caught by gauges cannot be differentiated from true snowfall (Fig. 9.1).

Measuring snow as it falls

Measuring snow with manual raingauges

If the snowfall does not overcap or bury the gauge and the wind is light, it is possible to use conventional manual raingauges to measure snow reasonably well.