All-weather precipitation gauge: weather instruments tour
At automatic weather stations, a single gauge is used to measure the amount of both rainfall and snowfall. The 2 main types of all-weather precipitation gauges used by Environment Canada are the Fischer and Porter weighing gauge and the Geonor. Both operate basically the same way and will tell us the amount of precipitation in millimetres. The metal pickets surrounding the gauge are designed to disrupt the wind flow, slowing it down so that precipitation can be captured rather than carried across the top of it.
An opening on the top of the cover directs precipitation into a bucket inside--in the picture at right, the cover is removed to show the blue bucket. This bucket sits on a weighing mechanism and as more precipitation flows into the bucket, the bucket gets heavier. The weight gain tells us how much precipitation has fallen. To keep the water from evaporating out of the bucket and confusing our readings, a small amount of oil floats on top of the water.
Snow is trickier to capture than rain. If you look closely at the picture above on the left, you'll see an anemometer mounted on a red post at the same height as the precipitation gauge. It's important to compensate for wind strength at that level, as strong winds will carry the snow flakes across the top of the gauge, allowing less than half the snow that's falling to actually be captured. Because of the colder winter temperatures, anti-freeze is added to the bucket to melt the snow so that its water content can be measured accurately.
At automatic stations, there's no human observer to decide whether the rain is freezing on impact or to measure the snow on the ground. That's why we sometimes supplement the Geonor with additional equipment like a snow depth sensor.
The silver capsule dangling from the left end of the crossbar in this picture is an ultrasonic snow depth sensor. It sends a high-frequency pulse of sound toward the ground. Because we know the speed of sound, the length of time required for the pulse to travel to the ground and back up to the sensor tells us how far it is to the snow surface, and by extension, how much snow is on the ground. The change in readings from one day to the next also indicates how much new snow has accumulated.
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