Rain gauges: weather instruments tour

standard rain gauge
graduated cylinder inside rain gauge
This small white rain gauge is used at observing sites where someone is available to check the contents at least once a day. The whole thing stands about 40 cm high. The top is a funnel 12 cm in diameter and it channels the rainfall into a graduated cylinder inside. The observer lifts the funnel out, finds the top of the water column in the cylinder, and reads the corresponding number from the scale on the side.

This plastic cylinder will hold 25 mm of rain water. If more than that amount of rain falls between observations, the overflow will be caught in the white outside container. The white container can be lifted off its mounting post to pour the contents into the cylinder. The excess rainfall can be measured--25 cm at a time--in this same cylinder to arrive at a total.

A second type, the tipping bucket rain gauge, can be used at either manned or automatic stations. It can be connected to send electronic impulses to either a chart recorder for people to look at or to a computer system for transmission as part of an automatic weather report. Although it looks much the same as the standard rain gauge, it's bigger and the mechanism inside is completely different. Let's cut one open and have a look inside!

tipping bucket rain gauge
bucket mechanism inside tipping bucket gauge
The whole gauge stands 70 cm high, with a funnel on top that's 25 cm in diameter, about twice the size of a standard rain gauge. The picture on the top right shows the miniature teeter-totter that is positioned under the spout of the funnel , with a scoop-shaped bucket on each end. As soon as 0.2 mm of rainfall has collected in the bucket that's in the "up" position, the weight of the water will cause it to tip down, spilling the water out and raising the other bucket into the "up" position. If you look closely, you'll see a small hole in the bottom of the gauge cover that allows the water to run back out onto the ground. Every time the bucket tips, an electrical signal is sent, showing that another 0.2 mm of rain has been measured. The number of "tips" in a 5-minute period (or any other time period) tells us the rainfall intensity.
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