Rainfall is measured as a depth, not a volume: the number of millimetres of water that would have collected on a flat, level surface if none of it drained or evaporated. One millimetre of rain is one litre of water per square metre. That is why the collecting aperture of the gauge has to be a known, precise size, and why a gauge with a distorted or partly blocked funnel under-reads no matter how good its electronics are.
This section covers everything that records precipitation, from a £10 analogue garden gauge to professional tipping-bucket assemblies and the rain sensors built into complete weather stations. Siting matters more than specification with this element, so the guidance below is worth reading before you buy.
How rainfall is measured
The simplest gauge is a graduated tube with a funnel on top. The funnel is wider than the tube, which multiplies the depth and lets you read small amounts accurately. You empty it and read it manually, usually once a day at a fixed time.
Electronic gauges almost all use a tipping bucket. Rain enters the funnel and runs into one of two small seesaw-mounted buckets. When that bucket has collected a fixed amount it tips, empties, and swings the second bucket into position, and the tip closes a reed switch. The station simply counts tips. Each tip on a typical domestic gauge equals 0.2 millimetres, or 0.01 inches on North American models, so the resolution of your gauge is exactly the size of one tip.
Tipping buckets have one known weakness: in very heavy rain a small amount of water is lost during the moment the bucket is in motion, so extreme rainfall rates read slightly low. Better gauges correct for this in firmware.
Siting a rain gauge
A rain gauge needs open sky. The standard guidance is that the gauge should be at least twice as far from any obstruction as that obstruction is tall — so a four-metre hedge means the gauge sits at least eight metres away. Under a tree, beside a wall, or on a fence line in the lee of a building, you are measuring the obstruction rather than the rain.
Mount the rim level, and keep it low: roughly 30 centimetres to a metre above short grass. Higher means more wind flowing over the aperture, and wind is what causes under-catch, as raindrops get carried past the funnel rather than into it. Never mount a rain gauge on a roof or a high pole if you can avoid it. Keep the funnel clear of leaves, moss and insects, and check it seasonally.
Snow, and heated gauges
An unheated tipping bucket cannot measure snow as it falls; the snow simply sits in the funnel until it melts, at which point it registers hours or days late. Where winter precipitation totals matter, a heated gauge melts the catch as it arrives so the tips happen in real time. If you only need a seasonal total, an unheated gauge and patience will do.
Rate versus total
Most stations report two rainfall figures and they answer different questions. The total is accumulated depth over a day, month or year, and it is what you want for garden, allotment and drainage planning. The rate, in millimetres per hour, is calculated from the interval between recent tips and is what matters for flash flooding, runoff and irrigation control. A short, violent downpour can produce an alarming rate and a trivial total.
What we recommend
For a simple garden gauge that needs no power, the TFA Black Point Analogue Rain Gauge and TFA Analogue Rain Gauge 47.1000 both read directly in millimetres. To get the reading indoors, the TFA Drop 47.3005.01 Wireless Rain Gauge and TFA 47.3006.01 Wireless Rain Gauge send totals to a console, and the TFA Monsun 47.3003 Wireless Rain Gauge adds an outdoor temperature reading. If you want rainfall as part of a full station, the Technoline WS 1700 Complete Weather Station and ACCUR8 DWS7100 7-in-1 Solar-Powered WiFi Weather Station include a tipping-bucket gauge, and for professional-grade catch accuracy the Davis Wireless Vantage Pro2 is the reference point.
Related measurements
Dedicated gauges are grouped in Rain Gauges, combined rain and temperature units in Rain & Temperature, and professional assemblies in RainWise. Rainfall makes most sense read against pressure, so see Weather Forecast, and against wind, in Wind. Replacement funnels, buckets and sensors are in Sensors, Spares & Accessories, and the other elements are indexed under Elements.