A rain gauge (pluviometer) is the instrument used to measure rainfall — the depth of liquid precipitation collected over a given period, expressed in millimetres (1 mm = 1 litre per square metre). The simplest design, dating back centuries, is a cylindrical funnel that channels rain into a measuring container. Modern versions add automation and precision for continuous, unattended monitoring.
Several types are in operational use: the standard manual gauge (a funnel with a 200 mm diameter and a graduated cylinder, read daily at fixed times — still the reference for many networks), the tipping-bucket gauge (a funnel directs rain into a small seesaw bucket that tips at 0.1 or 0.2 mm, each tip generating an electronic pulse — the most common automated type), the weighing gauge (measures the weight of accumulated precipitation — handles snow and mixed precipitation), and optical disdrometers (laser sensors that measure individual drops, providing drop size distribution as well as rainfall rate).
Accurate rainfall measurement is surprisingly difficult. Wind is the biggest source of error: at gauge height, wind turbulence deflects small drops around the funnel opening, causing under-measurement of 5–20 % (worse for snow). Shields such as the Alter or Tretjakov windshield reduce this error. Evaporation, splash, and wetting losses also contribute. Weather radar provides spatial rainfall estimation over wide areas but must be calibrated against ground-truth rain gauges. Globally, ~11,000 rain gauges report to the WMO network.