The types of rain gauges most often used by official forecasters and airports consist of four main elements. The primary housing is a large cylinder which holds a funnel and a measuring tube. The other element is a measuring stick that is not a part of the rain gauge itself. The funnel sits at the top of the cylinder and collects rain water as it falls. The rain water runs from the funnel into a measuring tube which is also inside the large cylinder.
The design of the measuring tube is exactly one-tenth the cross sectional dimension as the width of the funnel opening. The measuring tube is reduced to account for the amplification of the height of the water, enabling more precise measurements. For example, 1/10 inch of rainfall through the funnel reads as 1 inch of rain water in the measuring tube. A rain gauge can measure up to 1 inch of water inside the tube before it will overflow into the surrounding cylinder. In order to retrieve an accurate rainfall measurement, observers empty the water in the measuring tube and pour in the overflow water from the cylinder, one inch at a time. Meteorologists insert the measuring stick into the measuring tube to obtain even more accurate readings. Readings from the measuring stick are accurate to within 1/100 inch.
Rain gauges work to assist meteorologists as well as other interested observers in determining how much rain falls in any given area at a particular time and over time. Knowing the amount of rainfall helps predict flash flooding, water levels, flooding and drought conditions that impact everyone. Local weather reports do not offer location specific rainfall measurements for every homeowner or farmer. The amount of rainfall can vary significantly between neighborhoods, even if they are adjacent.
A variety of personal rain gauges available commercially work to help individuals measure the amount of rainfall in their own backyard. These gauges may be complicated instruments that weigh the rain water or simple tubes with measurement indications.
Rain gauges can also work to measure other types of precipitation, such as sleet, hail or snow. In order to accurately measure other precipitation, observers will remove the funnel and the inner measuring tube and allow the precipitation to collect in the outer cylinder. The material is allowed to melt and then poured into the measuring tube for the final, accurate measurement. The amount of precipitation measured in a rain gauge does not equal snowfall amounts. The ratio of snowfall to water can vary by more than 20:1 or by less than 8:1.