How Does a Rain Gauge Work?
There are many kinds of rain gauges. A standard design consists of a wide funnel at the top that feeds into a narrow tube, where the rainfall is measured with a special ruler. Let’s suppose one inch of rain falls. If the rain fell into a flat pan, the depth would be one inch. The rain gauge funnels that water into the tube. The mouth of the funnel is ten times the area of the tube.
For every inch of rain that falls into the funnel, the water rises ten inches in the measuring tube. This type of rain gauge determines measurements to the nearest 1/100 inch. Most rain gauges generally measure the precipitation in millimeters equivalent to liters per square metre. The level of rainfall is sometimes reported as inches or centimeters.
Rain gauge amounts are read either manually or by automatic weather station (AWS). The frequency of readings will depend on the requirements of the collection agency. Some countries will supplement the paid weather observer with a network of volunteers to obtain precipitation data (and other types of weather) for sparsely populated areas.
In most cases the precipitation is not retained, however some stations do submit rainfall (and snowfall) for testing, which is done to obtain levels of pollutants.
Rain gauges have their limitations. Attempting to collect rain data in a hurricane can be nearly impossible and unreliable (even if the equipment survives) due to wind extremes. Also, rain gauges only indicate rainfall in a localized area.
For virtually any gauge, drops will stick to the sides or funnel of the collecting device, such that amounts are very slightly underestimated, and those of .01 inches or .25 mm may be recorded as a trace.
Another problem encountered is when the temperature is close to or below freezing. Rain may fall on the funnel and ice or snow may collect in the gauge, blocking subsequent rain.
Rain gauges should be placed in an open area where there are no obstacles, such as buildings or trees, to block the rain. This is also to prevent the water collected on the roofs of buildings or the leaves of trees from dripping into the rain gauge after a rain, resulting in inaccurate readings.