why the glass measuring cylinder of the rain gauge is inside an outer container
Rain gauge The standard NWS rain gauge, developed at the start of the 20th century, consists of a funnel emptying into a graduated cylinder, 2 cm in radius, which fits inside a larger container which is 20 cm in diameter and 50 cm tall. If the rainwater overflows the graduated inner cylinder, the larger outer container will catch it. When measurements are taken, the height of the water in the small graduated cylinder is measured, and the excess overflow in the large container is carefully poured into another graduated cylinder and measured to give the total rainfall. Sometimes a cone meter is used to prevent leakage that can result in alteration of the data. In locations using the metric system, the cylinder is usually marked in mm and will measure up to 250 millimetres (9.8 in) of rainfall. Each horizontal line on the cylinder is 0.5 millimetres (0.02 in). In areas using Imperial units each horizontal line represents 0.01 inch.