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An instrument called rain gauge is used to measure rainfall. It consists of a typical copper container, in which there is a copper collecting can containing a glass jar, and a copper funnel that fits on to the top of the container. The gauge is sunk into the ground so that the top of the funnel is about 30 cm above ground level.
Rain falling over the funnel collects in the glass jar. This is emptied, usually every 24 hours, and measured in a tapered glass measure, graduated in millimeters. The tapered end of the jar enables very small amount of rain to be measured accurately.
The gauge is placed in an open space so that no run-off from trees or buildings or other objects can get into the funnel. Also, the outer case is sunk in the ground to prevent the sun's heat from evaporating any of the rain collecting in the glass jar. It projects about 30 cm above ground level to prevent any rain from splashing up from the ground into the funnel. The rainfall is recorded for a place, either for a day or for a week or a month or a year can be shown on a map. This is done by using lines called isohyets. An isohyet is a line on a map which passes through all places having the same rainfall.Most of Britain's rainfall is bought by depressions which move from the Atlantic which means that the western side of the country experiences the depressions first. In part this explains why western Britain is wetter than eastern Britain but it must be remembered that most of the highland, the mountains and the hills are in the western zone.
Although the amount of rain that a region receives annually is important, of greater importance is the frequency of rainfall , the duration of each fall of rain and the amount that falls.