Isohyets: World circa 1889

Rainfall maps. Isohyets are lines denoting equal volumes of rainfall (Greek). The term rainfall includes all the forms in which water is precipitated on the Earth's surface - rain, snow, hail, and sleet. It is measured by observing the depth of water in a vessel with vertical sides, with an open surface that lies in a horizontal plane. The snow entering such a guage is melted, and the depth of water it forms is measured.

The temperature and pressure maps of the World are based on the observations of 1539 and 1280 land stations repectively during fifteen years. If we assume only two daily observations at each station, this means that the world maps of annual temperature summarises about 17,000,000 observations, and those of annual pressure at least 14,000,000 observations, excluding all observations made at sea. The climate maps of the countries shown in greater detail are usually based on returns from many more stations for much longer periods. For instance, the annual temperature map of the British Isles summarises at least 4,250,000 observations, and that of temeprature nearly 5,000,000 observations.

The rainfall maps of the British Isles and central and southern Europe and South Africa are based on returns for a definite period. The other maps are made for the longest means available, but have been specially treated. The rainfall maps are based on returns from many more stations than the others. The annual map of the British Isles summarises the daily observations at nearly 760 stations during 25 years, or nearly 7,000,000 observations. About 1100 stations were used to make the rainfall maps, compared with 100 for the temeprature maps of India. The annual rainfall map of the World summarises the records of nearly 9000 land stations, two-thirds of them in Europe.