Isotherms: South Africa & S.E Australia circa 1889

Isotherms: South Africa & S.E Australia. Isotherms are lines denoting equal temperature (Greek). The temperature of the air at any place depends on many factors, of which altitude, latitude, distance from the sea, temperature of the sea, and exposure are the chief. Altitude and latitude are the most important. The mean temperature falls about 5.5 degrees C. for every 1000 metres of ascent; but in the Northern Hemisphere, between 30 and 70 degrees N., the mean temperature decreases only 0.75 degree C. for every increase of a degree latitude; i.e 0.0068 degree C for every km nearer the pole. The temperature therefore falls 800 times more in the same distance in a vertical temperature direction than in a horizontal one.

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.