THE scientific Renaissance in the 17th century provided, for the meteorologists of the time a wide range of new instruments to play with. Readings' from barometers and thermometers in particular quickly built up into a great bulk of weather data, but it was by no means clear what anyone could do with it. There were no recognised methods for extracting salient conclusions from such a vast array of numbers.
At first the data were arranged in tables, and a perusal of the figures allowed some conclusions to be drawn. But this had obvious limitations, and in due course efforts were made to try to represent the data pictorially to transform the information in the numbers into some kind of diagram in which a pattern could be seen.
The first to try this method was Edmund Halley of the famous comet in 1686, he drew a map which showed the prevailing maritime winds both in and near the tropics, "whereby tis possible the thing may be better understood than by any verbal description whatsoever".
Halley went on to study terrestrial magnetism, and in 1701 he published another famous map which featured lines joining places with the same magnetic declination thereby inventing one of the most important tools of modern science, the isopleth, or a line joining points having the same value of any element of interest.
Although Halley's maps were widely noticed at the time, more than 100 years were to elapse before his techniques became widely used in, meteorology. Then Alexander von Humbolt saw them as a way of picturing the distribution of heat over the Earth's surface; he drew lines on a map which he called isotherms, joining places having the same average temperature.
He introduced his isotherms, to the Academie des Sciences in Paris in 1817, and so enthusiastic was the reception that, by 1820, meteorologists all over Europe were drawing lines of equal barometic pressure, of annual precipitation and the frequency of thunderstorms, as well as of temperature and the deviation of the temperature from normal.
Most isopleths, like the generic term itself, have names that begin with the Greek prefix "iso-" meaning "equal". Readers of Weather Eye will be most familiar with the isobar, a line joining points of equal pressure, but there are many, many more - like isohyets, isokels, isonephs and isotachs, that relate to rainfall, sunshine, cloud and wind, respectively. Strangely, it was another 60 years before anyone thought of using graphs to illustrate the data. The term itself, and the method of depiction it describes, did not appear until the 1880s.