kelvin (K)		Lord Kelvin (William Thomson) (1824–1907)

William Thomson was a Scottish physicist and engineer. He made a wide range of contributions to meteorology, but is best remembered as the originator of the kelvin* temperature scale.

In 1848 Thomson proposed a temperature scale based on the fundamental laws of thermodynamics. The construction of his scale was such that there had to exist a point in it at which no "caloric" (heat) could be transferred. This absolute cold (now known as absolute zero) was an idea that had been discussed as far back as Boyle, in his 1665 work New Experiments and Observations touching Cold. Laplace and Lavoisier also considered the concept during their work on heat and calorimetry in the 1780s, as did Dalton in his 1808 work A New System of Chemical Philosophy.

Thomson was given the title First Baron Kelvin in 1892, and came to be known as Lord Kelvin, just as his temperature scale became widely known as the kelvin scale. In modern scientific usage (Système International, or SI) these temperatures are referred to as kelvins, with the symbol K. 0 K (absolute zero) is equal to -273.16°C on the Celsius scale and -459.67°F on the Fahrenheit scale.

Thomson considered vortices in fluids, and did some work with von Helmholtz in this area. This work led to Thomson's theorems on fluid circulation which were published in 1869. The two men are also remembered for their study of a form of atmospheric instability now known as Kelvin-Helmholtz instability.

Thomson studied atmospheric moisture and precipitation, and in 1862 gave the first correct description of the saturated adiabatic process. In such a process, the rate of cooling with height of a rising parcel of saturated air is less than that of a rising unsaturated parcel. These ideas are important in studies of convection and stability in the atmosphere. This work was later extended by Heinrich Hertz, who in 1884 published a diagram for calculating the changes in a parcel of moist air rising adiabatically in the atmosphere. This diagram was the precursor of all modern thermodynamic diagrams.

Thomson also did some preliminary work on the laws that govern atmospheric pressure tides, following pioneering studies in this area by Laplace. This work was later generalized by Rayleigh and Margules.

Thomson recognized the existence of atmospheric electric fields in various weather conditions including fair weather (Franklin's well-known work on atmospheric electricity was mainly associated with thunderstorms and lightning). This followed the pioneering work of Giambatista Beccaria who made quantitative measurements of atmospheric electricity in various weather conditions in Italy in the 1770s, and wrote in 1775 that "if, when the rain has ceased ... a strong excessive [i.e. positive] electricity obtains, it is a sign that the weather will continue fair for several days; if the electricity is but small, it is a sign that such weather will not last so much as that day, and that it will soon be cloudy again, or even will again rain" (Beccaria, G., 1775: Della elettricite terrestre atmosferica a cielo serno, Turin). Some 8 decades later Thomson also strongly believed that knowledge of atmospheric electricity could be useful in weather forecasting. He described his ideas in a paper presented to the British Association for the Advancement of Science in 1859 (Thomson, W., On the necessity for incessant recording, and for simultaneous observations in different localities, to investigate atmospheric electricity. 29th meeting, British Association for the Advancement of Science, Aberdeen, September 1859). Thomson stated in a lecture to the Royal Institution in 1860 that "there can be no doubt but the electric indications, when sufficiently studied, will be found important additions to our means for prognosticating the weather; and the speaker hopes soon to see the atmospheric electrometer generally adopted as a useful and convenient weather glass".

To make his own measurements of atmospheric electricity, Thomson invented the water dropper electrometer. In this instrument, the water droplets in a spray from water in an insulated metal tank acquired the local electric potential of the atmosphere which could then be measured using a normal electrometer. This was a major contribution to the science of atmospheric electricity measurement.

The water dropper electrometer was adopted at the British Met Office observatory at Kew, where Thomson established a program of continuous measurements of atmospheric electricity in 1861. He found that the electrification of the atmosphere in clear sky conditions could be explained through the presence of atmospheric positive charges. Thomson's instrument was later adopted at other weather observatories, including the one at the top of the Eiffel Tower, and was also used in some European scientific balloon flights of the early 20th Century.

Kelvin's electrometer measured relative changes in the electric field. C.T.R. Wilson (who had demonstrated an early version of his cloud chamber to Kelvin in the late 1890s) also worked in the area of atmospheric electricity, and as part of his research in the early 1900s made improvements to Kelvin's measurement techniques and designed updated electrometers.

In honour of his scientific work, Kelvin's name was given to the SI (International System of Units) unit of thermodynamic temperature. Kelvin is the absolute temperature scale, and a change in temperature of one kelvin is the same size as a change of one degree Celsius.

^*Note that the kelvin temperature scale, when spelled out, is not capitalized, to distinguish the unit name from the person Kelvin. The symbol K, however, is capitalized.

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