Taking up the Dobson choice


METEOROLOGISTS, by and large, conform to Oliver Cromwell's assessment of the Quakers: "A people risen and come up that I cannot win either with gifts, honours, offices or places.

But there is one accolade toe which even weather people are susceptible, albeit that is usually awarded only after death, and that few can reach the heights of eminence needed to achieve it: to have a unit of meteorological measurement bear one's name.

It is an honour rarely accorded nowadays, if only for the reason that most of the common units have been named already. Pascal of hectopascal fame, for example, lived 350 years ago; the eponymous Celsius and Fahrenheit are only slightly later; and even Beaufort of the Beaufort Scale was a man of the early 19th century. But Dobson of the Dobson Unit, or DU for short, died only 20 years ago - on March 11th 1976 to be precise.

Gordon Dobson was the pioneer of ozone measurement. He was born in 1880 and devoted most of his life to the subject, his greatest contribution, perhaps, being the invention of the Dobson speclmphotometer which was for many years the standard instrument for measuring ozone in the atmosphere.

It does so by comparing the amount of the sun's radiation that manages to penetrate to ground level at two separate ultraviolet wavelengths one that is strongly absorbed by ozone and another that is not.

These combined readings at the earth's surface can be used to estimate the total amount of ozone in the column of air directly above the point where the measurements are taken.

Ultimately, Dobson's name came to be given to the units in which we measure atmospheric ozone. He calculated that if all the ozone in the atmosphere were brought down to the earth's surface, it would form a layer only 3 millimetres thick.

Becoming rather fond of this analogy, he converted all his readings into "thickness" units, the idea caught on, and nowadays ozone concentration is expressed in Dobson Units - the hypothetical thickness in hundredths of a millimetre of a "concentrated" ozone layer.

One DU corresponds to an average atmospheric concentration of ozone of approximately one part per billion. As we know, however, this ozone is not distributed uniformly through the vertical column, about 90 per cent of it being in a relatively narrow zone some 15 to 20 miles above our heads - the so called "ozone layer".

Three hundred DUs is around average readings over 400 are often recorded in high latitudes in summer, while values as low as 95 DU have been obtained in the middle of the Antarctic springtime "ozone hole".