Unpronounceable displaces unfashionable

Nowadays the amount of ozone in the atmosphere above a certain point is specified in terms of a unit both obscure and unpronounceable…

Nowadays the amount of ozone in the atmosphere above a certain point is specified in terms of a unit both obscure and unpronounceable, an "m atm-cm". Three hundred "m atm-cm" is near enough to normal; readings over 400 m atmcm are often recorded in the higher latitudes in summer, while values as low as 90 m atmcm have been obtained in the middle of the Antarctic springtime "ozone hole".

Meteorology, however, was less linguistically challenging when I was young. We had never heard of m atm-cms, but we knew the amount of ozone that it represented; we called it a Dobson Unit, after Gordon Dobson who in those days was still alive and well, and fit and hale and hearty. He died, unfortunately, 23 years ago today, on March 11th, 1976, and it was after his departure that the m atm-cm was thrust upon us.

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 spectro-photometer 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 measurements, combined with 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 at which the observation has been made.

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In due course, Dobson's name was 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 the ozone concentration came to be expressed as the hypothetical thickness in hundredths of a millimetre of a "concentrated" ozone layer. These were Dobson Units, or DUs for short, one of which corresponds to an average atmospheric concentration of ozone of approximately one part per billion.

O tempora, O mores! Meteorologists used to love chattering on about their Dobson Units. But now that the m atm-cm has taken over this particularly rapid, unintelligible patter isn't generally heard, and if it is it doesn't matter.