Among the various applications of meteorology to the practical requirements of everyday life, the observation of fog, as a meteorological element, has. been associated primarily, in the past, with locomotion on land and with navigation on the sea. The first scale of fog intensity used by meteorological observers in this country was, in fact, based on the effect of different degrees of atmospheric obscurity on navigation. The scale was drawn up in 1903 as the result of an investigation into the conditions of formation of London fogs, and subsequently modified by the Meteorological Office, in conjunction with the Admiralty and Trinity House (see Table I.). It included five specifications of fog intensity ranging from a clear atmosphere free from fog or mist, represented by “ of,” to a thick fog, denoted by “ sf.”

Within the last twelve years, while the importance of fog as an obstruction to road and rail transport and to shipping has in no way diminished, the reporting and forecasting of fog has assumed a new significance in relation to air navigation.

The Air Mail route from Cairo to Baghdad is now securely established. Seven years ago the major portion of it, from Transjordan to Iraq, was no more than a project in certain far-seeing minds. The maps of this region between Amman in Transjordan and Ramadi, a little market town on the Euphrates, 65 miles west of Baghdad, were until 1921 painfully reticent. The countries of Egypt, Palestine, and to some extent Transjordan, had been fairly accurately mapped for some time, and were flown over extensively during the war by ourselves and the Germans. The landmarks are generally speaking abundant, and from the navigational point of view they therefore present no outstanding difficulties.