Light, Air, and Fog. by G
13 LIGHT, AIR, AND FOG. BY G. V. POORE, M.D., F.R.C.P. Read at the Sessional Meeting, February 8th, 1893. IN considering the climatological conditions which are favourable to the formation of fog in London one cannot do better than examine actual facts; and, therefore, I have recourse to the returns to the Registrar-General for the week ending December 26th, 1891, for the purpose of making a critical study of the notable fog of Christmas, 18i)1, its cause and consequences. In order that fog may be formed, the air must be laden with moistu1’e, and consequently in London the winds which bring the fog are those which come from the eastward, over the sea and up the estuary of the Thames. Fogs are necessarily phe- nomena which depend upon stillness of the air, for they are quickly dissipated by even a moderate breeze. In order that the moisture-laden air may precipitate its moisture in tlie form of fog it needs to be c7ailled, and, therefore, our winter fogs only occur in cold weather. December 20th, 1891, at Greenwich was a clear frosty day with very light air from the north-east. On the following day the wind became due east and dropped to a dead calm, and then commenced a dense fog which continued till the evening of Christmas Day, when the wind shifted to the south-west and the frost and fog came to an end. In the week ending December 26th six days were frosty, and a dense fog continued for the greater part of five days.
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