Smoke in Relation to Fogs in London 1

Abstract

LONDON fogs are produced by the mechanical combination of particles of water with particles of coal or soot, and require for their fullest development the following conditions: a still air, a temperature lowest at or near the ground in comparison with an elevation of some hundreds of feet, saturation or partial saturation of the air within a moderate distance of the ground, absence of clouds overhead, and free radiation into space. The artificial darkness and peculiar colouring occur with greatest effect at times when a very large quantity of coal is being burnt in domestic fireplaces, and cannot as a rule prevail during the night between 10 p.m. and 5 a.m., or to any great degree in warm summer weather. The early hours of summer mornings are the only ones in which clear views of the whole city are possible. Next in clearness come fine Sunday afternoons in summer, when fires are allowed to go down. The hours of greatest density are those following the time of greatest cold on the earth's surface and of the lighting of large numbers of kitchen and other fires. Thus about 8 to 10 a.m. is frequently the period of thickest and darkest fogs. It may be noted that on Sundays, when factory fires are inoperative, fogs in winter have been densest; on one Christmas Day there was absolute darkness during the whole day, thus showing the dependence of light-absorbing matter on kitchen and domestic fires. Many distinct conditions may alter the time of maximum density.