Long term trends in London fog

The frequency, and weekly and seasonal distribution of London fog may be obtained from non-instrumental observations which stretch back into the seventeenth century. Despite difficulties due to differences in individual observer's definitions of fog and perceptual bias, a coherent picture emerges. The fogs of the late seventeenth century have a surprisingly high frequency (20 per year) and a rather unusual seasonal distribution that may arise from the Little Ice Age climate. The eighteenth and early nineteenth century seems to have been a period with a relatively low frequency of fogs (about 10 per year), but most of the nineteenth century exhibits a dramatic rise in the incidence of fogs which rose to nearly 70 per year by the 1890's. These changes were followed by an equally dramatic reduction in fog frequency throughout the present century. These trends are attributable to changes in the distribution and strength of London's air pollution sources. The weekly distribution of fogs shows no really pronounced "Sunday-effect" which might implicate domestic sources for the pollution, despite contemporary comment on this phenomenon. The yellow colour and proverbial celebrity of London fog is also discussed. © 1981.