Attention in this column yesterday was focused on smoke as it ascends a chimney from a fire. What could be more logical than to follow the smoke out into the open air, and see what happens to it then?
As in almost every other area of life, there is a weather lore concerned with the smoke from chimneys. You will be told that when the smoke rises vertically from the stack it is a sure sign of good weather and that when the plume arranges itself almost in the horizontal, the weather will be poor.
Chimney stacks, alas, are not much good as weather forecasters, but the behaviour of the smoke does tell us something about atmospheric conditions at the time, and the different patterns it makes have been given names by those who like to classify such things.
Particularly in the case of very tall chimneys, like those of factories or power stations, two factors govern the behaviour of the smoke: the strength of the wind at the time and the thermal structure of the atmosphere.
If there is no wind and if the air temperature decreases in the normal way with height, the plume of smoke, being warm and buoyant, rises vertically. Usually, however, there is a wind, and then more interesting patterns emerge.
When the atmosphere is unstable - where the temperature falls very rapidly with height - combined with the wind such conditions result in what is called a looping plume: the trail of smoke spreading from the chimney undulates sharply up and down, so that the plume is very high over some spots - perhaps twice as high as the chimney itself - and at other places it dips down to touch the ground below.
If the atmosphere is "stable" - or thermally stratified, with no great tendency for vertical motion - "coning" takes place: the plume of smoke spreading downwind from the chimney resembles a cone with a more or less horizontal axis.
And in a very stable situation, "fanning" is observed: the smoke spreads out laterally in the shape of a fan, with very little vertical diffusion.
But the most interesting patterns occur when an "inversion" exists - when the temperature, in contrast to the usual situation, increases with height over a shallow layer of the atmosphere.
The inversion acts like a barrier to any vertical diffusion of the smoke, and when, as in most cases, the chimney is below the inversion, the plume has a flat top; the smoke fills the space downwind of the chimney below the inversion to pollute the surrounding area, a condition aptly known as "fumigation".