Imagine yourself on a bright clear morning, observing a tall factory chimney from which smoke is billowing forth in great quantities. Its behaviour will depend mainly on two factors - the strength of the wind 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, will rise vertically, and the air near the ground on all sides will remain clear and unpolluted. Nature, however, is rarely so obliging.
Usually there is a wind, and the smoke drifts with it horizontally away from the top of the chimney. When the air temperature drops off quickly with height - a condition very conducive to the development of rising currents of air - it results in what is called a looping plume; the trail of smoke spreading away from the chimney undulates sharply up and down, so the plume is very high over some spots - perhaps twice as high as the chimney - and at other places it dips down to touch the ground below.
If the atmosphere is stable - thermally stratified, with no great tendency for vertical motion - coning takes place; the plume of smoke spreads out downwind of the chimney-top, resembling a cone with a more or less horizontal axis. In a very stable situation, fanning is observed, where the smoke spreads out laterally in the shape of a fan, with little vertical diffusion. But the most interesting patterns occur when an inversion exists - when the temperature actually increases with height over a shallow layer of the atmosphere. The inversion acts like a barrier to the diffusion of the smoke; if, as in most cases, the chimney is below the inversion, the plume has a flat top, and the smoke fills the space downwind of the chimney below the inversion to pollute the surrounding area. This condition is aptly known as fumigation.
The relevance of the smoke this Tuesday morning is that its behaviour in all these different circumstances bears some resemblance to what may happen to airborne moisture droplets expelled from the respiratory systems of animals infected with the foot-and-mouth virus. These droplets may be carried along by the wind for several miles to infect otherwise healthy stock, thriving in conditions of high relative humidity. The infected particles, unlike the smoke, have no significant intrinsic heat, but otherwise they will be subject to much the same atmospheric processes as the plume of smoke - looping, coning or fanning on the way as they proceed merrily upon their deadly journey.