By Philip Eden

Farmers are able to change the weather. It has been done, and it can be demonstrated pretty conclusively how these changes were achieved. We are not talking here about large-scale deforestation in Brazil, the draining of marshes in Russia, or over-grazing in the sub-Saharan savannah in Africa. We are looking at English arable farmers chiefly in the Midlands, East Anglia and southern England; we are looking at the late-1970s and 1980s; and we are of course looking at relatively localised and short-lived changes in the weather.

Stubble-burning is the subject - that bane of late-summer life between the mid-1970s when it became commonplace among the cereal farms of central, southern and eastern England, and the late-1980s when it was finally banned by the Conservative government of the day.

During summers when the weather in August and September was changeable with brisk gusty winds and regular falls of rain, the smoke from stubble fires was generally a minor nuisance. Occasionally, a fire got out of control, most likely on a windy day. Although the burning of cereal stubble was not allowed when the forecast was for strong winds, corners were occasionally cut, and forecasts occasionally went wrong. Billowing clouds of smoke sweeping across motorways and other major roads could be dangerous, but such events were rare.

It was during the years when August and early-September were quiet and settled that the impact on the weather of stubble burning was greatest. In 1983 and 1984 the effects were remarkably persistent and potentially quite serious. During those years high pressure dominated the weather during the relevant weeks, bringing warm and often sunny weather with fitful breezes.

Under a summer high pressure system there is normally a temperature inversion between 1000 and 3000 metres above the ground, maintaining a mass of stagnant air beneath the inversion, and rain is usually absent. These circumstances conspire to limit the dispersal of smoke particles through a deep layer of atmosphere by turbulence, or across a large geographical area by strong winds. And there is rarely any rain to wash the haze away.

The net result during the Augusts of 1983 and 1984 was a progressive concentration of carbon particles in the lower layers of the atmosphere, and although the larger fragments eventually fell to earth under the influence of gravity, the smaller ones remained aloft in the form of an unpleasant, dense, acrid haze. This haze in turn reduced the strength and duration of sunshine in many inland parts of England.

There was a further effect which also diminished the sunshine duration: the extra heat at the base of the smoke columns helped the polluted air to rise above the condensation level, creating large cumulus clouds which otherwise would not have developed. On some occasions the strength of the warm rising air currents was sufficient to break through the temperature inversion allowing the clouds to grow into fully-fledged cumulonimbus clouds ... end result - rain.

© Philip Eden