Confusing concepts of weather and climate


`All professions", said George Bernard Shaw, "are conspiracies against the laity". Likewise, all professions have their thorny problems that have been debated in their ranks for generations. Artists, for example, tease out the precise distinction between culture and civilisation, and theologians, we are told, used to argue about the number of angels that would fit on the head of a pin. When meteorologists have little better to do, they try to distinguish between "climate" and "the weather".

At a practical level, we experience the weather every day, and even without thinking about it very much, we know precisely what it is. In a more formal sense, a meteorologist might tell a member of the laity that "weather comprises short-term variations in such properties of the atmosphere as temperature, rainfall, humidity, cloudiness and wind, and several other factors, all described as they change from minute to minute, hour to hour, or even from one month to the next". And you might possibly have an inkling what was meant.

But the long-term manifestations of the weather are considered to be part of "climate", and this turns out to be a much more woolly concept. The father of modern climatology, Alexander von Humboldt, defined climate in 1845 as "all changes in the atmosphere affecting the human organs". Later climatologists, sensing perhaps that there was something unsatisfactory, indeed even humanly chauvinistic from the point of view of plants and animals, about this definition, came to think of climate as "the average of the weather", a description that is still quite often used.

Meteorologists nowadays, however, feel this is misleading because it implies that deviations from the average condition are in some way abnormal, ignoring the fact that in many cases, the unique and characteristic feature of the climate of a particular area may be the way in which it deviates sharply and habitually from average values.

Moreover, simple averages of some weather elements are quite meaningless: one cannot sensibly speak of an "average wind direction", for example, but the changing frequencies of northeast and southwest winds are of obvious importance.

The concept of climate nowadays embraces statistical concepts, such as the range and variability of the various climatic elements. One of the more recent definitions is: "The synthesis of weather events over the whole of a period statistically long enough to establish its statistical ensemble properties, largely independent of any instantaneous state" - more than enough, I suspect, to continue to confuse the laity.