Considering the clouds


KING Solomon of old was very wise. Nor, indeed, did he hesitate to give the world the benefit of his advice, since we have it on reliable authority that "he spake three thousand proverbs, and his songs numbered one thousand and five". He is also said to have written Ecclesiastes, in which he seems to warn his readers that material rewards are unlikely to be showered on those who pursue a career in meteorology: "He that observeth the wind shall not sow; and he that considered the clouds shall never reap".

Despite this divinely inspired discouragement, weather people spend a great deal of their time analysing the wind, and considering the structure of clouds in detail. The latter is a formidable task. As the 19th century art critic John Ruskin put it: "Nature never lets one of the members of even her most disciplined groups of clouds be like another; but though each is adopted for the same function and in its great features resembles all the others, not one out of the millions with which the sky is chequered is without a separate beauty and character".

In each individual case, the weather observer isolates three factors of importance: what type of cloud it is, its height above the ground and, most importantly, how much of it there is.

Experience and training are required for the observer to be able to recognise the different types of cloud, since there are more than 30 categories, ranging from stratus on or near the ground, to cirrus four or five miles above our heads. For assessing the height, a ceilometer is often used - an instrument, which employs a laser beam to calculate the distance of the cloud above the ground - although a skilled observer can estimate the height of a low cloud to within 50ft, and that of even a very high cloud to within 1000ft or thereabouts. The amount of cloud is reported in a unit called the okta.

An okta is a coverage of one eighth of what is rather grandly called the "celestial dome". Until not too long ago, cloud, amounts were reported in tenths, but flying in the face of the emerging trend for decrimilisation, meterologists decided in 1949 to adopt the okta system. One advantage of this is that it always allows the amount of cloud in the sky to be included in a weather report as a single figure. It is also convenient that the number 8 has a multitude of factors: it is useful at times for the observer to be able mentally to divide the sky into two halves, each of 4 oktas, or even quadrants - each of two oktas and by examining each zone separately arrive at a more accurate assessment of the whole.