"Every cloud engenders not a storm," announces the Duke of Clarence with somewhat misplaced optimism in Shakespeare's Henry V. Nonetheless, meteorologists like to keep an eye on them. They give us valuable clues about the thermal structure of the atmosphere, and it may also be possible to tell from a distant cloud report if a front is approaching the area, if thunderstorms are likely, or if frost or fog may be expected after dark.
But how does an observer on the ground assess this mass of grey on high? To complete a weather report, the observer must decide on three things: how much cloud exists, what type of cloud it is, and its height above the ground. The amount of cloud is recorded in okta - one okta being a coverage of one-eighth of what is rather grandiosely called the "celestial dome". Until not too long ago, cloud amounts were assessed in tenths, but flying in the face of the emerging trend for decimalisation meteorologists decided in 1949 to adopt the "okta" system.
One advantage 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 okta, or even quadrants - each of 2 okta - and by examining each zone separately arrive at a more accurate assessment of the whole.
Assessing the amount of cloud comes easily with practice, but experience and training are required for the observer to be able to recognise the different types. Cloud is classified into more than 30 different categories, ranging from stratus on or near the ground, to cirrus four or five miles above our heads.
And the last piece in the jigsaw is the cloud height. A skilled observer can estimate the height of the base of low cloud to within 50ft, and that of very high cloud to within 1,000ft or so. For more accurate assessment, however, particularly at night, a ceilometer is often used.
This ground-based instrument sends a laser beam vertically upwards in very short pulses. The beam of light is scattered in all directions when it hits a cloud layer overhead, and some of the light is redirected downwards towards the ground.
The time taken for the light to travel upwards to the cloud, and then return to the ceilometer, can be used to by a computer in the instrument to calculate the height of cloud precisely.