Remarks on the clear air


THE distance a meteorologist can see is called "the visibility". It has nothing to do with the distance away of the horizon, nor for that matter does it bear much relation to how far anybody else might see. It is carefully defined as "the furthest horizontal distance at which a person of normal sight can distinguish and identify an object for what it is known to be, in normal conditions of daylight illumination" - a concept that is not at all as simple as it seems.

The definition is framed to exclude extraneous matters from the reckoning - irrelevant variables like the eyesight of the observer, the amount of light (if any) that may be present at the time, and the degree of contrast that may exist between an object and its background. What it does try to isolate as the most important factor is the "turbidity", or transparency, of the atmosphere.

The air is never completely clear rays of light heading towards an observer from some distant object undergo a continuous process of attenuation caused by impurities or water droplets suspended in the atmosphere. And even if the air were perfectly clear, the maximum visibility would still be only about 150 miles, because the molecules of the air itself attenuate the light. In practice, it is quite unusual to be able to see clearly a distance of more than 40 miles.

If, therefore, you listen to the sea area forecast on the radio, and hear the encouraging news "visibility unlimited", it should not be taken literally: it merely means that visibility is something in excess of 50 miles, the highest numerical value with which meteorologists concern themselves, and that it will not be a limiting factor for any normal activity that might be undertaken.

More frequently, however, visibility is reduced by haze, or mist, or fog. Haze is a reduction in visibility caused by tiny solid particles suspended in the air - little motes of dust or smoke. The latter two are variations on the same theme: in the case of mist or fog, visibility is reduced by tiny droplets of water suspended in the air. The distinction is that when the visibility is less than 1,000 metres - or 1,100 yards - the condition, purely by convention, is described as "fog": if visibility is greater than 1,000 metres, the obscuring medium - is called a mist. Sometimes, of course, to add to the confusion, people refer to a light drizzle as a mist, a gaffe that makes any thinking meteorologist recoil in horror.