Weather and Visibility

Visibility, to most of us, is not really too much of a problem most of the time. It really only becomes a problem when we get fog, and then this interferes with driving our cars.

Fog is basically cloud at ground level, and so the distance we can see in it is obscured by millions of tiny water droplets. On still cold nights this poor visibility is most often found in valley bottoms, where the dense air collects, and condenses out into cloud droplets.

At other times, when it is humid and raining, the poor foggy visibility is found on hills, where the cloud forms as it ascends the hill tops. Hence, the phrase you will occasionally hear on the weather forecast of "patches of hill fog".

Although poor visibility can be a nuisance for all of us, there are some activities where it is positively dangerous, and one group of people, where it can be life-threatening, are aircraft pilots.

We used to have an ingenious method of measuring the visibility at night. A light, of known candle power and distance, was placed on the airfield. We would then look at it through an instrument that looked, and behaved, like a slide rule, with sliding piece made of smoked glass. This was graduated from clear to completely opaque, and by looking at the light through it and, moving the slide until it just disappeared, enabled us to calculate the visibility.

Fog, for the general public, is defined as a visibility of less than 200 metres, and dense fog less than 50 metres, but at sea and in the air it has different parameters. Here, fog is a visibility of less than 1000 metres and dense fog less than 200 metres.

For outdoor photographers the visibility is the most important part of the weather forecast. They become very good amateur meteorologists and can look at a synoptic weather map to see when, and where, they would get the best visibility.

In this country we get this with a northerly wind. These winds bring air straight down from the Arctic, and can give almost unlimited visibility with objects miles away very sharp and in focus, whereas the worst visibility can come from the southeast. This wind comes to us from the industrial areas of the Ruhr, which has large amounts of pollutants trapped in under the temperature inversion. We can also get a lot of fog on hills and mountains in the western side of the country when there is a moist southwesterly wind blowing.