|
In
theory, on a perfectly clear day the visibility can exceed
200 kilometres. (The visibility is always measured in metric
units.)
In
reality, the visibility is usually much lower as a result
of natural pollutants in the air - dust, pollen, salt crystals
etc.
Man-made pollutants such as car fumes and heavy metals from
factory chimneys also add to the ‘haze’ that is normally suspended
in the atmosphere.
The
best visibility usually occurs when the wind is blowing from
the North. The air coming from this direction is usually relatively
unpolluted, having originated over the North Pole.
Similarly,
most our poorest visibility is usually found when the wind
blows from the East.
Easterly winds bring air from polluted regions of Eastern
Europe. That pollution is added to as the air travels across
the industrial areas of Northern France, Belgium, the Netherlands
and even London before reaching the West Country.
The biggest culprit, the one that really knocks down the visibility,
is water vapour. To put it simply, warm air can hold masses
of the stuff, whereas cold air cannot.
That is why the water vapour in our breath condenses in front
of us on winters' days. So, usually, the quickest way to reduce
visibility is to cool the air.
Fog can form in several ways but the most common type is what
we call ‘radiation’ fog.
This usually forms on fine nights in the autumn when the nights
are long but by day the air is warm enough to hold a lot of
moisture. Under clear skies the temperature falls and the
air cannot hold that moisture.
Without
any wind we would just get a heavy dew or a little shallow
fog as all the moisture falls to the ground. But add in a
light breeze and it can just stir the air enough to cause
the fog to deepen. And then you’ve got problems.
When
the visibility falls below 10 kilometres we call it ‘mist’
or ‘haze’ depending on the humidity level. Below 1000 metres
we are in ‘fog’. Thick fog is when the visibility is less
than 200 metres.
Fog is a bigger killer than you might think. In 1977 at Tenerife,
a KLM 747 was taking off in thick fog at the same time the
a Pan Am 747 was taxiing on the same runway. 583 people were
killed - no survivors.
All the regions motorways and major routes have seen serious
accident caused by people travelling too fast in fog. Its
easily done though. The lack of contrast that fog produces
fools our sensory perception, so everything looks to be moving
more slowly than it really is.
That
is why the met office issues specific warning for fog on the
forecasts. 200 metres is considered as 'fog' for motoring
purposes. 50 metres is ‘thick fog’ when driving becomes positively
dangerous.
It
is amazing how quickly the visibility can change - from thick
fog at dawn to a sparkling 50 kilometres just a few hours
later.
|
