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In
theory, on a perfectly clear day the visibility can exceed 200 kilometres.
(The visibility is always measured in metric units.)
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| The
best visibility usually occurs when the wind is blowing from
the north |
In
reality, the visibility is usually much lower as a result of natural
pollutants in the air - dust, pollen, salt crystals etc. Manmade
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.
Water
vapour is the biggest culprit
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.
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 have got problems. |
| Richard
Angwin |
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 have got problems.
Mist
or haze
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's
why we issue 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.
by
Richard Angwin
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