have all seen a foggy day when it is difficult to see, and no matter
how fast you run the fog seems just as thick and just as far away
from you. This is because there is no insulating blanket of air
above to keep the earth's heat in. The air closest to the ground
cools rapidly, with all the water vapour in the air condensing as
tiny floating droplets of water.
is simply how far you can see. In weather terms it is how clear
and air are, taking into account fog, mist and urban pollution.
Fog and mist are like clouds on the ground. They are made up of
lots of tiny water droplets, which is why you can sometimes get
wet when walking through fog. When there is a light breeze, clear
skies and damp air, moisture in the air condenses
near the ground making fog or mist.
can be estimated using descriptive words such as: good, poor, foggy
etc. How far you can see is also used to describe visibility:
Fog – less than 1km
Poor – 1 to 5km
Moderate – 5 to 10km
Good – more than 10km