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F
is for …
FOG
may just look like fog, but did you know there are lots of different
types? They are formed in very different ways, but the definition
is the same - an obscurity, caused by water droplets and/or smoke
suspended in the lower layers of the atmosphere, giving visibility
of less than 200 yards (180m).
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| Fog
on the motorway |
One of the most common is Radiation Fog. For this to form you
need damp air, clear skies overnight, and light winds. Under clear
skies the moist air cools down and the moisture condenses into droplets.
A gentle breeze stops the moisture from settling on the ground as
dew, and keeps it suspended as fog.
Advection
Fog is known in the northeast as fret or haar. Relatively warm,
moist air, often from Central Europe, passes over a cooler surface
such as the North Sea, and the moisture in the air condenses out
into water droplets. Many a lovely spring or summer day is blighted
around our east coasts by fret pushing inland a little way and blotting
out the sun.
Hill
Fog is basically cloud whose base has lowered to envelop the
hilltops.
Upslope
Fog is formed when moist air is forced to rise up and over high
ground. As it rises it also cools, and so the moisture condenses
out into droplets.
Frontal
fog forms at or near to fronts, when precipitation falling from
relatively warm air above the frontal surface falls into cooler
air below. The raindrops evaporate and cause the cooler air to become
saturated with moisture.
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| Santiago
skyline under a thick cloud of smog |
Smog
is a mixture of fog and pollution. Pollutants can enhance the formation
of fog as they act as tiny nucleii around which the water droplets
can form more readily.
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| Horses
exercise in frost , Middleham, North Yorkshire |
FROST
occurs when the temperature of air in contact with the either the
ground or our temperature-measuring equipment is below zero degrees
Celsius, the freezing point of water. In these cases we say there
is 'ground frost' and 'air frost' respectively.
Our perception of cold is defined not only by air temperature, but
by wind speed too, and is classified as “slight/moderate/severe/very
severe” depending on how the two combine in a given situation.
Fohn
Winds are warm, dry winds that blow down the leeward slopes
of mountains. The air that goes up the mountain may be cool and
moist, but during the process of rising it is compressed and cooled,
losing moisture. This means that it can warm up to a greater extent
as it comes back down the other side of the mountain, so the net
effect is a warming and drying of the air. Originally named in the
Alps, this effect can sometimes be felt on the eastern side of the
Pennines where, in stable westerly breezes, temperatures may be
pushed up a few degrees higher than they are on the western side.
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| Funnel
Cloud in USA |
Funnel
Cloud
This is a column of tightly and fast-spinning air, with very low
pressure at the centre, which extends downwards from the base of
a convective cloud, sometimes all the way to the ground. It is often,
though not always, at the core of a waterspout or tornado.
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