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A
is for …
Anemometer
An anemometer is an instrument which measures wind speed. It does
this either by using the wind’s kinetic energy to turn cups
or vanes, measuring the pressure exerted by the wind on a pressure
tube or plate or by measuring the wind’s cooling power.
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| Anemometer
with weather vane |
The most
common type is the cup anemometer which consists of 3 or 4 conical
or hemispherical cups mounted on a vertical axis. The rotating cups
generate a voltage which then registers on a dial normally calibrated
in knots, miles per hour or metres per second. Ideally, an anemometer
should be mounted on a 10 metre mast on flat ground, away from obstructions
such as trees and buildings to reduce the amount of turbulence, so
the middle of Grizedale Forest Park isn’t the ideal spot to site
one!
Anomalous
Radio Propagation (anaprop)
Radio waves (including signals such as TV pictures) are transmitted
almost horizontally and normally travel about as far as the horizon,
being limited by the curvature of the earth.
In
abnormal atmospheric conditions, such as in subsidence under an
area of high pressure or anticyclone, the waves can be refracted
downwards which has the effect of extending their normal range.
This phenomenon is known as "anaprop" and means that when
an anticyclone is dominating the weather, radio and TV signals can
be interrupted by signals from a long way off, so you may tune in
only to find you get Roger Bennet on Radio Bristol instead of Richard
Nankivell on Radio Cumbria!
Airmass
This is a distinct parcel of air, hundreds or even thousands of
kilometres across, in which temperature and humidity are relatively
constant in all horizontal directions.
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| Michael
Fish, Meteorologist |
Air
masses are separated from each other by transition zones called
"fronts" in which temperature and humidity change markedly
over a small area.
Airmasses
are named after, and gain their characteristics from, their source
regions. They bring distinctive weather conditions to us in Cumbria
- here are some of the common ones.
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| Wet,
wet, wet |
Polar
Maritime air originates in the North Atlantic.
As such it is cool and moist, but it warms slightly on it’s
way south and so becomes rather unstable, often bringing showers
from the NW.
Tropical
Maritime airmasses form in the mid-Atlantic area around Bermuda.
The air is therefore warm and moist. It tends to bring drizzle/low
cloud/mist to western parts of Britain.
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| Storm
clouds |
Tropical
Continental air comes from Africa originally, and as you might expect,
starts off hot and dry. However, it picks up moisture on its way
north and can therefore bring the classic British summer scenario
of three nice days ending in a good old thunderstorm! It can also
bring mysterious "red rain" which gets its colour from
dusts of the desert caught up in the atmosphere.
Polar Continental air comes from two sources: Eastern Central Europe,
or the Siberia area. In summer it is largely warm and dry, and also
rather polluted, having crossed much of industrial Europe on its
way to us. In winter it is very cold, and having picked up moisture
crossing the North Sea, brings the threat of snow, although the
Pennines offer us some protection.
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| Dressed
for the cold |
Arctic
Maritime airmasses originate around the North Pole. Unsurprisingly
they are extremely cold to start with. However they do warm a little
and pick up moisture as they travel south. This renders them unstable
and they often bring snow to northern areas like ours.
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