|
A
katabatic is a wind that occurs on steep-sided valleys such
as you find across much of the West Country like Cooper's
Hill in Gloucestershire.
After
a fine day during the late autumn and early spring - once
the sun sets - the air at the top of a large hill cools down
quickly.
You
see, temperatures decrease with height anyway. So the air
near the top of the hill cools quickly, certainly more quickly
than at the bottom of the hill. And of course, cold air is
denser than warm air - so the air at the top of the hill flows
downhill.
You
can feel a cool draught blowing downhill on cold nights. Wind
speeds are generally light, between 5 and 10 miles per hour.
In
other parts of the world these winds are much stronger. On
the Antarctic ice shelf speeds of over 200 miles per hour
have been recorded.
Of
course, by day, as the top of the hill warms, the air becomes
less dense and the flow is reversed - that’s an anabatic wind.
Paragliders and hang-gliders can take advantage of this breeze
on the steepest of hill.
There
are lots of local winds around our region. On a Points West
forecast I can only hope to give a general wind speed and
direction. But our winds are funnelled up the Bristol Channel;
squeezed between the moors and they develop off the sea. And
to that list you can add the katabatic.
|
