|
 On
Sunday 26th January 2003 a new meteorological record was set in
Eastern Scotland: Aboyne, in Aberdeenshire, recorded a maximum temperature
of 18.3°C. This was the equal-highest temperature for January anywhere
in the UK.
This unusually high temperature
was due to the 'Föhn effect'. The name comes from a German term
given to a wind local to the Alps, which is most noticeable in late
winter and spring.
The Föhn wind draws warm
air up from the south rising up over the Alps. As the air lifts
it slowly cools, and moisture condenses over the mountains. Dry
air changes temperature much more quickly than moist air, so as
the air in the Föhn descends down the leeward side of the Alps,
it warms more rapidly than it had cooled. Thus it reaches a higher
temperature at the bottom of its descent than it held before it
started rising.
As a result, in leeward locations
under the influence of the Föhn temperatures can rise by as much
as 15 to 20 degrees Celsius (27 to 36 degrees Fahrenheit) in just
one hour, and will be considerably warmer than locations on the
windward side of the Alps.
This rapid and significant
increase in temperature can cause problems. The most serious is
at upper levels: with the Föhn, snow can start to melt rapidly,
and this can trigger dangerous avalanches. At lower levels the very
dry nature of the Föhn increases the fire risk to wooden buildings,
such as the typical Alpine holiday chalets.
The Föhn effect helped temperatures
leeward of the Grampians to rise significantly on the 26th January
this year, giving Aboyne its record. The UK was in a westerly flow,
which brought mild, moist air in from the Atlantic.
Temperatures were already
quite mild for January: 12 to 14°C (54 to 57°F) across most parts
of the country. The air was forced up over the Highlands, causing
it to condense and hence lose most of its moisture. On its descent
down, it warmed up more readily and Aberdeenshire bathed in temperatures
between 14 and 17°C (57 to 63°F), and 18.3°C (64.9°F) in Aboyne.
The Föhn effect is most commonly
seen in the Alps, but also in the Rockies, in the USA, (where it
is called the Chinook) and other mountainous regions around the
world. It can last for two or three days, bathing leeward locations
in spring-like conditions as the dry air means it is also cloud-free.
Unfortunately the spring-like
conditions experienced in Aboyne did not last for long: at midday
on the 30th the air temperature was -2°C (28°F), and with a 20 knot
(23 mph) wind, there was a wind chill equivalent temperature of
-12°C (10°F)!
|
