Geoscience Reference
In-Depth Information
Wales and the Pennines have little effect. There are
also areas of negative enhancement on the lee side
of mountains. The sheltering effects of the uplands
produce low annual totals on the lee side (with
respect to the prevailing winds). Thus, the lower
Dee valley in the lee of the mountains of North
Wales receives less than 750mm, compared with
over 2500mm in Snowdonia.
The complexity of the various factors affecting
rainfall in Britain is shown by the fact that a
close correlation exists between annual totals in
northwest Scotland, the Lake District and western
Norway, which are directly affected by Atlantic
depressions. At the same time, there is an inverse
relationship between annual amounts in the
Western Highlands and lowland Aberdeenshire,
less than 240km to the east. Annual precipitation
in the latter area is more closely correlated with
that in lowland eastern England. Essentially, the
British Isles comprise two major climatic units
for rainfall - first, an 'Atlantic' one with a winter
season maximum, and second, those central
and eastern districts with 'continental' affinities
in the form of a weak summer maximum in
most years. Other areas (eastern Ireland, eastern
Scotland, northeast England and most of the
English Midlands and the Welsh border counties)
have a wet second half of the year.
The occurrence of snow is another measure of
altitude effects. Near sea level, there are on average
about five days per year with snow falling in
southwest England, 15 days in the southeast and
35 days in northern Scotland. Between 60 and
300m, the frequency increases by about one day
per 15m of elevation and even more rapidly on
higher ground. Approximate figures for northern
Britain are 60 days at 600m and 90 days at 900m.
The number of mornings with snow lying on the
ground (more than half the ground covered) is
closely related to mean temperature and hence
altitude. Average figures range from about five
days per year or less in much of southern England
and Ireland, to between 30 and 90 days on the
Pennines and over 100 days on the Grampian
Mountains. In the last area (on the Cairngorms)
and on Ben Nevis there are several semi-perma-
nent snow beds at about 1160m. It is estimated
that the theoretical climatic snowline - above
which there would be
net
snow accumulation - is
at 1620m over Scotland. Since 1987 all but three
years up until 2000 have seen below-average
(1961-1990) snow cover duration.
Marked geographical variations in lapse rate
also exist within the British Isles. One measure of
these variations is the length of the 'growing
season'. We can determine an index of growth
opportunity by counting the number of days on
which the mean daily temperature exceeds a
threshold value of 6°C. Along southwestern coasts
of England the 'growing season' calculated on this
basis is nearly 365 days per year. Here it decreases
by about nine days per 30m of elevation, but in
northern England and Scotland the decrease is
only about five days per 30m from between 250 to
270 days near sea level. In continental climates
the altitudinal decrease may be even more gradual;
in Central Europe and New England, for example,
it is about two days per 30m.
B NORTH AMERICA
The North American continent spans nearly 60
°
of latitude and, not surprisingly, exhibits a wide
range of climatic conditions. Unlike Europe, the
West Coast is backed by the Pacific Coast Ranges
rising to over 2750m, which lie across the path of
the mid-latitude westerlies and prevent the
extension of maritime influences inland. In the
interior of the continent, there are no significant
obstructions to air movement and the absence of
any east-west barrier allows air masses from the
Arctic or the Gulf of Mexico to sweep across
the interior lowlands, causing wide extremes of
weather and climate. Maritime influences in
eastern North America are greatly limited by
the fact that the prevailing winds are westerly,
so that the temperature regime is continental.
Nevertheless, the Gulf of Mexico is a major source
of moisture supply for precipitation over the
eastern half of the United States and, as a result,
