Geoscience Reference
In-Depth Information
Figure 10.12 Mean orographic
enhancement of precipitation over
England and Wales, averaged for
several days of fairly constant wind
direction of about 20 m s -1 and nearly
saturated low-level airflow.
Source : After Browning and Hill (1981),
reprinted from Weather , by permission of
the Royal Meteorological Society. Crown
copyright ©.
Approximate figures for northern Britain are sixty
days at 600 m and ninety days at 900 m. 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 thirty and fifty 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-permanent snow beds
at about 1160 m. It is estimated that the theoretical
climatic snowline - above which there would be net
snow accumulation - is at 1620 m over Scotland.
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 30 m of elevation, but in northern
England and Scotland the decrease is only about five
days per 30 m 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 30 m.
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
2750 m, 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 airmasses from
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