Geoscience Reference
In-Depth Information
Atlantic. The Rhône and Ebro valleys are funnels
for strong airflow in the rear of depressions located
in the western Mediterranean, generating the mistral
and cierzo winds, respectively, in winter (see C.3, this
chapter). Throughout western Europe, the mean velocity
of winds on hilltops is at least 100 per cent greater than
that in more sheltered locations. Winds in open terrain
are on average 25 to 30 per cent stronger than in shel-
tered locations; and coastal wind velocities are at least
10 to 20 per cent less than those over adjacent seas (see
Figure 10.1).
contrast between western Norway and Sweden. There
are numerous indices expressing this continentality,
but most are based on the annual range of temperature.
Gorczynski's continentality index (
K
) (Note 1) is:
A
K
= 1.7 —— - 20.4
sin ϕ
where
A
is the annual temperature range (°C) and ϕ is
the latitude angle (the index assumes that the annual
range in solar radiation increases with latitude, but in
fact the range is a maximum around 55°N).
K
is scaled
from 0 at extreme oceanic stations to 100 at extreme
continental stations, but values occasionally fall outside
these limits. Some values in Europe are London 10,
Berlin 21 and Moscow 42. Figure 10.2 shows the vari-
ation of this index over Europe.
An independent approach relates the frequency of
continental airmasses (
C
) to that of all airmasses (
N
) as
an index of continentality, i.e.
K
=
C/N
(per cent). Figure
10.2 shows that non-continental air occurs at least half
the time over Europe west of 15°E as well as over
Sweden and most of Finland.
A further illustration of maritime and continental
regimes is provided by a comparison of Valentia (Eire),
Bergen and Berlin (Figure 10.3). Valentia has a winter
rainfall maximum and equable temperatures as a result
of its oceanic situation, whereas Berlin has a consid-
erable temperature range and a summer maximum of
rainfall. A theoretically ideal 'equable' climate has been
defined as one with a mean temperature of 14°C in all
months of the year. Bergen receives large rainfall totals
due to orographic intensification and has a maximum in
autumn and winter, its temperature range being inter-
mediate between the other two. Such averages convey
only a very general impression of climatic characteris-
tics, and therefore British weather patterns are examined
in more detail below.
2 Oceanicity and continentality
Winter temperatures in northwest Europe are some
11°C or more above the latitudinal average (see Figure
3.18), a fact usually attributed to the presence of the
North Atlantic current. There is, however, a complex
interaction between the ocean and the atmosphere. The
current, which originates from the Gulf Stream off
Florida strengthened by the Antilles current, is primarily
a wind-driven current initiated by the prevailing south-
westerlies. It flows at a velocity of 16 to 32 km per day
and thus, from Florida, the water takes about eight or
nine months to reach Ireland and about a year to reach
Norway (see Chapter 7D.2). The southwesterly winds
transport both sensible and latent heat acquired over the
western Atlantic towards Europe, and although they
continue to gain heat supplies over the northeastern
Atlantic, this local warming arises in the first place
through the drag effect of the winds on the warm surface
waters. Warming of airmasses over the northeastern
Atlantic is mainly of significance when polar or arctic
airflows southeastward from Iceland. The temperature
in such airstreams in winter may rise by 9°C between
Iceland and northern Scotland. By contrast, maritime
tropical air cools on average by about 4°C between the
Azores and southwest England in winter and summer.
One very evident effect of the North Atlantic Current
is the absence of ice around the Norwegian coastline.
However, the primary factor affecting the climate of
northwestern Europe is the prevailing
onshore
winds
transferring heat into the area.
The influence of maritime airmasses can extend deep
into Europe because there are few major topographic
barriers to airflow and because of the presence of the
Mediterranean Sea. Hence the change to a more conti-
nental climatic regime is relatively gradual except in
Scandinavia, where the mountain spine produces a sharp
3 British airflow patterns and their
climatic characteristics
The daily weather maps for the British Isles sector
(50 to 60°N, 2°E to 10°W) from 1873 to the present
day have been classified by H. H. Lamb according to
the airflow direction or isobaric pattern. He identified
seven major categories: westerly (W), northwesterly
(NW), northerly (N), easterly (E) and southerly (S) types
