Geoscience Reference
In-Depth Information
penetrate into the heart of Siberia. Thus the
Siberian high pressure is quasi-permanent at this
season, and when it extends westward severe
conditions affect much of Europe. In summer,
pressure is low over all of Asia and depressions
from the Atlantic tend to follow a more zonal
path. Although the storm tracks over Europe do
not shift poleward in summer, the depressions at
this season are less intense and reduced air-mass
contrasts produce weaker fronts.
Wind velocities over Western Europe bear a
strong relationship to the occurrence and
movement of depressions. The strongest winds
occur on coasts exposed to the northwest airflow
that follows the passage of frontal systems, or at
constricted topographic locations that guide the
movement of depressions or funnel airflow into
them (
Figure 10.1
). For example, the Carcassonne
Gap in southwest France provides a preferred
southern route for depressions moving eastward
from the 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, respec-
tively, in winter (see C.1, this chapter). Through-
out Western Europe, the mean velocity of winds
on hilltops is at least 100 percent greater than that
in more sheltered locations. Winds in open terrain
are on average 25-30 percent stronger than in
sheltered locations; and coastal wind velocities are
at least 10-20 percent less than those over adjacent
seas (see
Figure 10.1
).
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.1).
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 North
Atlantic, this local warming arises in the first place
through the drag effect of the winds on the warm
surface waters. Warming of air masses over the
northeastern Atlantic is mainly of significance
when polar or arctic air flows 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 air masses can
extend deep into Europe because there are few
major topographic barriers to airflow and owing
to the presence of the Mediterranean Sea. Hence
the change to a more continental climatic regime
is relatively gradual except in Scandinavia, where
the mountain spine produces a sharp contrast
between western Norway and Sweden. There
are numerous indices expressing this continent-
ality, but most are based on the annual range
of temperature (see Note 1). Gorczynski's
continentality index (
K
) is:
A
K
= 1.7 ----- -20.4
sin
°
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
southwesterlies. It flows at a velocity of 16 to 32km
°
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 of
°
