The British Isles has a maritime
climate, characterized by mild winters
and relatively cool summers, which is a consequence of its proximity
to the sea. This is because water has a much larger heat capacity
materials making up the land. As a consequence, it takes more heat
energy to raise the temperature of water one degree, and there is more
heat energy to give up when the water cools by one degree, when
compared with rock and soil. Consequently bodies of water warm up
and cool down more slowly than adjoining land. The nearby sea thus
prevents coastal areas becoming as cold in the winter as inland areas and
also helps maintain temperatures well into the autumn.
In contrast, inland areas on the great landmasses at the same latitude
have a more extreme climate, with very cold winters and hot summers;
the features of a continental climate
. Whereas most of the British Isles
lowland is normally above freezing for most of the winter, average
mid-winter temperatures for Moscow and Hudson Bay (both continental
climate situations) are nearer −15°C.
|Figure 2.6 Cloud formation and rainfall caused by
(a) fronts and (b) higher ground (orographic rain).
Note: warm air caused to rise over cold air or
higher ground forms cloud when the air reaches
the dew point of the air mass.
The North Atlantic Drift
, the ocean
current flowing from the Gulf of Mexico
towards Norway, dominates the climate
of the British Isles (see Figure 2.5).
The effect of the warm water, and the
prevailing southwesterly winds blowing
over it, is particularly infl uential in
the winter. It creates mild conditions
compared with places in similar latitudes,
such as Labrador and the Russian coast
well to the north of Vladivostok, which are
frozen in the winter.
The mixing of this warm moist air stream
and the cold air masses over the rest
of the Atlantic leads to the formation
of a succession of depressions
regularly pass over the British Isles
bringing the characteristic unsettled
weather; with clouds and rain where
cold air meets the moist warm air in the
slowly swirling air mass. Furthermore,
the moist air is also cooled as it is forced
to rise over the hills to the west of the
islands giving rise to orographic rain
In both instances clouds form when the dew point is reached.
This leads to much higher rainfall levels in the west and north compared
with the south and east of the British Isles. In contrast, a rain shadow
is created on the opposite side of the hills because, once the air has lost
water vapour and falls to lower warmer levels, there is less likelihood of
the dew point being reached again (see Figure 2.6). Depressions are also
associated with windier weather.
The sequence of depressions (low-pressure areas) is displaced from time
to time by the development of high-pressure areas
usually bring periods of settled drier weather. In the summer these are
associated with hotter weather with air drawn in from the hot European
land mass or North Africa. In the winter, clear cold weather occurs as air is
drawn in from the very cold, dry continental landmass. In the spring, these
anti-cyclones often lead to radiation frosts
, which are damaging to young
plants and top fruit blossom.