Geoscience Reference
In-Depth Information
Seasonal changes in the Icelandic low and the Azores high, together with variations in cyclone
activity, control the climate of Western Europe. The eastward penetration of maritime influences
related to these atmospheric processes, and to the warm waters of the North Atlantic Current, is
illustrated by mild winters, the seasonality of precipitation regimes and indices of continentality.
Topographic effects on precipitation, snowfall, length of growing seasons and local winds are
particularly marked over the Scandinavian Mountains, the Scottish Highlands and the Alps. Weather
types in the British Isles may be described in terms of seven basic airflow patterns, the frequency
and effects of which vary considerably with season. Recurrent weather spells about a particular
date (singularities), such as the tendency for anticyclonic weather in mid-September, have been
recognized in Britain and Europe and major seasonal trends in occurrence of airflow regimes may
be used to define five natural seasons. Abnormal weather conditions (synoptic anomalies) are
associated particularly with blocking anticyclones, which are especially prevalent over Scandinavia
and may give rise to cold, dry winters and warm, dry summers.
The climate of North America is similarly affected by pressure systems that generate air masses
of varying seasonal frequency. In winter, the subtropical high pressure cell extends north over the
Great Basin with anticyclonic cP air to the north over Hudson Bay. Major depression belts occur at
about 45-50
°
N, from the central USA to the St Lawrence, and along the east coast to Newfoundland.
The Arctic Front is located over northwest Canada, the Polar Front lies along the northeast coast
of the United States, and between the two a maritime (arctic) front may occur over Canada. In
summer, the frontal zones move north, the Arctic Front lying along the north coast of Alaska, Hudson
Bay and the St Lawrence being the main locations of depression tracks. Three major North American
singularities concern the advent of spring in early March, the midsummer northward displacement
of the subtropical high pressure cell, and the Indian summer of September to October. In western
North America, the Coast Ranges inhibit the eastward spread of precipitation, which may vary
greatly locally (e.g., in British Columbia), especially as regards snowfall. The strongly continental
interior and east of the continent experiences some moderating effects of Hudson Bay and the Great
Lakes in early winter, but with locally significant snow belts. The climate of the east coast is
dominated by continental pressure influences. Cold spells are produced by winter outbreaks of high-
latitude cA/cP air in the rear of cold fronts. Westerly airflow gives rise to chinook winds in the lee
of the Rockies. The major moisture sources of the Gulf of Mexico and the North Pacific produce
regions of differing seasonal regime: the winter maximum of the west coast is separated by a
transitional intermontane region from the interior, with a general warm season maximum; the
northeast has a relatively even seasonal distribution. Moisture gradients, which strongly influence
vegetation and soil types, are predominantly east-west in central North America, in contrast to the
north-south isotherm pattern.
The semi-arid southwestern United States comes under the complex influence of the Pacific and
Bermudas high pressure cells, having extreme rainfall variations, with winter and summer maxima
mainly due to depression and local thunderstorms, respectively. The interior and east coast of the
United States is dominated by westerlies in winter and southerly thundery airflows in summer.
Hurricanes are a major element of the summer-autumn climate of the Gulf Coast and southeast
United States.
The subtropical margin of Europe consists of the Mediterranean region, lying between the belts
dominated by the westerlies and the Saharan-Azores high pressure cells. The collapse of the A
zores
