Weather systems in middle latitudes generally travel from west to east. Consequently, northwestern Europe receives air that has crossed the Atlantic, gathering moisture as it does so. The ocean is warmer than the land in winter and cooler than the land in summer, so Atlantic weather systems produce wet climates with cool summers and mild winters.
This type of climate affects the western Pyrenees and the mountains of western Norway, but it does not extend very far into the Alps. There are no major mountain ranges aligned north-to-south to force the air approaching from the west to rise and its water vapour to condense. Consequently, the maritime air penetrates far inland and the maritime climate of the west gives way fairly gradually to the continental climate of eastern Europe, with its drier conditions and greater extremes of temperature. Air reaching North America, in contrast, loses much of its moisture as it crosses the Rockies, leaving the plains to the east in a rain shadow so pronounced as to produce desert conditions.
Mountains strongly modify climates, however, and the mountain ranges that comprise the Alps produce many contrasts. Mild, moist air reaches the mountains from the west, bitterly cold arctic air from the north, dry continental air, hot in summer and cold in winter, from the east, and Mediterranean air from the south.
The topography and altitude are, if anything, even more influential. Temperature decreases with height, by an average 3.6°F for every 1,000 feet (6.5°C per kilometre), but air is heated only by its contact with the ground surface, and high in the mountains strong winds usually sweep the air away before it has had time to warm. The air is cold, but when the skies are clear the sunshine feels warm.
The warmest places are in the valleys exposed to the sun and sheltered from the wind, where the air remains in contact with the warm ground. On winter nights, however, the valleys are often cold. Cold, dense air sinks down the mountainsides to form pools in the valleys. That is why many alpine villages are built part way up the mountains, rather than on the valley floors. The coldest valleys are in the eastern Alps, where the climate is predominantly continental. The lowest temperature ever recorded in Switzerland, of -27°F (-33°C), was measured in the valley of the river Inn, near Davos.
High pressure often dominates the weather in winter and it can cause temperature inversions, where cold air is trapped beneath warmer air. At these times fog often forms in the pools of cold valley air and the fogs sometimes persist for weeks on end. When the low ground is blanketed by cold fog the higher slopes, above the fog, are warmer than the valleys.
When low pressure dominates to the west, the counterclockwise circulation of air brings warm air from the south and across the mountains. The air cools as it rises and expands, but as it descends on the opposite side of the mountains it sinks into a region of higher pressure where it is compressed and its temperature rises, producing a föhn wind. This is the alpine equivalent of the North American chinook. Föhn winds blow from either the south or the north, depending on the precise path followed by the weather system, and they can continue for up to three days. They bring a sharp and rapid rise in temperature, of up to 36°F (20°C) in 24 hours, but as they funnel through narrow valleys they can reach gale force.
At elevations above 5,000 feet (1,500 m) almost all precipitation in winter falls as snow. Snow usually lasts from November until May at the 6,500-feet (2,000-m) level, and can be more than 33 feet (10 m) deep, although there are some winters when little snow falls.
Mean winter temperatures have risen in recent decades as part of the general rise in global temperature. If the warming continues, it is likely that relatively snow-free winters will become more frequent and skiers will not find good snow conditions every year.
Like the Alps, the Pyrenees lie along an east-west axis and, also like the Alps, they form a climatic barrier that intensifies the contrast between continental and Mediterranean conditions. The climate of the western Pyrenees, as far south as Pamplona, is dominated by air approaching from the Atlantic. It is mild, often misty, and more precipitation falls in winter than in summer.
Ocean air, flowing across the French plain, also affects the northern side of the central Pyrenees. Farther east, damp winds from the Mediterranean, called "levanters", bring humid conditions to the French side of the mountains. On the Spanish side, the southern Pyrenees experience hot, dry summers but also cold winters due to the high elevation. Most of the precipitation falls in autumn and spring.
Snow falls in winter everywhere above about 2,000 feet (600 m). In the central Pyrenees it lies at that elevation usually for about three weeks. At 6,500 feet (2,000 m) the snow lies for about 24 weeks of the year and at 8,000 feet (2,500 m) for about 34 weeks.
Unlike the Alps and Pyrenees, the mountains of Norway and Sweden run along a north-south line and rise very steeply on the western side, but they are neither high nor continuous enough to produce a marked rain shadow in their lee. They do mark a sharp climatic division, however, between mild, wet, maritime weather to the west and more continental conditions in the central mountains, where the cold can be intense. Eastern Sweden has a mild climate due to the proximity of the Baltic Sea, but one that is colder than the climate of western Norway. The Gulf of Bothnia remains frozen from November to May and the mean January temperature at Haparanda on the coast is 10°F (-12°C), implying a temperature of about 0°F (-18°C) at 3,300 feet (1,000 m) elevation.
Norway lies directly in the path of the frontal depressions which cross the Atlantic. These make the weather very changeable and they often bring gales. There are three or four gales a month along the western coast in winter. Southwesterly gales are the most frequent, bringing rain that falls as snow in the mountains. Northwesterly gales always bring snow. Gales are rare inland, where calm conditions often prevail.
Northwestern Norway, well inside the Arctic Circle, has a very mild climate, due to the warm Norwegian Current, a branch of the Gulf Stream that washes its shores and brings currents of air from the south. In winter the sea off western Norway is up to 36°F (20°C) warmer than the average for that latitude. At the coastal town of Bodo, latitude 67.27°N, the mean temperature in February, the coldest month, is 28°F (-2°C). The weather is generally wet, but precipitation falls mainly as rain. In southern Norway, calm weather in winter often produces temperature inversions that trap cold air in the mountain valleys.
Snow falls in the northern mountains for up to eight months of the year and winter temperatures can fall below -22°F (-30°C). Conditions are milder in the south, where the January sea level temperature is only a few degrees below freezing and snowfall is irregular. Nevertheless, when the temperature at sea level is 23°F (-5°C), on mountain slopes at 5,250 feet (1,600 m) it is likely to be 5°F (-15°C) and the wet climate should guarantee abundant snow.