Topographic effects

In various parts of Europe, topography has a marked effect on the climate, not only of the uplands themselves but also of adjacent areas. Apart from the more obvious effects on temperatures, precipitation amounts and winds, the major mountain masses also affect the movement of frontal systems. Frictional drag over mountain barriers increases the slope of cold fronts and decreases the slope of warm fronts, so that the latter are slowed down and the former accelerated.

The Scandinavian mountains form one of the most significant climatic barriers in Europe as a result of their orientation with regard to westerly airflow. Maritime airmasses are forced to rise over the highland zone, giving annual precipitation totals of over 2500 mm on the mountains of western Norway, whereas descent in their lee produces a sharp decrease in the amounts. The upper Gudbrandsdalen and Osterdalen in the lee of the Jotunheim and Dovre Mountains receive an average of less than 500 mm, and similar low values are recorded in central Sweden around Ostersund.

Figure 10.10 The mean precipitation anomaly, as a percentage of the average, during anticyclonic blocking in winter over Scandinavia. Areas above normal are cross-hatched, areas recording precipitation between 50 and 100 per cent of normal have oblique hatching.

Source: After Rex (1950).

Figure 10.10 The mean precipitation anomaly, as a percentage of the average, during anticyclonic blocking in winter over Scandinavia. Areas above normal are cross-hatched, areas recording precipitation between 50 and 100 per cent of normal have oblique hatching.

Source: After Rex (1950).

Figure 10.11 The mean surface temperature anomaly (°C) during anticyclonic blocking in winter over Scandinavia. Areas more than 4°C above normal have vertical hatching, those more than 4°C below normal have oblique hatching.

Source: After Rex (1950).

Mountains can function equally in the opposite sense. For example, Arctic air from the Barents Sea may move southward in winter over the Gulf of Bothnia, usually when there is a depression over northern Russia, giving very low temperatures in Sweden and Finland. Western Norway is rarely affected, since the cold wave is contained to the east of the mountains. In consequence, there is a sharp climatic gradient across the Scandinavian highlands in the winter months.

The Alps illustrates other topographic effects. Together with the Pyrenees and the mountains of the Balkans, the Alps effectively separates the Mediterranean climatic region from that of Europe. The penetration of warm airmasses north of these barriers is comparatively rare and short-lived. However, with certain pressure patterns, air from the Mediterranean and northern Italy is forced to cross the Alps, losing its moisture through precipitation on the southern slopes. Dry adiabatic warming on the northern side of the mountains can readily raise temperatures by 5 to 6°C in the upper valleys of the Aar, Rhine and Inn. At Innsbruck, there are approximately fifty days per year with föhn winds, with a maximum in spring. Such occurrences can lead to rapid melting of the snow, creating a risk of avalanches. With northerly airflow across the Alps, föhn may occur in northern Italy, but its effects are less pronounced.

Features of upland climate in Britain illustrate some of the diverse effects of altitude. The mean annual rainfall on the west coasts near sea-level is about 1140 mm, but on the western mountains of Scotland, the Lake District and Wales averages exceed 3800 mm per year. The annual record is 6530 mm in 1954 at Sprinkling Tarn, Cumbria, and 1450 mm fell in a single month (October 1909) just east of the summit of Snowdon in north Wales. The annual number of rain-days (days with at least 0.25 mm of precipitation) increases from about 165 in southeastern England and the south coast to over 230 days in northwest Britain. There is little additional increase in the frequency of rainfall with height on the mountains of the northwest. Hence, the mean rainfall per rain-day rises sharply from 5 mm near sea-level in the west and northwest to over 13 mm in the western Highlands, the Lake District and Snowdonia. This demonstrates that 'orographic rainfall' here is due primarily to an intensification of the normal precipitation processes associated with frontal depressions and unstable airstreams (see Chapter 4F.3).

Even quite low hills such as the Chilterns and South Downs cause a rise in rainfall, receiving about 120 to

130 mm per year more than the surrounding lowlands. In south Wales, mean annual precipitation increases from 1200 mm at the coast to 2500 mm on the 500-m high Glamorgan Hills, 20 km inland. Studies using radar and a dense network of rain gauges indicate that orographic intensification is pronounced during strong low-level southwesterly airflow in frontal situations. Most of the enhancement of precipitation rate occurs in the lowest 1500 m. Figure 10.12 shows the mean enhancement according to wind direction over England and Wales, averaged for several days with fairly constant wind velocities of about 20 m s-1 and nearly saturated low-level flow, attributable to a single frontal system on each day. Differences are apparent in Wales and southern England between winds from the SSW and from the WSW, whereas for SSE airflow the mountains of north Wales and the Pennines have little effect. There are also areas of negative enhancement on the lee side of mountains. The sheltering effects of the uplands produce low annual totals on the lee side (with respect to the prevailing winds). Thus, the lower Dee valley in the lee of the mountains of north Wales receives less than 750 mm, compared with over 2500 mm in Snowdonia.

The complexity of the various factors affecting rainfall in Britain is shown by the fact that a close correlation exists between annual totals in northwest Scotland, the Lake District and western Norway, which are directly affected by Atlantic depressions. At the same time, there is an inverse relationship between annual amounts in the western Highlands and lowland Aberdeenshire, less than 240 km to the east. Annual precipitation in the latter area is more closely correlated with that in lowland eastern England. Essentially, the British Isles comprise two major climatic units for rainfall - first, an 'Atlantic' one with a winter season maximum, and, second, those central and eastern districts with 'continental' affinities in the form of a weak summer maximum in most years. Other areas (eastern Ireland, eastern Scotland, northeast England and most of the English Midlands and the Welsh border counties) have a wet second half of the year.

The occurrence of snow is another measure of altitude effects. Near sea-level, there are on average about five days per year with snow falling in southwest England, fifteen days in the southeast and thirty-five days in northern Scotland. Between 60 and 300 m, the frequency increases by about one day per 15 m of elevation and even more rapidly on higher ground.

Precipitation England

Figure 10.12 Mean orographic enhancement of precipitation over England and Wales, averaged for several days of fairly constant wind direction of about 20 m s-land nearly saturated low-level airflow.

Source: After Browning and Hill (1981), reprinted from Weather, by permission of the Royal Meteorological Society. Crown copyright ©.

Figure 10.12 Mean orographic enhancement of precipitation over England and Wales, averaged for several days of fairly constant wind direction of about 20 m s-land nearly saturated low-level airflow.

Source: After Browning and Hill (1981), reprinted from Weather, by permission of the Royal Meteorological Society. Crown copyright ©.

Approximate figures for northern Britain are sixty days at 600 m and ninety days at 900 m. The number of mornings with snow lying on the ground (more than half the ground covered) is closely related to mean temperature and hence altitude. Average figures range from about five days per year or less in much of southern England and Ireland, to between thirty and fifty days on the Pennines and over 100 days on the Grampian Mountains. In the last area (on the Cairngorms) and on Ben Nevis there are several semi-permanent snow beds at about 1160 m. It is estimated that the theoretical climatic snowline - above which there would be net snow accumulation - is at 1620 m over Scotland.

Marked geographical variations in lapse rate also exist within the British Isles. One measure of these variations is the length of the 'growing season'. We can determine an index of growth opportunity by counting the number of days on which the mean daily temperature exceeds a threshold value of 6°C. Along southwestern coasts of England the 'growing season' calculated on this basis is nearly 365 days per year. Here it decreases by about nine days per 30 m of elevation, but in northern England and Scotland the decrease is only about five days per 30 m from between 250 to 270 days near sea-level. In continental climates, the altitudinal decrease may be even more gradual; in central Europe and New England, for example, it is about two days per 30 m.

Was this article helpful?

0 0
Solar Power

Solar Power

Start Saving On Your Electricity Bills Using The Power of the Sun And Other Natural Resources!

Get My Free Ebook


Responses

  • DIETER KR
    What causes climate topographic effects?
    3 years ago
  • pandora
    How the topography of the alps effects weather?
    1 year ago
  • MALISSA
    What are the topographic impacts?
    3 months ago

Post a comment