Bill Burroughs on weather variations
Every autumn, keen skiers anxiously await news of early snow in the Alps which may provide a guide of what is to come. The last week has been stormy in the Alps, but the erratic snow seasons of the last 10 or so years are sufficient reason to justify this concern. And although the last few years have produced adequate snow records, the spectre of global warming still invades many skiers' nightmares.

The run of dry years around the end of the 1980s were seen as the harbinger of a warmer climate. Indeed, 1994/95 looked set to repeat this pattern, with record-breaking warmth in November and a dearth of snow. Then the huge falls of early January set things up for what proved to be a bumper season. Last year, cold, dry conditions, rather than warmth,got the season off to a poor start, and subsequent snowfalls were modest, especially in France, despite the fact that is was a distinctly cold winter.

The effect of global warming on winter snow-cover in the Alps will depend on changes in both temperature and precipitation. If rising temperatures are accompanied by increased precipitation, then the extra snowfall will cancel out the effects of any warming at higher levels. Overall, the amount of snow for skiers may remain unaltered or even increase.

Statistics collected in Austria and Switzerland since the late 19th century suggest that this compensating effect has occurred. Above around 1500 metres (5000 feet), there is no appreciable trend in snowfall over the last 100 years. At lower levels, there is a hint that higher temperatures have outweighed the effect of increased precipitation and reduced the amount of snowfall, cutting the length of the season a bit.

Incidentally, statistics collected since the 1930s in California for the purposes of managing water resources tell the same story. Despite a warming trend, seasonal snowfall in the Sierra Nevada has, if anything, increased over the last 60 years.

Statistics for the Alps also provide insights into the shorter term variations in snowfall. On every timescale, the key word is erratic. Runs of good years can be followed by a series of poor seasons, or a bad year, such as 1969, being followed by the bumper falls of 1970. Within any season, the same story applies, as the sudden improvement in January 1995 demonstrated, or conversely, after the magnificent falls of the autumn of 1992, there was nearly two months of drought before further snow gave a good end to the season.

There is no identifiable rhyme or reason to these fluctuations. Despite the impression of a pattern in the variations over the years, there are no reliable cycles that can be used to predict whether any year will be good or bad. Similarly, within any season, the shifts between settled, dry sunny periods and stormy, snowy intervals are unpredictable more than a few days ahead.

Examination of temperature records for the Alps show that snowy years do not necessarily feature cold winters. Much more important is the amount of precipitation. It follows that cold, dry winters such as 1964 can be just as disastrous as mild, dry ones such as 1989, and last year nearly fell into this category.

The ideal combination is a cold and stormy late autumn and early winter, such as those that set up the seasons of 1981/82 or 1992/93, followed by a relatively mild dry winter with plenty of settled, sunny weather to enable skiers to exploit the ample snow. By way of contrast, if the weather gets very cold, as spells of January 1987 and February 1991 demonstrated, it can be too bitter in the Alps to enjoy the snow in the high resorts.

Basic guidelines for skiers in search of snow remain the same - best options are the big, high resorts in high season. But at the lower resorts, especially those below 1000 metres (3300 feet) in Austria and below 1250 metres (4100 feet) in the French Alps, you may get little snow cover unless you have access to slopes above 1500 to 2000 metres (4900 to 6600 feet). Where there are not enough high-level slopes, early and late holidays are likely to become increasingly risky.

Bill Boroughs' book 'Mountain Weather: A Guide for Skiers and Hillwalkers' is published by Crowood Press, priced at pounds 10.99