Travellers Checks: Book early for when the school bell rings

For winter sports, the current villain is global warming, accused not only of eating into the glaciers but also of disrupting perceived wisdom about when is best to book.

According to the most recent UN report on climate change, the threat hangs most heavily over the Alps. During the past 150 years, the Alpine glaciers have shrunk by a third and lost half their volume. The permafrost line, currently around 2,500m, is getting higher, leaving less of the mountain permanently frozen. This loss of stability could result in landslides and endanger cable-car stations built on insufficiently secure foundations.

In the medium term, these factors are expected to push the winter snowline up from 1,200m to 1,800m, a change that would progressively rob old favourites such as Kitzbühel, Wengen and Megève of their livelihood. More immediately, the snow pattern is changing. Traditionally, the Alps delivered a white Christmas, but recently this has been patchy, with promising November falls often dwindling as the festive season approaches.

January and February offer better prospects, but spring sunshine brings a big melt in March, followed by April storms that can provide the season's finest conditions after the lifts have shut.

The new pattern is not helpful to families with young children. While they might enjoy a traditional jingle-bells Christmas in Austria or a convivial New Year in child-friendly Italy, it wouldn't necessarily involve much skiing.

If you want to be certain, you have to apply the altitude test, which means that the best bets are purpose-built high flyers in the Tarentaise valley, primarily Courchevel, Val d'Isère, Tignes, Val Thorens, Les Arcs and La Plagne, plus St Moritz and Saas Fee in Switzerland and Obergurgl and Obertauern in Austria.

Finland is also a definite winner, with Father Christmas licensed to pilot his reindeer sleigh through Levi, the riotous capital of Santa-land. But it pays to remember that the frozen wastes to the north of the Arctic Circle can only be cold and dark over the winter solstice.

If Easter had a fixed date, the tour operators could make the most of it, but with more than a month's leeway, they have to cope as best they can. In 2000 and 2001, the feast day fell in mid to late April when snow conditions were at their best. In theory, this should have been a bonanza for families. The days are long and the sun, when it shines, is hot, but it is generally thought that the second half of April is too late for a winter-sports holiday in the Alps. The 2002 Easter date, 31 March, is more promising for pulling in the crowds, the irony being that conditions may be better a month later when many resorts close for the summer rather than risk running lifts on empty. All this puts undue emphasis on half term, now the optimum time for families. In 2002, the break is spread over a fortnight, with packages starting on 9 and 16 February.

This is a welcome division of the pressure on the individual weeks, but an insuperable problem for families whose children have different half terms. However, there is no shortage of takers willing to pay premium rates for the privilege. "It's a nightmare," says Lizzie Norton, the owner of Ski Solutions. "You can still find places, but don't expect a wide choice in evergreen family favourites such as Lech, Zermatt and Méribel." Trust her: the time to book is now.