It is difficult to say when the weather is best. Bad weather in a ski resort is seriously bad. But if you have the right clothes, you can have huge fun skiing in even the worst weather, and after the storm there's the added advantage of powder snow.

The coldest period is between mid-December and mid-February, but that is also when the higher ski resorts are actually above the cloud layer and tend to have more sunny days. Val d'Isère locals used to reckon that a six-day holiday would have on average two days of snow, two grey days, and two perfect postcard days. Nowadays, the split seems more like one snowy day, two grey ones and three for the album. Has global warming had an effect? Judging by when the dam near Val d'Isère freezes over, no, it's as cold now as ever. But there is certainly more sun and less snow across the Alps now than thirty years ago, which is another reason one has to ski somewhere high like Val d'Isère.

Whenever you come, it could snow every day, or there could be endless sunshine. Bring clothes and equipment for every eventuality, from fleecy cagoules to quality sun-block, and you won't be caught out.

So when are you most certain to have good snow?

Good snow means enough to cover the rocks, without slush or ice. High in the Alps, fresh snow is likely at any time from late November until early May, but fresh snow is not essential. Most people enjoy skiing more on firm, squeaky winter snow that has been groomed and re-groomed than in powder, which is great for a few hours, but then turns the pistes into ploughed fields for a day or two.

At the very start of the season, the snow usually won't have much to lie on, except rocks and grass. It may be a bit thin. However, the sun is low, the days are short, the nights are cold, and there are very few people skiing. The lower slopes might not yet be as good as they will be, but up high you can easily have some of the best skiing of your life before the Christmas holidays.

By Christmas enough snow has normally fallen for the whole ski area to be in excellent condition. It is surprisingly uncrowded: people feel duty-bound to spend Christmas at home, en famille, rather than enjoying themselves!

You wouldn't go at New Year if you could choose. But if that's the only time you have off work or your family can ski together, that's when you have to go. The downsides are that it is both the most expensive and the busiest week of the year, so you're paying more for less than at any other time. You're subsidising the lucky blighters who'll be skiing the same slopes practically alone a fortnight later. But that's life. Your time will come. For the moment you're stuck with New Year, and in the right resort it is great.

During peak weeks even Val d'Isère's village feels crowded. There can be up to about 19,000 skiers in the resort. But high-speed lifts whisk them all astonishingly quickly up the mountains, where they spread painlessly across the huge number of long, wide runs.

There's another good thing about New Year and the other school holiday periods: the skiers are mostly families. Now families may be a pain in the neck when you're stuck behind one in a ski shop, or next to one in a restaurant. But they are good news on the slopes. Mum and Dad can't wait to be shot of the kids to go and calm their nerves with a second breakfast of caffeine or alcohol, so they bung the beasts in ski school. And ski school classes go slowly and predictably down the mountain. They may be hard to pass, but at least they're not going to take you out from above. One twenty-stone British snowplougher divebombing down from a big lunch can inflict more collateral damage than the USAF, and neutralise a whole piste. Children don't do that.

January is a fabulous month. The snow is bound to be deep enough now, however late the season began. It squeaks as you walk on it. Powder off-piste lasts for weeks. There's almost nobody around. Holidays are sold for less than they cost. Statistically it is probably a fraction colder than December or February, but a cold sunny day doesn't feel as cold as a warm, cloudy day. And the Alps don't do cold like the Rockies...

February is when all Europe has half term. France spreads its school holidays across four weeks, normally from around 7 February to 7 March. So the beginning of February is empty, because families aren't on holiday, but everyone else thinks they are!

The rest of February is similar to New Year: Val d'Isère feels very full, but the crowds disappear faster than when a busker takes his hat around. You should book ski school if you have kids or beginners, and just avoid that 9.30 slot if you don't. If you want a take-away pizza, you'll have to fight half of Paris for it. If you want gastronomy, there'll be space. The snow is still the crisp, carvy stuff people who only ski at Easter have never known, but you're starting to need serious sun screen.

March is many people's favourite month. The snow is now at its deepest. As the month goes on the lower slopes will be slushy in the afternoons and icy in the mornings, but that's why you've come to a high resort. The upper slopes feel like winter, the sun feels like summer. Anything less than Factor-60 is positively dangerous.

Easter is when you are really glad you picked a high-altitude ski area. The days are long and warm. If you pick your runs carefully, avoiding those which melted the previous afternoon and are still icy, the skiing is brilliant. What's more, April showers in Britain are snow showers at 6,000 feet, and there are more days of fresh powder, than in any other month. Best of all, the continentals are convinced that there's no skiing in the spring, and the slopes are never crowded.

April powder doesn't last long: after one sunny afternoon it starts to transform into the even more exhilarating spring snow. This isn't just snow that happens to be hanging around in spring. It is snow which has melted and re-frozen several times. Once the morning sun has been on it for a while the top layer starts to soften off, and it's like skiing on a billiard table: soft and velvety on top, firm underneath. To find perfect spring snow you may have to walk some distance, take a helicopter, or just hire a guide who has a few secret pitches. But it's worth it.

After the Easter holidays there are two or three weeks of long, warm days of powder or spring snow, winter conditions on the upper slopes and water skiing down low. Not all of the runs will be ready at the same time. Some will have shut for good. It's like November all over again. Wonderful conditions on a limited number of slopes, with no-one around and giveaway prices.

So when's the best time to come? Well, early December always has empty slopes and low prices, and often has fantastic snow; Christmas and New Year offer the best family holidays you'll ever have; the snow and the skiing are best in January, and the days, though short, are mostly sunny; the combination of winter snow and spring sun peaks in February; March has most accumulated snow, an instant suntan and no school holidays; April has everything from powder to picnics, long afternoons, high sun and amazing bargains.

Frankly, it doesn't matter when you come. It's still the best holiday in the world.