The authoritative online guide Henry’s Avalanche Talk (HAT) warns this week that in current conditions, a single off-piste skier can be enough to trigger a potentially lethal avalanche. “Over the past week there have been countless slides around Val d’Isère – sometimes occurring naturally but often triggered by skiers or boarders,” it says.

While noting great snow and excellent off-piste conditions after cold, clear nights, HAT says crusty surfaces are softening and humidifying on south facing slopes during sunny and warm days. “Natural slides and medium avalanches of humid snow may occur at all altitudes. These may sometimes take the whole snowpack with them.”

HAT also warns that natural avalanches of thick, dry, breakable or hardened windslab are possible on less sunny slopes and in areas of greater snowdepth – particularly near the Italian border.

Why is it so dangerous?

In recent weeks, more than five feet of snow has fallen in the Alps – chiefly in the west – affecting many of the French Resorts, now the most popular area for British skiers.

This season, the early snow was late in coming and once it arrived there was a period of warm weather – see the situation in Bourg St Maurice on 10 January. On that day, Val d’Isère was warmer than Edinburgh!

The conditions this season have produced a thick layer of ice under the new fall. Below this is a deep layer of depth hoar. This is the dangerous part.

Imagine a bowl of shiny, new ball bearings. They do not stick to each other but they are in a bowl so contained. Now place them on a plate and place another plate on top – the ice layer – and elevate the plates to 25 degrees. Now place five feet of snow on top of that, when the top plate snaps you have a very large and dangerous avalanche.

The default advice in these circumstances is “don’t ski off-piste”.But this is like telling teenagers not to stay up late! Skiers will ski off-piste – the trick is to know when to do this, and how to do it safely.

HAT always provides wise advice on mountain safety, and this week’s off-piste snow report, available online, contains links to an educational video on persistent weak layers, and an detailed explanation of avalanche danger ratings.

In a nutshell, in these conditions,avoid:

– Slopes over 25 degrees

– South facing slopes after midday, depending on the temperature

– East facing slopes from March.

If it’s warm don’t go there. And always watch out for skiers on steeper slopes above you.