Mount Everest pokes two-thirds of the way through the air of the Earth's atmosphere to the ozone-producing stratosphere, 5.5 miles (8.7 km) up. Stratosphere air contains little water vapour or dust; only wispy cirrostratus clouds streak the distant sky. Storm clouds, however, can and certainly do form around the landmass of Everest.

The mountain peak (29,035 feet) scrapes the jet stream. In the winter, the high-flying jet stream hurtles in from the north and batters Mount Everest with hurricane-force winds exceeding 177 mph (285 km/h).

Professor Moore thinks the weather patterns that day in May 1996 led to the atmospheric pressure falling so dramatically that in effect the stratosphere dropped onto the summit of the 8,848-metre mountain. Normally the peak sits just below the atmospheric layer.

That would be the equivalent of raising the summit by 500 m on a normal day - and would cut the available oxygen in the air, which on the summit is just one-third that at sea level, by 14%.

Although most climbers at the summit use supplementary oxygen, they still rely on that in the air to help them breathe. But above 8,000 m there is too little oxygen to sustain life, making it essential that people spend as little time as possible at such heights.

Professor Moore explained that the "jet streak" winds travelling at more than 100 mph up the sides of the mountain would have dragged a huge volume of air upwards, causing the air pressure to drop and leaving less oxygen available to the desperate climbers.

"At these altitudes climbers are already at the limits of endurance," he said. "The sudden drop in pressure could have driven some of these climbers into severe physiological distress."

If Everest  were 500 metres higher, some experts reckon it would be impossible to climb without oxygen: the distance from the start of the "death zone" to its summit and back would be too great for anyone to survive. The consequence therefore of effectively raising the summit of Everest that day would prove to be catastrophic.