ON THE COVER
Katabatics are associated with strong, sudden gusts, but they can be confused with other phenomena. Chris Beeson explains
What sort of wind? What is a katabatic wind, and what is not? Not all sudden gusts are katabatic, as Chris Beeson explains
Katabatic: the forces of gravity and air pressure gradient draw cool, dry air down from high pressure plains at altitude towards low pressure at the coast, funneling in river valleys and through passes. Wind speeds of over 100 knots have been recorded R ecently Barry Gray, a YM reader from Oman, wrote in to relate an alarming experience that happened while sailing the beautiful, mountainous Musandam peninsula in his Jeanneau Sun Odyssey 36i. After supper they were lying peacefully at anchor when they noticed the wind begin to build. They were clearing away the dishes when all hell broke loose. The boat was blown 180 degrees round her anchor and heeled to such an angle that Barry had to climb up the side of the companionway steps.
He described the winds as ‘katabatic’ and it seemed to tally with the notion we all have of katabatic winds: sudden, violent and linked to mountains. Then I did some more research and found that katabatic winds are a little more complicated than that.
Katabatic winds are found where there are elevated ice sheets or snowy high plateaux reasonably close to lower, relatively warmer areas, often coasts. At night the plateau radiates heat, cooling the air, increasing the cold, dry air mass and making it denser, until it spills over any natural barriers, like mountain ranges, and gravity drags the cold, dense, dry air down the slopes. If they have a valley to funnel down, they have been known to reach windspeeds of more than 100 knots.
They often occur when there is no wind, because any headwind may prevent the air rolling downhill, and also because wind tends to mix air masses, raising the average temperature.
With these criteria, it’s no surprise that katabatic winds are well known – named, even – in many colder parts of the world, like Antarctica, Greenland, Alaska, Norway and Tierra del Fuego. However they’re also found in less obvious places like France, Croatia, even California, and they’re there because the right conditions exist.
In France, cold air builds up over the Massif Central then rolls down the Rhône
The effect of turbulence can be seen on the water in the lee of Culver Down on the Isle of Wight