There are various causes of turbulence over ridges but one which probably exists more often than we may realise is an invisible form of the 'dust devil', a small tornado which may pick up loose material and move around the ridge causing havoc amongst pilots waiting to take off. These dust devils are sure indicators that thermals are breaking away from the ridge but anyone flying through one is certainly in for an interesting ride.

We're all familiar with the whirlpool effect of water going down the plug-hole of a sink. In this case any rotation (angular momentum) of the water is concentrated as it is drawn towards the plug-hole when the plug is pulled making the water rotate faster. This is similar to the effect of spinning ice skaters pulling in their arms to make themselves spin faster. In a narrow thermal any rotation of the air is 'concentrated' as the air converges and an inverted 'whirlpool' may be formed.

Our ridge is unlikely to be completely symmetrical and the wind is unlikely to be exactly straight on the hill. If the wind is blowing across the hill there will be a wind gradient in the horizontal plane which may create rotation about a vertical axis. This will be made worse by any irregularities in the shape of the slope. Air coming up one side of the ridge meeting air coming up at an angle from the other side is another good example. Air blowing around the corner of a spur may also form a rotor about the vertical axis.

Unfortunately there is no substitute for experience in recognising when this is likely to occur but here are a few ideas.

• In Europe dust devils are most often encountered on hot ridges or promontories.



• Be wary of scratching slopes when the wind is not directly on the side of the hill.



• Try to avoid scratching to the very top of an edge if you think dust devils may occur. If possible find a thermal which will take you above the ridge top without flying too close to the crest. This avoids the area where air is converging over the spine and ensures good terrain clearance in the area most prone to turbulence.

Take a funnel about six or eight inches in diameter. Put a finger over the spout and fill it with water. Float a match stick in the middle and wait until any rotation stops. Now release the finger from the spout and watch the rotation of the match. The water should run out without rotating. Repeat the experiment but this time stir the water before releasing the spout. This time the rate of rotation of the match should increase as the water runs out. This 'inverted thermal' shows how the slow rotation is 'concentrated' as the water converges producing a high rate of rotation. Imagine then what happens when a large, slowly rotating mass of air is concentrated in a 'tight' thermal.