White Squal

[gerkin]

I was out windsurfing a couple of months ago and a large non thundery rain/hail squal passed over.  What happened was very quick, but a remember being suddenly blasted by hail, being sucked up off the water and slammed violently back into the sea.  A couple of windsurfers not too far behind me saw it happen.  I spoke to them after the squal had passed, and they told me that as i slammed back into the sea, the spray went up in a vortex and not horizontal.

I have never encountered this before but can rain squals produce tornadoes ??!!

[Hippy]

The sort of wind shear instability you describe is usually found at the approach of a cold front and low pressure system. A full scale spinning cyclone is partly the result of up and down draughts in a thunderstorm interacting with the wind shear, making the wind shear form an upright vortex. On a larger scale, air around the low pressure area, already slowly spinning in an anti-clockwise direction if in the northern hemisphere, converges inward toward the thunderstorm, causing it to spin even faster.

Other processes can enhance the chances for cyclone formation. For instance, dry air in the middle atmosphere can be rapidly cooled by rain in a thunderstorm or an approaching cold front, causing strong down draughts which can start a cyclone. If you examine cyclone (or tornado if American) pictures you will see that the cyclone has formed at the boundary between dark up draught clouds and lighter down draught clouds, showing the importance of up and down draughts to cyclone formation. Also, an isolated strong thunderstorm just ahead of a squall line that then merges with the squall line often becomes cyclonic.

It sounds like the down draught of cold air from aloft at the squall line, bringing with it hail showed that the air was cold and dry, therefore light, but carried precipitation from a higher strata. This cold dry air pushed down and the heavier damp warm air from the surface would have been propelled violently upward into the low pressure. If the wind shear or squall line was moving fast enough relative to the ground, i.e. at 90 degrees from the up and down draughts a mini cyclone could be caused. You would be swept upwards by the movement of the outer up draught and then plonked back down by the central cold hailstone down draught, and then the phenomenon would have passed. The helical pattern of spray confirms this.

[Hippy]

Sorry, I got that last little bit in the wrong order. The cold air pushing down would be at the shear line and it is this that gives the initial shock. As this passes rapidly, the warmer damp air around you would be displaced into the low pressure and propel you upward. Because this up draught would be following the shear line and by inertia you would not, splash! gravity takes over and you land back on the surface.

[MargeB]

Trust you weren't hooked in!??! If there ever was an argument for not crossing over to kites. Now you don't want this to happen to you with a 16m Leading Edge Inflatable...