ON THE COVER

Kata­bat­ics are as­so­ci­ated with strong, sud­den gusts, but they can be con­fused with other phe­nom­ena. Chris Bee­son ex­plains

Yachting Monthly - - VIEW FROM THE HELM -

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

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