August 2012 Issue - Federated Mountain Clubs of NZ

advancing or retreating ‘stable’ environments,the strongest winds can be experienced, withhurricane force winds not unusual.The photo taken in Otago highlights aclassic lenticular cloud, which forms in suchenvironments when the wind becomes verystrong. As the air ‘pours’ over the ridge tops,then rapidly descends on the lee side, it causesthe air to bounce and create waves downstream(you can see this downstream of boulders in ariver). At the top of the wave, the water vapourbriefly condenses before it sinks again, creatingthese clouds. The air speed through these cloudscan be in excess of 180 kilometres per hour, yetthe cloud itself remains stationary as the waveremains stationary.This particular lenticular cloud is knownas the ‘Taieri Pet’ which is fairly common overOtago’s Taieri Plains. In this case, the windhas met a number of ranges upstream in justthe right sequence, which amplifies the wave,creating a greater area of lift in that region andhence causing a thicker, layered-looking cloudto form.If you are concerned about extreme winds overa particularly sensitive part of your trip, askyourself these questions.1. Is the park exposed to that particulardirection of wind? Will it be lying parallel orperpendicular to the flow? (The air speed willbe enhanced if the flow is directly againstthe range.)2. What does the weather map tell you? Doyou have a retreating or developing ridge ofhigh pressure? (This could be an indicatorof very strong winds – check the forecast forconfirmation.)Check the forecast. The forecast gives a generalview of the flow strength over the range, expectdifferences in the strength depending on yourlocation within the range, but if the forecast windis high, expect a rough day out on the tops!For more information on this topiccheck out Erick Brenstrum’s blog at http://blog.metservice.com/2009/09/ridge-top-winds/.If you have mountain weather questions, email them tobulletin@fmc.org.nz, and we will ask Leigh to answer them infuture weather columns.FMCAugust 2012 • FMC Bulletin 45