Wave clouds at sunrise, MacKenzie Basin, South Canterbury
When air is forced up and over a mountain range a series of standing waves is often set up downwind of the mountains, in much the same way as waves form in the surface of a stream as it flows over a submerged boulder. Standing waves in the atmosphere are frequently made visible by the formation of smooth, regularly-spaced rolls of cloud, caused by air rising, cooling and condensing near each wave crest before it sinks into the next wave trough causing the cloud to dissipate again. These regular lines of cloud, running roughly at right angles to the direction of the wind, may extend downwind for hundreds of kilometres before the waves finally dissipate.
The eastern side of New Zealands Southern Alps is famous for these so-called lee waves, thanks to the frequent and strong northwesterly winds that blow across the main divide of the Alps. Lee waves are keenly sought after by glider pilots who take advantage of the strong updrafts in the waves to soar to record-breaking heights. Omarama, in South Canterbury, has a reputation for offering some of the finest gliding conditions in the world and pilots from all over the globe travel here in search of the ultimate wave.
Spring and summer usually bring the strongest lee waves in this region but they can appear at any time of year, such as on the midwinter day in this photograph which was taken when a norwester was setting in ahead of an approaching cold front.