On Saturday, August 26, 2017 at 3:21:05 AM UTC+12, Dave Nadler wrote:
On Friday, August 25, 2017 at 11:13:23 AM UTC-4, John Cochrane wrote:
I got to fly in one in New Zealand, with Gavin in the back....
-- a hill, lenticular cap cloud, gap, then sort of a wall of turbulent cloud.

Cool! Did the wall look like the picture in Bernard's article?
What was the discontinuity that provoked the jump?

The hydraulic jump is more common than most people realise.

Often what is referred to as the 'primary' of the wave is closer to the trigger (back of the hill) than the wavelength of the rest of the wave train. The wave is actually not being triggered by the back of the hill. A hydraulic jump is being triggered on the back of the hill, and the wave train sets up behind the hydraulic jump.

We see this a lot at Omarama because the cool air from the ocean is being pushed onto the Southern Alps by the prevailing westerly. This is much denser than the air east of the divide and this density difference helps make a hydraulic jump.

On weak days we often get just the hydraulic jump, no wave train setting up behind. The jump is often very close to the cap cloud, just a few hundred meters downwind of the trailing edge of the cloud. I think this is because the back edge of the best triggers are very steep.