On Thursday, October 22, 2020 at 12:31:35 PM UTC-2, Martin Gregorie wrote:
On Thu, 22 Oct 2020 07:09:32 -0700, Tango Whisky wrote:

I had that one over Norther Germany (whis is as flat as flat can be).
Cloud streets with base at 1200 m and tops at 1800 m. Wave lift started
just under the base in front (upwind) of the cloud, and I managed to go
over the tops.
You need a wind shift of 30° or so at cloud base level, with the air
becoming stable (and the convective layer to be significantly unstable
so that the thermals are pushed far into the stable layer above).

I've seen that too, in Cambridgeshire, UK directly over our airfield, but
the bases were lower - only 1200-1400 ft with a fairly brisk breeze,
blowing. The first time I saw it, the streets had short branches almost
at right angles to the street. It was possible to slope soar 150-200 ft
up the front of a branch. The second time there were no branches but
there was lift enough to climb up and over the street and then glide down
on the other side. I don't recall there being sink on the way down and
don't remember whether there was a crosswind under the street, which was
more or less circular in cross section.


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Martin | martin at
Gregorie | gregorie dot org


The comments are interesting but a convergence zone usually takes surface air up to the inversion;
I am familiar with flatland ( thermal) wave and have used it numerous times..

I was too quick to dismiss the donut vortex ring model. Vortices can be stable entities.
If the core was more than 600 ft dia., the donut could be over 1000 ft across; some energy
has to be supplied to keep it circulating.
This could come from the surface wind pushing the vortex; slightly unstable air is then picked up
at the downwind side and ascends in the core ; it spills out at the sides and rear and descends.
This could be quite smooth flow as indeed it was. It would explain why exploring all four quadrants
only showed sink. (ie not wave).
a drifting vortex ring might act as a mechanical trigger for local patches of "buoyant but sticky"
surface air (a.k.a. super adiabatic) to join the ring and keep it sustained. (thanks Dave Frank)