Wave in a South Wind

Reprinted from Keep Soaring August-September 2008

By Geoff Neely

I had a deadline of 30 June to remove a control pushrod from the DG400 and I had been waiting for weeks for good weather for one last good flight before winter. I got it, par excellence, on 15 June in wave.

The surface temperature was several degrees short of the trigger temperature indicated by the BLIP temperature profile but the forecast wind was from 170T, increasing with height in the range 30kt to 45kt. Not a recognised wave direction but I drove out anyway. On the way I saw one classic multi-layered lenticular over the southern horizon but the cloud in our area was not unusual: regular crosswind streets of strato-cumulus. 

There was no prospect of thermalling into wave and if it had not been so close to the maintenance deadline I might not have bothered but I decided to motor into wind to a good height and see what happened. The surface wind was such that I seemed to pass 1,000 AGL abeam Sims’s hangar. 

The subsequent action all took place on a short east-west beat a couple of miles south of the highway between Carroll and the twin towers. I had to reduce power and level out under the base of a crosswind cloud street at about 5,000 QNH. After resuming the climb in the gap, I could not quite top the next cloud street at 7,500 QNH so I shut down (this takes a hundred feet or so), turned downwind across the gap and arrived about halfway up the upwind flank of a street. Sure enough weak but consistent lift took me slowly up the side until I could overlook the cloud. The lift continued into clear air; the variometer gave out a relaxed beep at a bit less than the frequency of a heartbeat and as usual in wave, this was the only indication that I was climbing, or even that I was moving in the dead calm air in invisible wave.

I spent much of the afternoon between 10,000 and 12,000 QNH and 4,000 ft above the cloud tops. The wave continued to that height but the air contained only so much moisture and the cloud formed a level base at the condensation level but the tops were limited.

 I do not like flying VFR on top and the sight of an unbroken layer of cloud is one I usually only see from the window of an airliner. Many a time I have looked down on cumulus tops far below over some foreign country and imagined what it would be like in the lift below the cloud.

I have not had such a successful wave flight since the big one to 23,000ft over the Snowy Mountains in 1982. On that occasion there were defined lenticulars to mark the way but this time the cloud was ill-defined although there were regular crosswind gaps. The gap below me remained stationary all afternoon. The tops were cumuliform but limited in height – a field of domes protruding from the top of the crosswind bands. Strangely, there were a few lenticular-form clouds embedded in the field of cloud but limited to the same height range. All of these were out of reach upwind or downwind.

I pushed upwind towards the next band of cloud but was not confident of remaining above the tops. Had I fallen below the tops I would have had to descend on the upwind side, dash back below cloud and arrive back perhaps too low to get up again. It became a game of toying with the cloud tops while always retaining an advantage that would get me back to my known beat. I tried to follow my street to Gunnedah but the cloud, and the lift, tailed off and I came back.

My GPS groundspeed into wind was 20kt and downwind it was 85 kt. I was reluctant to circle to get a GPS wind but tried to pick the crosswind heading by turning until my groundspeed was equal to what I guessed was my true airspeed. Although I was in clear air I was always above a cloud street and could explore for lift without being carried away downwind.

The field of cloud was a beautiful sight: the lumpy upper surface extended to the horizon in all directions, with a very few lenticulars for which I could see no reason. The moon was a couple of days short of full moon and during the afternoon it rose out of the haze into crisp, cold clear air and became brighter against the dark blue sky. In the direction of the winter sun the tops were lined with light from behind, looking through my amber sunglasses like burnished bronze. The shadows were sharp and black.

There were depressions aligned downwind in the cloud mass, one a steep sided valley with cloud streaming down the sides and another a broad shallow depression such as you would see in the outback. Over Gunnedah there was a clear sector along the wind line.

The field of cloud extended as far upwind as I could see and I surmised that the wave was generated by the Dividing Range and perhaps was boosted by being in phase with the Duri Range. I could hear Warkworth gliders on the radio but there was no mention of any special weather.

Encouraged by my gloating transmissions, Jenny got Garry to launch her but she did not tow to my position and did not contact wave.

At one time the cloud looked better towards Tamworth so I called Tamworth Tower and asked for a clearance to 9nm TW. He offered a clearance not above 8,500 but I told him I did not want to come that low, thanked him and said I was clearing the area.

Eventually the lift fell off and I began that losing battle that we all know so well. Leaving it as late as I could, I skimmed over the top downwind towards home and flew to a low lenticular that had been stationary to the west of Sport and Rec. Here was more beauty. I skimmed the upwind flank but there was only reduced sink. The view into the low sun was wintry with cold-looking haze and the silhouette of the Carroll Range. I flew close to the flank of the cloud and the shadow of the aircraft in a triple halo kept pace with me like a Doppelganger.

Wisps of cloud formed behind me and I broke off and turned for home. I had been enticed further downwind that I had intended (wave and slope soaring require vigilance) but I still had comfortable height.

Below cloud I entered a different world: in contrast with clear sunlight I was now in wintry air below a broken layer of blue-black cloud bases. There were deep shadows on the ground but in places the low sun and slight mist gave a soft white light that showed up white tree trunks and the rich green of new grass.

Everybody had left the office and my calls for a ‘roo patrol went unanswered but with no traffic and height in hand I was able to have a good look, land beside my car and dash for warm clothing.