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Unexplained lift at Ringstead, August 8th, 2013

Returning from the Golden Gate to the Fort, the buoyant air continued out to sea, and I found myself up to 2800 feet, a half-mile offshore.

— From Flying the Shear at Funston by Geoffrey Rutledge, in the May 2009 edition of Hang Gliding & Paragliding.

The article by Geoffrey Rutledge describes a flight starting at Fort Funston, the cliffs of which are only 200 feet high. He then cruised the adjacent coastline of the San Francisco Bay Area. The article includes some theory that might help explain lifting air I encountered at Ringstead on the Dorset coast in England on August 8th, 2013.

The Ringstead cliffs face south-west, but the lift on the cliffs on August 8th, 2013, was not great. The lift turned off completely a couple of times during the day, sending paragliders and one high-performance hang glider to the beach. It turned back on in late afternoon and it increased with enough of a south component to enable hang gliders and paragliders to fly the more south-facing cliffs east to Lulworth Cove and back.

Paragliding Marine Grant Oseland describes the conditions on that day as “…from too windy, to too light, to too far off to the south and then too far off to the west…” (See his report with photos of his cliff run to Lulworth Cove and back in Eye in the Sky — in the block of entries for Thu 08 Aug 2013.)

During my flight, the wind veered (swung west). The lift on the cliff reduced while the wind increased to make it difficult for the slow-flying paragliders to penetrate. Eventually, everybody else landed. About a half hour after I launched, orographic cloud began to form between where I was (over the cliff) and the top landing field (immediately behind the take-off hill). Some orographic cloud is visible in the distance in the accompanying photo taken earlier in the same flight.

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Not wanting to be caught in cloud, I headed back towards the top landing field. It was then that I entered a slow but smooth climb over the shrub-covered slopes to the right (west) of the cliff. The orographic cloud in the vicinity dissipated and I continued to climb over the fields inland from the coastline. Eventually I reached 900 ft above my launch altitude, or about 1,300 MSL.

Wind lines on the sea in the photo indicate a surface wind somewhat west of south-west. When this photo was taken, I was flying directly into wind at more than 1000 feet MSL. (I was straight and level, but the wide angle of the camera lens might make it appear that I was in a left turn.) The wind there is clearly more like due west or even north of west. This wind shift with altitude seems more than the rule-of-thumb that (in the northern hemisphere) the wind veers about 20 degrees at 1000 ft compared to ground level.

The air was smooth and, flying at what felt to me the right speed for minimum sink rate (in a slow-flying type of hang glider) I was barely moving over the ground. Eventually, I pulled on extra speed and flew into sinking air just before arriving above the cutting of the road down to the beach. The road is out of view to the right of the photo, but the emergency bottom landing field (RAF Ringstead*) is the one containing the circle of tents. I then flew downwind and inland to the take-off hill, above which there was only sinking air at my height. That last observation precludes ridge lift as a possible cause of my climb (apart from the improbability of a small ridge providing lift to nearly ten times its height). After nearly an hour in the air I top-landed in the turbulence we always encounter behind the launch ridge in a west wind.

* RAF Ringstead: Barely visible in this photo, at the left end of the emergency landing field (where the bushes and trees start — and partly cut by the bottom edge of the photo) is an earth mound that contains the reporting station for the radar aerials sited on these fields during World War 2.

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What caused the lift over these fields, which slope only slightly? One possibility is an atmospheric wave effect from Portland Island, directly off shore. (It is a much larger land mass than it appears in my photos.) However, on this day the wind was well off from Portland, so that seems an unlikely cause.

Maybe the Geoffrey Rutledge article about the Funston convergence might shed some light:

…when the prevailing wind is a strong northwest condition, the southwest winds at Fort Funston may be the eddy caused by Point Reyes, a land mass protruding out to sea north of the Golden Gate.

Here is my diagrammatic interpretation of that, based on the assumption that the land roughness slows the wind more than does the smoother sea, which causes the wind direction to swing in accord with the grey wind arrows:

And here is my equivalent interpretation of the Ringstead convergence:

It strikes me that a minor change either in the basic wind direction or thermal effects from the solar heating of the land (at Point Reyes in the first case and at Portland and Weymouth in the second) could cause rapid and major changes down-wind, affecting flying conditions at these coastal flying sites.

Nevertheless, the smoothness of the air in that lift zone is more characteristic of wave than of convergence.

Internal link

Overview of Ringstead (a sight-seeing guide, not a flying site guide)