Cold lakes: ground inversion season on the Downs
It's that time of year again. Prompted by a question in the members' Telegram group this week, we've gone back to the club's own ground inversion warning: Steve Purdie's safety notice from 2012, which Dave Lewis brought back in Windsock in November 2022. It's still the best short explanation we have, so we've built on it.
New under Before you fly: Cold lakes on the Downs. An animated explainer, with what the cold lake means at each of our sites, quoted from the site guides.
The short version
After a clear, calm night the ground loses its heat and chills the air touching it. That cold, heavy air drains into the low ground and sits there as a "lake", anything from a few feet to a thousand feet deep, with the real wind blowing over the top. On our patch, that's the Weald below the north-facing ridges, the Ouse levels under Caburn and the Cuckmere below High & Over.
Where it catches pilots
- The calm morning: it's light on take-off, but the first climbs lift you into a rough shear layer and the wind above it.
- The mix-out: when the thermals break the lake up, expect ten minutes to half an hour of rough air. In Steve's words, sensible pilots will be on the ground having coffee.
- The evening lull: after a windy day the wind near sunset hasn't stopped; it's gone up.
- The landing: the glider can dive as it drops into the cold, still layer. Come in with extra speed, and legs down on a hang glider.
On our sites
- Devil's Dyke, Ditchling, Firle and Bo Peep: you launch over the Weald, which is where the lake collects. The bottom field can be dead calm while the ridge blows, and the guides already warn of steep gradients at take-off and over the trees.
- Mount Caburn and High & Over: the valleys are the lake's rivers. Caburn's guide says "the wind direction is frequently different at the top and the bottom landing field"; High & Over's, that the wind at the foot of the river cliff "can switch without warning".
- Beachy Head and Newhaven: first thing, the cold air drains off the land and out to sea, so expect an offshore breeze until the real wind swings onshore. Beachy's guide warns of "a separating inversion layer, which often forms at or around take-off height".
The forecast can help now
The new What to expect card, on Conditions and every site guide, reads the cold lake off the forecast. It shows how strong the lake is at first light, roughly when it should mix out, and the evening lull after a windy day. It's in beta and the model misses shallow lakes, so clouds moving well over a windsock that barely stirs still beats any forecast.
Read the originals: Steve's 2012 safety notice and the Windsock article (November 2022, pages 6–8). The rest of the magazines are in the Windsock archive.
Fly safe.