How the cold lake forms
- Night. The ground loses its heat, and cold air drains into the Weald.
- Dawn. Calm in the lake — the real wind blows over the top.
- Mid-morning. Thermals stir the lake up: 10–30 minutes of rough air.
- Midday. Mixed out — the wind reaches the hill, climbs go higher.
- Evening. The lull is the wind going up, not stopping.
By day the sun heats the ground, the ground heats the air touching it, and that air rises as thermals. After sunset it runs the other way. With no cloud to hold the heat in, the ground radiates it to space and gets cold, and so does the air touching it. Cold air is heavy: it drains downhill and pools in the low ground, growing thicker and colder through the night.
“We now have a lake of heavy, cold air over the land with the real air doing its own thing over the top.”Steve Purdie, club safety notice, August 2012 — printed again in Windsock, November 2022 (Dave Lewis, ‘Tis the Season… To Be Jolly Careful of inversions)
Forecasters call it a ground inversion: a layer where the air gets warmer with height instead of cooler. It can be anything from a few feet to a thousand feet deep. Inside it the wind decouples from the wind above — light, often draining down the slope whatever the real wind is doing — while just over its top the real wind blows as hard as it did all day. Between the two is a shear layer, and that is where the trouble is.
Steve’s original safety notice (2012) · the Windsock article (November 2022, pages 6–8) · the Windsock archive
The three traps — and the landing
- The calm morning. It’s light and flyable on take-off. The first decent climb lifts you to the top of the lake and into the shear layer — a battering — and if you get through it in one piece there may be a lot more wind above.
- The mix-out. Once the thermals are strong enough to punch through, they stir the whole thing up and bring lumps of the windy air above down to the hill. It usually takes ten minutes to half an hour, and it is not a time to be flying. In Steve’s words, “sensible pilots will be on the ground having coffee”. Afterwards the thermals slow while the fields catch up, then the climbs go all the way.
- The evening lull. It’s been windy all day and near sunset the wind drops to flyable. It hasn’t stopped — it’s gone up. Climb a little and you meet it again, through the shear layer, as strong as it was all afternoon.
On our sites
Cold air drains downhill like water, so the lake collects wherever our ground is lowest. The general pattern is below; the quotes are each site guide’s own, and the guide is always the final word.
The north-facing ridges — Devil’s Dyke, Ditchling, Firle, Bo Peep
Every one of them launches over the Weald, and the Weald is where the cold air collects. On a still, clear morning the bottom-landing fields can sit in the lake while take-off sits above it or right in its shear layer. The guides already warn of steep gradients near the ground, and a cold lake is one more reason to respect them:
- Firle — “the take-off area usually has a steep wind gradient. Walk to the very edge to gauge actual wind speed.”
- Ditchling — “The trees on the lower slopes create a steep wind gradient with associated lee turbulence — keep enough height to cross them safely.”
- Devil’s Dyke — “In northerlies … the take-off can sit in wind-shadow with a steep gradient.”
- Bo Peep — “Always walk to the top of Bostal Down to assess the true wind.”
If the bottom field is cold, damp and dead calm on a sunny morning while the ridge is blowing, you are looking at the lake. Plan the landing for it.
The valley sites — Mount Caburn and High & Over
Our valleys are the lake’s rivers: on still nights cold air drains down them towards the sea. Both guides describe the wind at the bottom doing its own thing:
- Mount Caburn — “The wind direction is frequently different at the top and the bottom landing field — check carefully before a final approach.” The landing field below is “waterlogged for most of the winter”.
- High & Over — “if the flow is up or down the main valley, you are probably at the wrong site”, and “the wind at the foot of the river cliff (below the left-hand take-off) can switch without warning.” In winter the landing field is often flooded.
The coast — Beachy Head and Newhaven
The cold lake is a land thing. On a clear, calm morning the cold air draining off the Downs flows out to sea — Steve’s “plenty of wind from the north as the cold layer flows out to sea, just like a river” — which at our cliffs means an offshore breeze first thing. Wait for the real wind to swing onshore. Once it does, the air arriving off the sea has never sat over cold ground: in winter the Channel is often the warmest thing around, and the air off it is more likely to be buoyant than stuck.
- Beachy Head — “Adverse temperature-gradient conditions can cause severe turbulence from velocity differences between the wind above and below a separating inversion layer, which often forms at or around take-off height; check the wind is constant in speed and direction at take-off and further down the slope.”
- Newhaven Cliffs — the lift “depends on the air being cooler than the sea rather than on wind strength alone”. And mind the tide: nothing west of the point from three hours before to two hours after high water.
The signs
Steve’s list, which is hard to improve on:
- It’s been a warm, breezy day, and in the evening the wind slows as it cools.
- The skies are fairly clear, letting the heat escape.
- It isn’t windy on the ground, but the clouds are moving well.
- The forecast is for wind, but it isn’t windy on take-off.
- The upper-level forecasts show wind, but the simple forecasts for ground-level wind show much less.
- It’s clearly shown on the forecast soundings.
- Plenty of wind on take-off but not much lift — get a bit low, slope-land, and there’s no wind at all: you’re in the lake, and only the top of the hill was sticking out into the breeze.
- Someone takes off half-way up, climbs 100 ft, takes a series of collapses and starts going backwards.
How the club forecasts it
The What to expect card — at the top of Conditions and on every site guide — reads the cold lake off the forecast profile for today and the next five days: how much warmer the air is a few hundred feet up at first light, the wind above it, roughly when it should mix out, and the evening lull after a windy afternoon. On a member’s site guide, the Skew-T shows it too: the temperature trace leaning right straight off the ground.
The model sees the lake through levels a few hundred feet apart, so it finds the deep ones and can miss a shallow one hiding below its first level. The card is labelled beta for exactly that reason. Clouds moving well over a windsock that barely stirs is the tell, whatever any forecast says.
With thanks to Steve Purdie, who wrote this up for the club in 2012, and to Dave Lewis, who brought it back in Windsock in 2022.