Is that lee or not?

What do you see in the picture? Take a few seconds before you read on.

The situation

We were flying near the Schilthorn and wanted to cross the Lauterbrunnental towards the Jungfrau. A client on a coaching, me in front. On the other side a southwest-facing slope, above it at over 4000 m a ragged cumulus cloud that showed a northwest wind.

Before a crossing like that, two questions are in the air. Is there wind below cloud base too, at about 3000 m? And are there thermals over there that I can use?

In a northwest wind, the mountain I am flying towards is in the lee. Its slope faces southwest and gets the sun, it is still in the wind shadow. So the question is a different one: does the wind at altitude reach down below cloud base, or is the mixed layer below it decoupled? That is a difference you are better off checking than guessing.

So I waited. Before the crossing. In my thermal, climbing and looking at the cloud over there on every circle. Freeze frame, circle, look, circle again.

What I observed

Three things, independent of each other.

First, the cloud. At the top it leans to the right as seen from us, towards the southeast. Up there the wind increases with height.

But I never look at a cloud as a whole. I pick out a single feature and look at it very closely. Here it was a small wisp of cloud hanging down out of the base. I followed it over five circles: is it moving? Is it changing?

Then the second question: where is the cloud forming anew? At the top on the front edge, on the left, new wisps kept forming and were combed to the right straight away. That is a cloud the air is flowing through. At the bottom that did not happen: new supply, but no combing. There it looks as if there were no wind in it.

These so-called “small wisps” below the cumulus keep condensing at its base. The cloud keeps building from below. These wisps are small and short-lived. How they move is a more direct picture of the air at cloud base level. For us they drifted slightly to the left, into the cloud. So against the wind direction the combed top suggests.

Second, my own circle. I was climbing at 2900 m before the crossing and hardly drifted. You can see that on the instrument too: on the vario the circle shifts along the track and many instruments calculate a wind direction and a figure in km/h from it directly.

My own drift is the most direct wind information I have in the air. But it needs to be read too: it shows me how the thermal is displaced, not cleanly the wind. Strong thermals rise more steeply and drift more slowly, weak ones faster. The figure on the vario is therefore exactly as good as the circle it comes from. A good reference point, not a measurement.

Third, the live readings. The station at the Silberhornhütte near the target of our crossing, at 2700 m, was calm. It stands on the side I am flying towards, almost at my height. That is why it counts here at all.

This reading needs to be read too. A station measures one point, at one height, at a site with its own character. The catch here is a special one: calm in the wind shadow can mean there is no wind. It can also mean the station itself is sheltered. A low reading from a lee is therefore exactly the reading you should trust least. I took it as a third clue, not as an all-clear.

All three together meant for me: below cloud base the wind at altitude probably does not reach down.

Probably. Not for certain. The difference matters to me. A cloud that holds still shows you the air at cloud base level, at this one spot.

Wind shows in the cloud, you just need to know what to look for. Either there is no cloud at all. Or it sits offset from the trigger point instead of above it. Or it is combed and torn. In the picture you see that at the top, not at the bottom.

The wisps at the top can drift. The block below is what counts.

Why I flew anyway

Not because of the cloud. Because of the way back. And because I fly ahead.

The way back was calculated before we set off. I do that with XCTrack and burnair Go: I set a point on the thermal I am coming from. Then the instrument shows me the glide ratio I need to get back there. On that day it meant: at 2000 m I am back with a glide ratio of 8.

This figure is a straight line through the air, nothing more. Terrain, wind and lift or sink along the way are not in it. That is why the 8 is not a rule you can take over either. It was my figure, for this line, knowing what it looks like along the way. What you can take over is the approach: set a figure before you set off and leave it enough air below that wind and sink don't eat it up.

If you know where your Plan B or C is, you fly the crossing differently.

I fly ahead. The client a little behind me. What the air over there is doing, I show before she is in it. Otherwise I would have turned back straight away.

The radio was the line for turning back. Had it been lee, I would have noticed it first and said: turn round, back to the old thermal. In time, not at the point where it hurts.

The cloud told me that the crossing was plausible. The way back and the order we flew in allowed me to try it. Both together, in that order.

My crossing decisions always follow the same pattern: observe, analyse, decide. Here that meant: one feature on the cloud, for five circles. A way back as a figure. And only then the decision. Every crossing is different, the pattern is the same every time.

The part I like about this method: the closer we got to the cloud, the longer and better we could observe. The decision is made across the whole crossing, I check it again with every look.

What would have made me turn back

Three things I told myself before setting off:

The lower block starts to move. The wisps at the top can drift. The block below is what counts.

No more cloud forms over there. That is no proof of wind. Mostly the air has simply become too dry, then it keeps rising, just without a cloud. I still keep it as a reason to turn back, because it tells me when to look more closely. When in doubt, I turn back.

The way back gets tight. As soon as the glide path to the old thermal falls below the figure I set myself beforehand, I could turn back. Even if the cloud is still standing.

The reverse case: no cloud at all

If nothing at all forms over a sunny slope, the air is usually just too dry. It rises, it is just not enough for a cloud. There may also be an inversion above it, the slope may not trigger at all or high cloud takes the sun away from it. And yes, it can also be wind tearing the thermal apart. Check anyway.

You typically see that at passes. There the valley wind and/or the wind at altitude meet and have to go through the same “gap”. Where it gets narrow, the wind gets faster. Whatever blows through then tears the thermal apart before it even comes together. Thermals need calm. They form where the air can warm up undisturbed and rise unhindered.

When you fly alone

That day my client had two things you don't have when you fly alone: someone ahead of her and a radio that gets a call to turn back to everyone within seconds.

The observation still works the same way: the cloud, your drift, the cloud forming again over there. What changes is the buffer. Without someone flying ahead you are the feeler yourself and the margin has to be bigger: decide earlier, take more height with you, calculate the way back more generously.

Whoever flies in first pays for the information. In coaching that is my job. On your own it is yours.

So, was it lee?

No. But the answer came later than you might think.

Nothing happened on the crossing itself. No shaking, no sinking out and no good sign either. Just gliding.

Then over the slope very gentle lift. The first good sign. And not because there was lift. You find lift in the lee too. Because it was calm.

The real climb came over a distinct trigger point, a steep ridge. The rest of the slope was uniform and gave the thermal no reason to break away from the terrain. That is part of the answer too: the cloud told me I could expect something over there. Where I had to start, it did not tell me. That was in the terrain.

And then nothing more. No sign of lee below the thermal, none on the onward flight towards the Eiger.

And yet: the fact that it worked out does not make the decision right. Had there been rough air or sink over there, the way back would still have been there. That was the part that mattered.

A decision and its outcome are two different things. In cross-country flying you decide under uncertainty: you never know for sure whether the cloud in front of you is still working, whether the valley wind has already set in, whether you will make the crossing. But that is exactly what makes it so exciting.

Your turn

You saw the picture before you knew my answer. What would you have done, crossed or waited? And what would you have based it on?

Write to me: info@streckenfliegen.com. I read every email.

Clouds also tell you exactly where to fly. If the cloud sits offset from the trigger point, the thermal between them stands at a slant. Where you aim then, I draw in a short video: Spotting thermals with wind drift.

P.S. That was my first newsletter. From now on one comes once a month, with a takeaway as an article or as a video. Drop me a quick line on how this one came across: too long, too detailed, just right? And the thing with the trigger point at the end, that the cloud says whether and the terrain says where: would that be a topic that interests you? Then I will take it next.