Freesteel Blog » High altitude wave flight

High altitude wave flight

Monday, April 24th, 2017 at 8:15 pm Written by:

I had a very agreeable flight on Saturday in Wales, marred by a crappy going down performance on Bradwell on Sunday, just to put me in my place as not having any talent.

Here’s the good place to be, about a mile up from the ground:

It was a confused forecast, which meant a lot of people stayed away. Turns out it was because there was wave everywhere. I flew Tim’s U2 (mid-performance glider), but was put off by the fact of everyone pointing out that I was too high on the bar, and so wouldn’t be able to steer it so well. Hang-loops, unlike climbing harness loops, do not come with a buckle, so you can’t do anything about it.

There was a lot of lift straight off the hill, and then Carl said over the radio, “I think I’ve contacted wave, just to the right of the mound in front of the hill.”

There were lenticular (French for “slow”) wave clouds everywhere and no cumulus. I was wearing a skinny pair of gloves and had no base bar mitts (which I do have on my own glider).

It was indeed the famous phenomenon, smooth gentle rising lift, giving me no practice turning at all. Pretty soon I was on top of the world, with most of north Wales a wrinkled blanket below me. And I was shivering in my harness.

This is my journey into the sky in the space of half an hour, first in some very buoyant thermals, and then in the wave phenomenon.

The wave starts at 12:34.

I spotted something weird in the temperature trace:

I did wonder why my mittens didn’t freeze solid; there’s a sudden step up of 2 degrees C as I entered the zone.

More striking is the humidity:
This is a decline from 80% to below 20%. Both humidity sensors registered this, so it’s not some artifact of the device.

Now, wave is explained everywhere as a rebounding oscillation of air going over a mountain range.

The invaluable RASP website has a map for wave effects allegedly showing where the wave bars are going to be:

To me the zones don’t look particularly associated with a mountain range. They begin out at sea. Are they an emergent phenomenon of the weather atmospheric simulation?

RASP also provides this altitude, dewpoint temperature chart, which I didn’t understand. It’s supposed to help tell you whether the thermals are going to be strong (according to the slope of the red line) and where the cloud layer is going to be (according to when the red and blue lines touch).


For a while I didn’t really get it, until I read that the constant temperature lines are orange and diagonal to the right, not vertical, so that the graph can skewed back onto the same page.

Here’s my dewpoint from temperature and humidity function:

def DewpointTemperature(tempseries, humidseries):
    A, B, C = 8.1332, 1762.39, 235.66
    svp = 10**(A - B/(tempseries + C))*133.322387415
    pvp = svp*humidseries/100
    return -C - B/(numpy.log10(pvp/133.322387415) - A)

Putting it all together, viola! Something approaching the same graph!


According to my measurements the transition was a whole lot steeper in real life.

I stayed in the zone for another two hours, because I could. No rush to get back to earth, as the air was so smooth and comforting, in spite of the fact that I seemed to be entering and exiting the low humidity zone repeatedly without noticing.


However, the temperature profile remained stable in that zone even as the humidity was step-changing.

As far as I am concerned, this rules out the immiscible layers of air rolling over one another to form the wave, or I’d have felt buffeted about by clear air turbulence. No, I think the wave phenomenon is mapped over the stratus, and there is something gentler here than it appears.

Whereas the thermic air rises all the way from the ground to the clouds as one packet of gaseous matter, the wave air itself might not be translating itself very far vertically. It is possible that it is simply always there. What we perceive is a geometric fact of it existing when its flow angle happens to exceed the glide angle of the glider. Then we go up in it in the same way that a surfer skims along the breadth of a wave by angling his board along the beach and skimming, even though no water is carried with him on his journey.

And that’s how I was granted this privileged visit to the upper atmosphere over Wales.

And on the next day I went to Bradwell, and screwed up and went down, while everyone else flew all around the hills.


I was not happy! A post mortum should happen in due course, unless I fly again and have some more interesting data to look at.


There was that point in the high flight where the humidity was flipping up and down every minute as I flew around in the layer.
The air was a bit rocky, but I would not call it turbulent.

Here is the plot in XY according to above and below the humidity 30% mark, shifted back by 10 seconds to account for an approximate delay in the sensor.

c = numpy.array(["r","b"])[(utils.InterpT(pQ, fd.pS.hS.shift(-50)[t0:t1])>30)+0]
plt.scatter(pQ.x, pQ.y, color=c, marker=".", s=1)


Because I am never going to find it again, here is a close-up of a few of those humidity transitions to justify the 10 second delay estimate. The transitions to low humidity look very smooth an exponential, indicative of a totally abrupt transition without any mixing.
The response curve is very different when the device is wetting; its curve seems to be linear for a time, before it curves into an exponential shape, as if there is a maximum rate of liquid uptake in the sensor that gets saturated up to a certain level.


(I wonder if this kind of observation is going to be known to the people in the beautiful building known as Sensor City.)

This is the plot of the temperatures of the track logger, rotated east by 20 degrees (the prevailing wind as calculated by the WindRose) shown from the side:

sx, sy = pQ.x*math.cos(g) + pQ.y*math.sin(g), pQ.y*math.cos(g) - pQ.x*math.sin(g)
c = numpy.array(["r","b"])[(utils.InterpT(pQ, fd.pS.hS.shift(-50)[t0:t1])<30)+0]
plt.scatter(sy, pQ.alt, color=c, marker=".", s=1)


This represents 30 minutes of flight, so there is a bit of time for the structures to evolve. Also, the glider is chasing lift, so the motions are not random. I can actually believe there are zones tending to the bottom right corner at about a 5 to 1 slope (the coordinate scales are not equal) if you imagine there is also some structure in the X (towards you) dimension. (This came out better when I sorted the points in that order.)

What can I conclude?

It definitely looks like the layers are out of order, as if there has been a finger-like incursion of wetter air into the warmer and drier high altitudes, trapping drier air below it, through which I have passed on my journey. By the way, the calculations gives a 1% difference in density due to that 2degC warmth (the water humidity makes a 0.1% difference in density the other way, because wetter air is less dense since a H2O molecule weighs less than the N2 and O2 molecules). I do not know if this is important.

I could speculate forever, but here are my questions to someone who knows what they are talking about:

1) What causes the stratification of the air layers in the first place, and how sharp is the boundary?

2) Are wave effects transposed geometrically onto this stratification without any reference to them so it's just a matter of where they happen to be when it starts up?

3) Is the seemingly striking evolution of the lenticular wave clouds merely the product of this early mixing as clear layers of wetter air fold over and are transported up?

4) What is the total geometry of the structure, outside of the zone of lift, in terms of how far the air is actually being transported vertically?

5) Can trapped warmer upper air convect into a layer of cold lower air that has been folded over the top?

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