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When Joseph Kittinger in Excelsior III dwelt at his peak altitude of 102,000 ft (31 km), he reportedly saw clouds at his altitude. What in the world could these clouds have been? Polar stratospheric clouds don't form as much south as Kittinger has been, do they? And noctilucent clouds form much higher, in the mesosphere. So what clouds could that have been? Kittinger says they had been seen at earlier times. Have they been observed by near-space balloons or SR-71 pilots afterwards?

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One possibility is nacreous clouds, very thin clouds likely made of water ice condensed on fine particles carried into the stratosphere. The reported altitude would be at the edge of the range quoted below.

Nacreous clouds, also known as mother-of-pearl clouds, are stratospheric; they occur between 15 and 30 km. Large volcanic eruptions emit dust particles in the lower stratosphere. These may combine with ice to produce nacreous clouds. In fact, in the year following Mt Pinatubo eruption in 1991, many nacreous clouds where spotted by airline pilots flying in the twilight.

The term "nacreous clouds" is often treated as synonymous with "polar stratospheric clouds", but they can form at non-polar latitudes. Besides volcanic eruptions, they can emerge from upper-air flows over mountains:

[Nacreous clouds] are most common in two situations. One situation is when strong winds (and winds increasing with height) cross a long mountain ridge, such as the Rocky Mountains in North America. The resulting high-amplitude gravity waves may propagate into the stratosphere. Nacreous clouds there are the stratospheric equivalent to lee wave clouds (Ac lenticularis) in the troposphere.

And in thunderstorms:

Large thunderstorm systems (mesoscale convective systems, MCSs) may also trigger nacreous clouds. The residual kinetic energy of convective updrafts within MCSs repeatedly poke through the tropopause. The combined effect of these overshooting tops is to bulge the tropopause upwards, and also to diffuse some water vapour in the lower stratosphere. The combination of water vapour transfer and uplift may result in a nacreous cloud above the remnants of an MCS.

A photograph of nacreous clouds taken by Hans Nilsson of Sweden:

enter image description here

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  • $\begingroup$ The link shows photos of polar stratospheric clouds and noctilucent clouds, that were already mentioned in the question. I don't know at what latitude Kittinger was. Wouldn't the stratosphere be too warm there? $\endgroup$
    – gerrit
    Apr 22, 2022 at 8:22
  • $\begingroup$ But they are not actually just polar, see my additional documentation. $\endgroup$ Apr 22, 2022 at 9:28
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    $\begingroup$ Interesting! NB: Your source has a typo in the photographer's name; I have corrected this in your post (I know the photographer, but see his academic homepage for confirmation). $\endgroup$
    – gerrit
    Apr 22, 2022 at 9:33
  • $\begingroup$ @gerrit thanks. The typo was actually not mine; the link had the name misspelled. $\endgroup$ Apr 22, 2022 at 11:13
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Polar stratospheric clouds occur at same level as Ozone layer. Otherwise known as Nacreous clouds. enter image description here

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