The Trindade Isle Case

December 12, 1959

Mr. Richard Hall, Secretary
Washington, D. C.

Dear Mr. Hall:

In reply to your letter of December 7, I regret to say that I do not have access to any
meteorological data for the equatorial area within which the Brazilian photo was reportedly
taken in January, 1958.  Surface and upper air data is only widely published for the middle
latitudes of the Northern Hemisphere.

However, I do not believe this lack of data is very relevant to the question of whether the
photographer could have been unknowingly photographing an aircraft "enveloped in its
vapor trails". The cut you sent is of poor quality, of course, but it reveals so large an angular
diameter of the "object" that had it been an aircraft the photographer could not have been in
any doubt as to its identity. Furthermore, it bears on resemblance, in my opinion, to the type
of condensation trails that aircraft can form in any altitudes, let alone low altitudes in the
tropics. In the latter case, exhaust condensation trails are simply not physically possible for
reasons I'll not try to explain here. However, given extremely high relative humidities, and
given an aircraft of very high wing loading(e.g. a fighter) making, say, a sharp pull-out from
a dive, perceptible condensation trails can form in the two tip-vortices that trail back from
each wing-tip. In addition, rarefaction at the propellor tips under such hypothesized high
humidities can form perceptible helical trails. Photographs of both of these types of
dynamically produced condensation trails are in the literature. The former type is not so
commonly reproduced in photographs, so you might be interested in examining a beautiful
example of it to be found in Ludlam and Scorer's "Cloud Study", p.77. It's a shot of a
British light-bomber in a pull-out in humid air, and shows two narrow tubes of vapor
streaming back from each wing tip.

Aircraft do not become "enveloped" in either of the above two types of dynamically
produced condensation trails (nor do they become enveloped in the much more common
exhaust trails peculiar to very low ambient temperatures).

I enclose the clipping, assuming it is your only file copy. If you care to send out the
glossy (if it is appreciably sharper), I might be able to make some further comment
on the basis of an examination of it.


J. E. McDonald

*NOTE: Some but not all grammar, typing and spelling errors have been corrected from the original letter.

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