336 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1959 
Imperial Valley one of the watermasters told me he had twice seen 
trains puffing along in the sky, lifted into sight over the massive bulk 
of the sandhills. From a ranch house on the edge of the mesa some 
20 miles south of Yuma, the owner frequently sees the little town of 
San Luis across the border in Mexico, which normally is not visible, 
lifted into sight, together with its highway and speeding cars. A 
ridge east of the New Yuma Hospital normally hides homes and 
ranches beyond it. Two or three times a year, on the average, they 
are lifted into sight. In eastern Colorado rolling hill country, ranchers 
frequently see on still mornings homes and farms as much as 20 miles 
away. The picture is lifted up, into sight, but not otherwise changed. 
In the mirages studied so far, we knew where the lenses were—in 
the street, dry lake bed, or on the ridge—and could go to them, take 
their temperatures, etc. In our study of other types, that advantage 
is lacking. Their lenses are in the temperature inversion layer aloft, 
a few feet to several thousand feet above the ground, well out of reach. 
Balloon-carried instruments giving temperature, humidity, and pres- 
sure are sent up four times a day from many places over the world, 
frequently passing through these layers, but their data, as bearing on 
temperature inversion layers, have not been tabulated. As far as 
known, no balloon run has been made up through a mirage-forming 
layer to give us a definite picture there. These layers are commonplace 
over all parts of the world, almost as much so as clouds, and as tran- 
sient. We know that the amount of the inversion varies greatly, as 
does the thickness of the layer and its area. From the mirage itself 
we may draw certain conclusions. For the building up of a certain 
effect, the lens and its sustaining layer must have certain qualifications. 
It seems probable that much less than a tenth of 1 percent of the in- 
version layers ever become mirage forming, and so it is evident that 
this is one of their minor activities. 
Aloft, as over the asphalt pavement, the lens picks its location 
carefully, and having picked it, uses it over and over again. It is 
evident that in all cases, the terrain, including the surface over which 
the lens is to form, is of the greatest importance. The mirage will 
come into being because, at the proper place between object and 
observer, the terrain makes possible the formation of an effective air 
lens, on the ground or aloft. Of course the weather must cooperate. 
OPERATION DISTORTION 
Operation Distortion, carried on in some particular temperature- 
inversion layer aloft, functions to make some object or scene look as 
it does not. The primary effect it achieves is a stretching up. This 
may be followed by a lateral stretching, mostly internal, as between 
the stretched-up parts of the object or scene—houses in a town, hills 
