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Their Heads in the Clouds

“The base of the lowest cloud, or ceiling, can be considered an important meteorological observation, although emphasis on its measurement comes primarily from aircraft navigational needs. In the middle 1920’s aviation had increased to an extent which required the Weather Bureau to measure ceiling with small balloons and ceiling light projectors. In the absence of these devices, the cloud height was estimated by eye. The importance of ceiling for aircraft operations, in particular the importance of having it recorded in cases of litigation in connection with aircraft accidents, led to the development of the practical instrument now used—the automatic ceiling projector. Light modulated (to be detectable in daytime) is projected vertically and scanned automatically from some distance away. The angle of the scanner when it is pointing at the spot on the base of the cloud is recorded continuously. Thus ceiling is measured automatically and could be telemetered.”

Automatic ceiling projector.
Ceilometer computer. [1]

In this configuration,[2] the detector is scanned up and down the vertical light beam and the scan angle of maximum detected illumination is recorded. Given the displacement between the light transmitter and receiver and the measured angle, ceiling height can be calculated. Danish scientist, inventor, and meteorologist, Poul la Cour, is the likely inventor of the modern ceilometer in 1871.

By the mid 1960’s, the use of pulsed laser radar (LIDAR / LADAR) for atmospheric measurements was established firmly.[3]

Cloud height measurement technology, 1966.

Today, it’s done a bit differently, but most techniques still use laser radar in one form or another. Over time, we’ve developed better materials, tools, and understanding. We learned from the experience of the engineers that came before us.

References:

1. Paul Meissner, “Design and Operation of the Ceilometer Computer,” National Bureau of Standards, 64 (1960).

2. C.A. Douglas and R.L. Booker, “Visual Range: Concepts, Instrumental Determination, and Aviation Applications,” National Bureau of Standards Monograph 159 (1977).

3. J.E. Masterson, J.L. Karney, W.E. Hoehne, “The laser as an operational meteorological tool,” Bulletin American Meteorological Society, Vol. 47, No. 9, SEP 1966.