The McGill University Laser Ceilometer

The downtown campus of McGill University operates a laser ceilometer round the clock. The vertically-pointing instrument returns data every 30 seconds. The ceilometer, thanks to very useful computer software, gives profiles of returned power from the ground up to 4 kilometers high, as well as a record of cloud base height. The returned power is a measure of the amount of the initial energy sent up by the ceilometer that is scattered back to the instrument by various atmospheric constituents being present overhead. Because of its relatively short wavelength corresponding to the infra-red portion of the electromagnetic spectrum, the ceilometer is sensitive to particles as small as aerosols, such as dust, and as large as precipitation, such as snowflakes. From the energy returned to the ceilometer from such particles, various atmospheric parameters can be calculated. One such parameter is the atmospheric optical extinction coefficient, which is closely related to visibility. Hence, the ceilometer is a very useful instrument that can be used for research or experimental purposes, as well as operational applications (often being used at airports).

For further reading, see :

• Zuev, V.E., 1982: "Laser Beams in the Atmosphere". Trans. from Russian by J.S. Wood. Consultants Bureau, New York, 504 pp.
• Collis, R.T.H., and P.B. Russel, 1976. "Lidar Measurements of Particles and Gases by Elastic Backscattering and Differential Absorption" in E.D. Hinkley (ed). "Laser Monitoring of the Atmosphere". Springer-Verlag, Germany, 71-151.