"Supporting the in situ observations to be made by the DOE Gulfstream-1 (G-1) aircraft (image at right) will be profiles of aerosol backscatter and extinction in the vicinity of these clouds to be made with a High Spectral Resolution Lidar (HSRL) onboard the NASA Langley Research Center's King Air Be-200."
-quote from Cumulus Humilis Aerosol Processing Study (CHAPS) The Atmospheric Science Program
-Image right from ARM Image Library
Lidar remote sensing
The basis for lidar remote sensing lies in the interaction of light with gas molecules and particulate matter in suspension in the atmosphere (aerosols).
More particularly, a lidar uses a laser (emitter) to send a pulse of light into the atmosphere and a telescope (receiver) to measure the intensity scattered back (backscattered) to the lidar.
By measuring the scattering and attenuation experienced by the incident pulse of light, one can investigate the properties of the scatterers (concentration of gaseous species, aerosol distribution and optical properties, cloud height) located in the atmosphere.
The light scattered back to the detector comes from various distances, or ranges, with respect to the lidar.
Because the light takes longer to return to the receiver from targets located farther away, the time delay of the return is converted into a distance (range) between the scatterers and the lidar, since the speed of light is a well-known quantity.
By pointing the laser beam in various directions and at various angles with respect to the ground surface (scanning), a ground-based lidar system can gather information about the three-dimensional distribution of aerosols in the atmosphere.
(Excerpt from: rap.ucar.edu)