The ceilometer is designed to find the cloud ceiling level, that is, the altitude of the base of clouds. This is a typical image produced by the ceilometer. The upper panel indicates the altitude of cloud base over a 12 hour period, (midnight to noon on Jan 22). The black dots show levels where it can clearly see a cloud, and the purple dots are where the cloud level is ambiguous.
The lower panel shows the strength of the echoes for the same period. This is the raw data used by the ceilometer to estimate the cloud altitude. The green line at 3 km just after midnight (00) indicates a cloud (also see upper panel). After 2 am, a front moved overhead, the cloud level dropped and snow started falling (the orange and yellow area on the lower panel). The ceilometer successfully identified the cloud level above the snow. At about 7 am, the snow and low cloud dissipated. Other clouds appeared later in the morning.
The green line at the lower right hand side of the image, shows a profile of reflectivity versus altitude sampled just before noon. The cloud level (a sharp bump in reflectivity marked with a red line) is obvious. Below this the reflectivity increases gradually towards the ground. This increase is probably due to low level haze. As illustrated by this example, although it is designed to observe clouds, the ceilometer can also be used to study smog or haze, and rain.