The Vaisala Ceilometer CL31 measures cloud height and vertical visibility. This small and lightweight measurement unit is ideal for mobile operation. The CL31 employs a pulsed diode laser LIDAR technology (LIDAR = Light detection and ranging), where short, powerful laser pulses are sent out in a vertical or near-vertical direction. The reflection of light -- backscatter -- caused by haze, fog, mist, virga, precipitation, and/or clouds is measued as the laser pulses traverse the sky. The resulting backscatter profile, or signal strength versus the height, is stored and processed and the cloud bases are detected. Knowing the speed of light, the time delay between the launch of the laser pulse and the detection of the backscatter signal indicates the cloud base height.
Ceilometer CL31 is able to detect three cloud layers simultaneously. If the cloud base is obscured due to precipitation or ground-based fog, CL31 reports vertical visibility. No field adjustments are necessary. The embedded software includes several service and maintenance functions and gives continuous status information from internal monitoring; it also is designed to give the full backscatter profile.