In order to measure the height as well as the amount of clouds, so called ceilometers (cloud base sensors) are applied. A laser ceilometer consists of a vertically pointing laser and a receiver in the same location. A laser pulse with duration on the order of nanoseconds is sent through the atmosphere. As the beam travels through the atmosphere, tiny fractions of the light are scattered by aerosols. Generally, the size of the particles in question is similar in size to the wavelength of the laser. The reflected beam is measured and by time of traveling the wave height is determined for the first and in case there are, second and third cloud base.