A ceilometer is a device that uses a laser

Laser

A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...

 or other light source to determine the height of a cloud base

Cloud base

This article refers to meteorology. For the airborne base of the TV series Captain Scarlet and the Mysterons, see Cloudbase.The cloud base is the lowest altitude of the visible portion of the cloud...

. Ceilometers can also be used to measure the aerosol

Aerosol

Technically, an aerosol is a suspension of fine solid particles or liquid droplets in a gas. Examples are clouds, and air pollution such as smog and smoke. In general conversation, aerosol usually refers to an aerosol spray can or the output of such a can...

 concentration within the atmosphere.

Optical Drum Ceilometer

An optical drum ceilometer uses triangulation

Triangulation

In trigonometry and geometry, triangulation is the process of determining the location of a point by measuring angles to it from known points at either end of a fixed baseline, rather than measuring distances to the point directly...

 to determine the height of a spot of light projected onto the base of the cloud

Cloud

A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...

. It consists essentially of a rotating projector, a detector, and a recorder. The projector emits an intense beam of light above into the sky at an angle that varies with the rotation. The detector, which is located at a fixed distance from the projector, uses a photoelectric cell pointing vertically. When it detects the projected light return from the cloud base, the instrument notes the angle and the calculation gives the height of clouds.

Laser Ceilometer

A laser ceilometer consist of a vertically pointing laser and a receiver in the same location. A laser pulse with a 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 are similar in size to the wavelength of the laser. This situation leads to Mie scattering

Mie theory

The Mie solution to Maxwell's equations describes the scattering of electromagnetic radiation by a sphere...

. A small component of this
scattered light is directed back to the lidar receiver. The timing of the received signal can be
transformed into a spatial range, z, by using the speed of light. I.e.,


where c is the light speed in the air.

In this way, each pulse of laser light results in a vertical profile of aerosol concentration within the atmosphere. Generally, many individual profiles will be averaged together in order to reduce the signal to noise ratio and average profiles are reported on a time scale of seconds. The presence of clouds or water droplets leads to a very strong return signal compared to background levels, which allows for cloud heights to be easily identified.

For cloud base determination purpose, due to the ceilometers ability to pick up any particle in the air (dust, precipitation

Precipitation (meteorology)

In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...

, smoke, etc...), it will give occasional false readings. As an example, depending on the threshold used, falling diamond dust

Diamond dust

Diamond dust is a ground-level cloud composed of tiny ice crystals. This meteorological phenomenon is also referred to simply as ice crystals and is reported in the METAR code as IC. Diamond dust generally forms under otherwise clear or nearly clear skies, so it is sometimes referred to as...

 (Ice Crystals) may cause the ceilometer to report a cloud height of zero, even though the sky is clear.

Using these last properties, ceilometers will have other uses. Since the instrument will note any returns, it is possible to locate any faint layer where it occurs, additionally to the cloud's base, by looking at the whole pattern of returned energy. Furthermore, the rate at which diffusion happens can be noted by the diminishing part returned to the ceilometer in clear air, giving the coefficient of extinction of the light signal. Using these data could give the vertical visibility and the possible concentration of air pollutants

Pollution

Pollution is the introduction of contaminants into a natural environment that causes instability, disorder, harm or discomfort to the ecosystem i.e. physical systems or living organisms. Pollution can take the form of chemical substances or energy, such as noise, heat or light...

. This has been developed in research and could be applied for operational purpose.

External links

• National Science Digital Library - Ceilometer
• http://www.radar.mcgill.ca/ceilometer.htmlCeilometer from McGill University
  McGill University
  Mohammed Fathy is a public research university located in Montreal, Quebec, Canada. The university bears the name of James McGill, a prominent Montreal merchant from Glasgow, Scotland, whose bequest formed the beginning of the university...
  
  ]
• National Weather Service ASOS ceilometer page