The ceiling projector or cloud
searchlight
A searchlight (or spotlight) is an apparatus that combines an extremely bright source (traditionally a carbon arc lamp) with a mirrored parabolic reflector to project a powerful beam of light of approximately parallel rays in a particular direc ...
is used to measure the height of the base of
cloud
In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may co ...
s (called the ceiling) above the ground. It is used in conjunction with an
alidade
An alidade () (archaic forms include alhidade, alhidad, alidad) or a turning board is a device that allows one to sight a distant object and use the line of sight to perform a task. This task can be, for example, to triangulate a scale map on site ...
, usually positioned 1000 ft (300 m) away and wherever possible set at the same level.
The projector is normally set at 90°, although 71° 31' may be used, in relation to the terrain. The projector consists of a 430-watt
incandescent
Incandescence is the emission of electromagnetic radiation (including visible light) from a hot body as a result of its high temperature. The term derives from the Latin verb ''incandescere,'' to glow white. A common use of incandescence is ...
bulb set in a weatherproof housing. Inside the housing are two
mirror
A mirror or looking glass is an object that Reflection (physics), reflects an image. Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the ...
s; the first, above the bulb, reflects the light downwards to the second mirror, that then reflects the light upwards to the cloud. Both mirrors are focused to produce a high intensity beam of light that renders a visible spot on the base of the cloud.
The alidade is mounted on a post at a height of 5 ft (1.5 m) from the ground. It consists of an arm with a pointer and open sight at one end and a rubber eyepiece at the other. The arm is mounted onto a curved scale that is marked both in meters and the coded cloud height (feet). The observer looks through the eyepiece and sets the sight onto the spot projected on the cloud and reads the height from the attached scale.
When the cloud is thin the beam of light may penetrate into the cloud. The observer should read the scale where the light first enters the cloud and not at the top. However, a remark may be made as to how far into the cloud the light was able to penetrate as this may be useful. In the case of
fog
Fog is a visible aerosol consisting of tiny water droplets or ice crystals suspended in the air at or near the Earth's surface. Reprint from Fog can be considered a type of low-lying cloud usually resembling stratus, and is heavily influ ...
or
blizzard
A blizzard is a severe snowstorm characterized by strong sustained winds and low visibility, lasting for a prolonged period of time—typically at least three or four hours. A ground blizzard is a weather condition where snow is not falling b ...
conditions the observer should read the scale where the beam disappears.
See also
*
Ceilometer
A ceilometer is a device that uses a laser or other light source to determine the height of a cloud ceiling or cloud base. Ceilometers can also be used to measure the aerosol concentration within the atmosphere. A ceilometer that uses laser light ...
*
Trigonometric function
In mathematics, the trigonometric functions (also called circular functions, angle functions or goniometric functions) are real functions which relate an angle of a right-angled triangle to ratios of two side lengths. They are widely used in all ...
*
Triangle
A triangle is a polygon with three Edge (geometry), edges and three Vertex (geometry), vertices. It is one of the basic shapes in geometry. A triangle with vertices ''A'', ''B'', and ''C'' is denoted \triangle ABC.
In Euclidean geometry, an ...
References
*Department of Transport (Canada) - Meteorological Branch - Ceiling Projectors and Associated Equipment - Manual 70
{{Meteorological equipment
Meteorological instrumentation and equipment
Cloud and fog physics