C

Capacitance rain gauge- A probe consisting of a stainless steel rod covered by a Teflon sheath set inside a cylindrical rainwater collector. The water surrounding the probe forms the outer plate of a coaxial-type capacitor while the metal rod forms the inner plate. The Teflon sheath serves as the dielectric. As the water height in the collector rises, the surface area of the capacitor increases, increasing the total capacitance. The total capacitance is measured and converted to an analog voltage directly proportional to the height of the water in the collector.

Capillary collector- An instrument for collecting liquid water from the atmosphere.

Cavity radiometer- A modern, self-calibrating pyrheliometer or device for measuring direct solar radiation.

Ceiling balloon- A small balloon used to determine the height of the cloud base.

Ceilometer- An automatic, active, remote-sensing instrument for detecting the presence of clouds overhead and measuring the height of their bases.

Chronograph- A clock-driven device for recording the time of occurrence of an event. It is often used, for example, in conjunction with a contact anemometer and a wind vane to obtain a record of wind speed and direction as a function of time.

Chronometric radiosonde- A radiosonde with a carrier wave switched on and off in such a manner that the interval of time between the transmission of signals is a function of the magnitude of the meteorological elements being measured.

Chronothermometer- A thermometer consisting of a clock mechanism the speed of which is a function of temperature.

Cistern barometer- A mercury barometer in which the lower mercury surface is larger in area than the upper surface.

Clinometer- An instrument for measuring angles of inclination. In meteorology, it is used in conjunction with a ceiling light to measure cloud height at night. With it the observer measures the angle included by the illuminated spot on the clouds, the observer, and the ceiling light.

Cloud condensation nuclei counter- Any of several devices that measure the number concentration of atmospheric particles upon which water vapor condenses at low values of supersaturation that are equivalent to those that occur with the formation of tropospheric liquid clouds.

Cloud detection radar- A radar designed for detection of cloud particles as distinguished from the larger particles that constitute precipitation.

Cloud drop sampler- A general term for instruments that collect cloud droplets to determine their size or impurities; includes devices with direct-impaction surfaces that either capture or leave an impression of the impinging droplets, such as the cloud gun, as well as bulk samplers that capture and collect cloud droplets for chemical analysis, such as the rime rod.

Cloud gun- A device for airborne or ground-based use that exposes a soot-coated glass slide for an instant to a cloudy airstream. Cloud droplets impact on and leave craters in the soot coating for subsequent measurement of droplet size spectra using a computer-driven image analysis system and calibrations that relate drop size to crater size.

Cloud height indicator- The general term for instruments that measure the height of cloud bases.

Cloud radar- A radar optimized to detect clouds.

Coherent radar- A type of radar that extracts additional information about a target through measurement of the phase of echoes from a sequence of pulses (or an extended observation interval, as in an FM–CW radar). The phase information may be used to improve the signal-to-noise ratio (see coherent integration), to estimate the velocity of the target through the Doppler effect, or to resolve the location of the target in a synthetic aperture radar.

Compensated scale barometer- Mercury barometer with a fixed cistern. Its scale graduations take account of the changes in the level of the free surface of the mercury in the cistern as a function of atmospheric pressure.

Constant level balloon- A balloon designed to float at a constant level of altitude, pressure, density, or entropy.

Continuous wave radar- A type of radar that transmits continuously instead of in pulses.

Cooling power anemometer- The general term for anemometers operating on the principle that the heat transfer to air from an object at an elevated temperature is a function of the air speed.

Cryogenic hygrometer- A special type of chilled mirror hygrometer capable of measuring very low frost points.

Cup anemometer- A mechanical anemometer with a vertical axis of rotation, usually consisting of three or four hemispherical or conical cups mounted with their diametrical planes vertical and distributed symmetrically about the axis of rotation.