Meteorological instrumentation

(engineering) Apparatus and equipment used to obtain quantitative information about the weather.

Devices that measure or estimate properties of the Earth's atmosphere. Meteorological instruments take many forms, from simple mercury thermometers and barometers to complex observing systems that remotely sense winds, thermodynamic properties, and chemical constituents over large volumes of the atmosphere.

Weather station measurements provide a description of conditions near the ground. In addition to the average regional conditions, these measurements also provide local information on mesoscale phenomena such as cold fronts, sea breezes, and disturbed conditions resulting from nearby thunderstorms. Traditional thermodynamic instruments are mechanical or heat-conductive devices relying on the expansion and contraction of metallic and nonmetallic liquids or solid materials as a function of temperature, pressure, and humidity. Among these are the mercury, alcohol, and bimetallic thermometers for measurement of temperature, mercury and metallic bellow (aneroid) barometers for measurement of pressure, human hair hygrometers, and wet/dry-bulb thermometers (called psychrometers) for measurement of relative humidity. Mercury barometers are simply weighing devices that balance the mass of the atmospheric column against the mass of a mercury column. On average, a column of atmosphere weighs the same as 76 cm (29.92 in.) of mercury. Psychrometers measure humidity by means of the wet-bulb depression technique. A moist thermometer is cooled by evaporation when relative humidity is less than 100%. The temperature difference between wet and dry thermometers is referred to as the wet-bulb depression, a well-known function of relative humidity at standard airflow speeds. A related method of humidity measurement is the chilled mirror technique (dewpointer). A polished surface is cooled to the temperature of water vapor saturation, at which point the cooled surface becomes fogged. Dewpoint saturation uniquely defines humidity at a known temperature and pressure. See also Barometer; Dew point; Hygrometer; Psychrometer.

Precipitation measurement devices may be described as precision buckets, which measure the depth or weight of that which falls into them. These gages work best for rainfall, but they are also used in an electrically heated mode for weighing snow. Rulers are routinely used for measurement of snow depth. Time-resolved measurements of rainfall are traditionally made by counting quantum amounts (0.01 in. or 0.25 mm) of rain with a small, mechanically controlled tipping bucket located beneath a large collecting orifice. Modern rain measuring is sometimes performed along short paths via drop-induced scintillations of infrared radiation, which is emitted by a laser. When the raindrop size distribution is needed, optical-shadowing spectrometers are employed, as are momentum-measuring impact distrometers, devices that measure the number density versus the size distribution of raindrops or other hydrometeors. See also Snow gage.

Wind measurements are performed by anemometers, some of which use wind-driven spinning cups for wind speed determination. Vanes are used in conjunction with cups for indication of wind direction. Alternatively, three-axis propeller anemometers may be employed to provide orthogonal components of the three-dimensional wind vector. Many hybrids of these basic approaches continue to be successfully employed. Fast-response sonic anemometers employ ultrasound transmission, where the apparent propagation speed of sound is measured. The difference between this measured speed and the actual speed for a fluid at rest is the wind speed. Such measurements are made on a time scale of 0.01 s and are used to determine the fluxes of momentum, water vapor, sensible heat, and other scalars in the planetary boundary layer. See also Anemometer; Wind measurement.

Balloon-borne vertical profiles or soundings of temperature, humidity, and winds are central to computerized (numerical) weather prediction. Such observations are made simultaneously or synoptically worldwide on a daily basis. The temperature and humidity sensors are lightweight expendable versions of traditional surface station instruments. Balloon drift during ascent provides the wind measurement. The preferred method of tracking these rawinsondes is to use global navigation aid systems such as Omega, Loran-C, and the Global Positioning System. Parachute-borne dropsondes are often released from aircraft in data-sparse regions. See also Loran; Satellite (astronomy).

Remote sensing, principally via electromagnetic radiation, is a mainstay of modern meteorology. Such devices typically operate in the optical, infrared, millimeter-wave, microwave, and high-frequency radio regions of the electromagnetic spectrum. Passive radiometers typically operate at infrared and microwave frequencies; they are used for estimates of temperature, water vapor, cloud heights, cloud liquid water mass, and trace-gas concentrations. These observations are made from the ground, aircraft, and satellites, usually measuring naturally emitted radiation. Radarlike, active remote-sensing devices are among the most powerful tools available to meteorology. Collectively, these instruments are capable of measuring kinematic, microphysical, chemical, and thermodynamic properties of the troposphere at high spatial and temporal resolution. Active meteorological remote sensors are principally deployed on land, ships, and aircraft platforms, as well as aboard satellites. Unlike passive instruments, active remote sensors can precisely resolve the distance at which a measurement is located.

At optical frequencies, lidars measure conditions in relatively clear air. Capabilities include determining the properties of tenuous clouds; determining concentrations of aerosol, ozone, and water vapor; and measuring winds through the Doppler frequency-shift effect. Millimeter-wave radars are used to probe opaque, nonprecipitating clouds. Polarimetric and Doppler techniques reveal hydrometeor type, water mass, and air motions. See also Lidar.

The best-known meteorological remote sensor is the microwave weather radar. In addition to measuring rainfall and tracking movement of storms, powerful and sensitive meteorological radars can measure detailed flow fields in and around storms by using hydrometeors, insects, and blobs of water vapor as reflective targets. These radars can also distinguish between rain, hail, and snow. When Doppler measurements are combined with the atmospheric equations of motion, thermodynamic perturbation fields, such as buoyancy, are revealed inside violent convective storms. At ultrahigh and very high radio frequencies, radars known as wind profilers measure the mean wind as a function of height in the clear and cloudy air. Superior to infrequent weather balloons, radio wind profiling methods permit continuous measurement of winds with regularity and high accuracy. When radio wind profilers are colocated with acoustic transponders, the speed of sound is easily measured through radar tracking of the acoustic wave. This permits the computation of atmospheric density and temperature profiles, on which the speed of sound is strongly dependent. See also Doppler radar; Meteorology; Radar meteorology; Remote sensing.