The Kew Pattern Barometer is the standard type of barometer used to measure atmospheric pressure. The barometer, the single most important meteorological instrument, measures atmospheric pressure. The invention of the mercury barometer in 1643 by Evangelista Torricelli depended on his realization that air has weight. He noted that if the open end of a glass tube filled with mercury is inverted in a bowl of mercury, the atmospheric pressure on the bowl of mercury will affect the height of the column of mercury in the glass tube. The greater the air pressure, the longer is the mercury column.
The Rainfall Gauge is used to measure the amount of rainfall over a specified area. The rain gauge is composed of three parts: a funnel, a measuring tube, and a 4-inch diameter overflow tube. The funnel directs the precipitation into the measuring tube and magnifies it by a factor of 10. This allows observers to report rainfall to the nearest 0.01″ (hundredth of an inch). The measuring tube, when full, will hold “one inch” of rainfall. When it rains more than one inch, the excess water collects in the overflow tube.
The Thermometer is used to measure air temperature. A thermometer measures temperature. Liquid-in-glass thermometers are the most common types of thermometers because they are easy to read and inexpensive to construct.
These thermometers are made from glass and consist of a glass bulb that is attached to a tube that is marked with a temperature scale. The liquid in the thermometer is usually alcohol or mercury. When this liquid warms up, the molecules in the liquid move faster and the volume expand. The expansion of the liquid causes it to move up into the bore and indicate the temperature. When the liquid cools, it contracts and moves back into the bulb, indicating a low temperature.
A liquid-in-glass thermometer with a constriction is used to measure the maximum temperature. These types are thermometers are also used to measure the body temperature of humans. As the temperature increases, the liquid moves up into the bore; however, as the liquid cools, it cannot freely move downward again because of a constriction in the bore. To lower the liquid in the bore, it must be reset by applying forces to the thermometer sufficient to push the liquid to bask into the bulb.
The Hand Anemometer is a small portable instrument that gives a direct reading of wind speed. Wind is air in motion. Differences in air temperature create differences in atmospheric pressure which, in turn, initiate air motion. This movement is modified by the rotation of the earth, which causes the wind to deflect to the right in the Northern Hemisphere. Another important factor is friction. High above the earth, the flow of air is smooth and continuous. At or near the ground this uniform flow is disturbed by the irregularities of the earth’s surface. Generally, the effect of these irregularities is to make the flow lighter but more variable, with lulls and gusts.
The Campbell-Stokes Pattern Sunshine Recorder is used to measure the duration of sunshine, (that is, the number of hours of sunshine per day). This is a tropical pattern suitable for use in the latitudes 5 Degrees South to 45 Degrees North. Bowl made of Tin-Bronze surface finished black painted and hard Lacquer, with a cut-glass sphere, mounted concentrically in a section of the spherical bowl, the diameter of which is such that when the instrument is exposed to sun rays they are sharply focused on a card held in the grooves of the bowl, the focused sunray’s burn a trace on the recording card which has a time scale printed on it. The length of the burn indicates the period of bright sunshine during the day. Complete in a wooden box with the recording cards for one year.
The purpose of the Stevenson Screen is to create a uniform temperature enclosure that closely represents the same temperature as the air outside. The screen is designed to shield thermometers from precipitation and direct radiation from the sun, while allowing air to circulate freely around them. It houses maximum and minimum thermometers and a wet and dry bulb hygrometer. It is made from wood with louvered sides to allow the free and even flow of air. The front panel is hinged at the bottom to form a door, which allows for maintenance and the reading of the thermometers. The whole screen is painted white to reflect radiation.
The instrument package is usually carried into the atmosphere by a balloon. Balloon-borne radiosondes can reach altitudes as great as 90,000 ft (27,400 m) above the earth’s surface. A radiosonde may also be carried by rocket, in which case it is known as a rocketsonde, or dropped by parachute (usually from an aircraft), in which case it is known as a dropsonde. Instruments included in a radiosonde are typically transducers for humidity, temperature, and pressure measurements; controls to improve return signals and determine which measurements are to be transmitted to data stations; and a radio transmitter. Radar also has been used to measure and transmit meteorological data from a radiosonde. These radiosonde observations are made as often as two times daily at some meteorological stations around the globe.
The Ceilometers is an instrument that measures cloud-bottom heights and vertical visibilities. The Ceilometer is a self-contained, ground-based, active, remote-sensing device designed to measure the cloud-base height at up to three levels and potential backscatter signals by aerosols. Model CT25K has a maximum vertical range of 25,000. The ceilometer transmits near-infrared pulses of light, and the receiver telescope detects the light scattered back by clouds and precipitation.
The Total Sky Imager is an instrument that measures Fractional sky cover and Sun obscuration by cloud. The Total Sky Imager provides time series of hemispheric sky images during daylight hours, and retrievals of fractional sky cover for periods when the solar elevation is greater than 5 to 10 degrees. The Total Sky Imager is an automatic, full-colour sky imager system that provides real-time processing and display of daytime sky conditions. An onboard processor computes both fractional sky cover and sunshine duration, storing the results and presenting data to users via an easy-to-use web browser interface. It captures images into standard JPEG files that are analyzed to infer fractional sky cover.