| Word | Description |
| Barographs | There are four types of Barographs. These are: the aneroid barograph (which includes the microbarograph); the float barograph; the photographic barograph; and the weight barograph. The aneroid barograph is the one that is most commonly used in weather stations. |
| Barometer | A barometer measures air pressure. The two most common types are mercury and aneroid. The mercury barometer is made of a corrugated hollow disk which has been is partially evacuated of gas and is prevented from collapsing by a spring fitted internally or externally. The tension of the spring will be nearly proportional to the difference between the internal and external pressures; a deflection is obtained both by the single or multiple capsules in series and mechanical linkages. The aneroid barometer is temperature compensated at a given pressure level by adjustment of the residual gas in the aneroid or by a bimetallic link arrangement. To record a representation of changes of pressure over time a barograph is used. Please see under 'barographs' in the technical glossary for more information. |
| Data loggers | To be added |
| Hygrometer | A hygrometer is an instrument that measures the water vapour content of the atmosphere. There are as many as six different ways of measuring this. The first is by using a psychrometer, which uses a thermodynamic method; the second set of instruments depends on a change of physical dimensions due to the absorption of moisture and uses either a hair hygrometer, a torsion hygrometer or a goldbeater's-skin hygrometer. The third group are those that depend upon condensation of moisture, such as a dewpoint hygrometer or a frost point hygrometer. Fourth is a class of instruments that depend upon the change of chemical or electrical properties due to the absorption of moisture such as an absorption hygrometer, an electrical hygrometer or a carbon-film hygrometer; fifth is a class of instruments that depends upon the diffusion of water vapor through a porous membrane, called diffusion hygrometers and finally sixth, a class of instruments that depend upon measurements of the absorption spectra of water vapor called spectral hygrometers. |
| Instrument shelter | An instrument shelter or thermometer shelter is usually a rectangular box like structure that has been designed to offer as much protection as possible to a range of weather related instruments from exposure to direct sunshine and precipitation, while providing enough ventilation, so as to offer the most accurate reading possible. Such instrument shelters are usually painted in a gloss white and have louvered sides with a 'double' roof. They are mounted on a four post stand to offer rigidity in the wind at about 1-2 metres above the ground with the door side facing to the north, so as not to catch direct sunlight. They may contain a variety of instruments but usually psychrometers, maximum and minimum thermometers and hygrothermographs. The traditional type of screen accepted for use by the UK Meteorological Office at its official accredited stations is called a 'Stevenson screen'. It was designed by Thomas Stevenson (1818-1887), a British civil engineer and father of the author Robert Louis Stevenson. |
| Lightning Detector | To be added |
| NAVTEX | NAVTEX (NAVigational TelEX) is a system for the broadcast and automatic reception of maritime safety information using a narrow-band direct-printing telegraphy. NAVTEX provides shipping with navigational and meteorological warnings and urgent information through automatic printouts from a dedicated receiver. NAVTEX is a component of the IMO/IHO Worldwide Navigational Warning Service (WWNWS) defined by IMO Assembly resolution A.706(17). It has also been included as an element of the Global Maritime Distress and Safety System (GMDSS). Since 1 August 1993, NAVTEX receiving capability has become part of the mandatory equipment which is required to be carried in certain vessels under the provisions of the International Convention for the Safety of Life at Sea (SOLAS). NAVTEX messages are transmitted worldwide from local stations. The number of stations grows month by month. The NAVTEX database provides details including name, position, range, and operational status of all known NAVTEX stations. It broadcasts weather and navigation warnings, in English on 518 kHz every 4 hours from transmitters all around the world and increasingly, local language and small craft information is also transmitted on 490 kHz. Although NAVTEX is essentially a marine product it is just at home on land near the coast where it will receive its signal from the nearest stations. It can easily be adapted for shore use by using a 12V mains adapter. |
| Precipitation sensors | Precipitation sensors detect real time present weather and report back the data using advanced optical technology using the principle of optical scintillation to measure precipitation. Present weather includes all forms of liquid, freezing and frozen precipitation such as rain, drizzle, snow, snow pellets, snow grains, ice pellets and hail, and can also include suspended particles that obstruct vision, eg) mist, fog, haze, dust and smoke. The idea is to approximate the human senses of sight and touch as closely as possible in order to generate accurate present weather values. Frozen and liquid precipitation is distinguished according to the ratio of the optical measurement value versus the amount of water measured on the capacitive precipitation sensor. |
| RADAR | Radar is an acronym for RAdio Detection And Ranging and is an instrument used for the detection and ranging of distant objects that are able to scatter or reflect radio energy. A radar consists of a transmitter, receiver, antenna and display, as well as equipment for control and signal processing. The most common type of radar systems are monostatic radars; these use the same antenna for both transmission and reception and depend on backscattering to produce a detectable echo from a target. Another less common type are bistatic radars which have the transmitter and antenna at one location and the receiver and antenna at another remote location. These radars depend upon forward scattering to produce a detectable signal. An object that lies in the path of the beam reflects, scatters, and absorbs the energy and a small portion of the reflected and scattered energy (called the target signal), travels back along the same path through the atmosphere and is intercepted by the receiving antenna. The time delay between the transmitted signal and the target signal is used to determine the distance or slant range of the target from the radar. The direction in which the focused beam is pointing at the instant the target signal is received (that is the azimuth and elevation angles of the antenna) determine the direction and height of the target. The information is presented visually as radar echoes on different types of radar displays. As individual rain particles scatter radio energy, weather radars, operating in certain radar frequency bands, can detect the presence of precipitation and other weather phenomena at distances up to several hundred kilometers from the radar, depending upon meteorological conditions and the type of radar. The heavier the precipitation the stronger the signal although some types of weather phonomena such as hail, can offer false signals, suggesting the precipitation is heavier than it actually is. |
| Radio sonde | A radiosonde is an expendable meteorological instrument package, usually carried aloft by a weather balloon, that measures various parameters such as temperature, humidity and pressure through from the surface right up to the stratosphere, then transmits the data via a radio signal to a ground receiving system. The temperature sensors measure temperature induced changes in the electrical resistance, capacitance, or voltage of a material whilst the pressure sensors are usually aneroid cells, which flex in proportion to the observed pressure changes. Some radiosondes also measure wind speed and direction. There is a network of official radiosonde stations based around the globe, making at least two observations a day and sometimes as many as half a dozen. These relay there data back to the main data networks around the globe and provide an important source of information about upper air conditions, that is then used to make weather forecasts. |
| Rain gauge | A rain gauge is an instrument designed to measure the amount of rain and includes recording, non recording, and rain-intensity gauges. |
| Stevensons screen | See: instrument shelter |
| Sunshine recorder | Radiation from the sun reaches us in two forms: direct radiation and that which is refracted from clouds or other obstructions. The distinction between sunshine and lack of sunshine is a relative arbitrary one however, largely dependent upon the type of sunshine recorder in use or upon the quality of human beings subjective estimates. Instruments can record both the duration of sunshine without regard to its intensity at a given location and with regard to its intensity. The former group of sunshine recorders can be classified into two groups; the first in which the timescale is obtained from the motion of the sun in the manner of a sundial (and these include the Campbell–Stokes recorder and Jordan & Pers sunshine recorder). The second class is where the timescale is supplied by a chronograph (eg the Marvin sunshine recorder). |
| Thermometer | There are a number of different types of thermometer. A glass thermometer contains a thermally reactive liquid in a graduated glass tube. It works by the fact that the glass and liquid have different expansion properties, and as the liquid expands at a greater rate it is forced through a small bore tube with graduated markings thus indicating a temperature rise. The most common types of glass thermometer are those containing mercury and alcohol. A maximum/ minimum thermometer works in the same way as a standard glass thermometer but is also capable of recording maximum and minimum temperature. This is done by including magnetic strips above the liquid, within a u-shaped glass tube. When the liquid expands it is forced through the bore of the tube and is pushed into a new position along the graduation either on one side to give a maximum temperature or the other (usually left side) to give a minimum temperature. After the max/min temperatures have been observed or recorded the max/min indicators can then be reset by moving a metal strip attached to a reset button away from the magnets which then fall back to the current level of the liquid indicating the present temperature. |
| Tide clocks | A tide clock is designed so as to keep a track of the Moon's apparent motion around the Earth. Along many coastlines the Moon contributes the major part (about 67 percent) of the combined lunar and solar tides but -importantly- not all of it. Tidal clocks keep the average rate between high lunar tides, which occur 12 hours 24 minutes apart. The bottom of the tide clock dial is marked 'low' to indicate the expected low tide and the top of the tide clock dial is marked 'high' to indicate expected high tide. On the left side of the dial marked 'rising' there is a count down of hours from 5 to 1. The right hand side of the clock is marked 'falling' and has a count of hours from 1 up to to 5. There is just one hand on the clock face with the number pointed to by the hand giving the time to or since the last (lunar) high tide. Generally tides tend to have an inherent lead or lag that is different in every location, so although tidal clocks are set for the time when the local lunar high tide occurs, this is often complicated because the lead or lag varies during the course of the lunar month, as the lunar and solar tides fall into and out of sync. The lunar tide and solar tide ebb and flow at the same time near the full moon and the new moon whilst being more unsynchronized near the first and last quarters of the moon so this is when the tide clock will be least accurate. So as ordinary tidal clocks only track a part of the tidal effect, and because the relative size of the combined effects is different in different places, they are in general only partially accurate. Navigators should use the lists of more accurate tide tables, which are available either in a booklet or on a computer. |
| UV (Ultra Violet) sensors | To be added |
| Visibility sensing | Visibility is the greatest distance in any given direction which it is possible to see and identify features and can be carried out by the unaided eye or using electronic sensing equipment. We usually use a measure of horizontal visibility. If carried by an observer using the naked eye during daylight hours we use a known measured and prominent object set against the sky and by night, a known light source. Nevertheless, observations of visibility are rather subjective estimates of the actual visual range at different times especially during periods of low light around dawn and dusk. By day we are essentially comparing contrast, while at night we are looking at the attenuation of flux density. In maritime locations fog horns or more recently buzzers give an indication of low visibility around coasts (usually in fog when visibility horizontally is less than 1000m). A visibility less than 200m is classed as being 'dense fog' and is potentially quite hazardous. Sensors within automatic weather stations can also detect visibilities, though in some situations the accuracy can be rather variable. We use them to detect poor visibilities eg fog and haze on our road networks, airports, harbour installations and observing systems at sea. They are normally calibrated with reference to accurate transmissometers, with sensors using a forward-scatter measurement principle to measure the meteorological optical range (MOR). On airfields, where runway visibility is especially important, estimates of the 'runway visual range' (RVR) are made frequently by automatic equipment and reported, in standard metar code, to aviators. |
| Weather satellite | A weather satellite is an artificial platform that has been placed into orbit around the earth, carrying instruments that gather weather and environmental data. The first weather satellite was Tiros 1, designed to test experimental techniques for taking television footage of weather patterns from orbit, and it was launched on April 1, 1960 from Cape Canaveral, Florida, USA. Primarily the data initially showed cloud cover at both visible and infra red wavelengths and this is still the most widely used function of weather satellite data. However we have come to rely on a vast range of other parameters available from satellite data to show us the extent of wild fires, the effects of pollution, sand and dust storms, snow cover, to help with ice mapping, portray the boundaries of ocean currents as well as monitoring volcanic ash cloud. As well as showing us the development and movement of weather systems across the globe, larger features such as El Niño and la Nina and their effects on the weather can be monitored from satellite images as well as the extent of the Antarctic ozone hole. |
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