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records 21 - 30 of 38
| image title | Automatic buoy |
| caption | Weather reports from across the oceans are provided by a large network of tethered buoys like this one on which is mounted a Marine Automatic Weather Station (MAWS). Ten MAWS are deployed on the western side of the UK continental shelf down into the Bay of Biscay and they form part of a wider world ocean network. In addition to wind speed and direction, air temperature and mean sea level pressure, MAWS report sea conditions. |
| code | MI017 |
| Section | Meteorological instruments |
| Subsection | Surface-based instruments |
| image credit | © NOAA |
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| image title | Anemometer |
| caption | An anemometer is a device for measuring wind speed. Shown here is a common form that uses three or more evenly-spaced spokes each of which ends in a simple cup shape, mounted on a freely-rotating vertical shaft. Another type (not shown) uses a wind-driven propeller on a horizontal shaft. In both cases, the speed of rotation changes roughly in proportion to the wind speed and can be precisely calibrated. The rotation speed is easily measured electronically, for example, by using the shaft to chop a beam of light passing between a light-emitting-diode (LED) and a photo-diode (light sensor). |
| code | MI012 |
| Section | Meteorological instruments |
| Subsection | Surface-based instruments |
| image credit | © The Open University |
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| image title | Aneroid barometer |
| caption | This barometer uses a sealed can shaped like a short fat cylinder. The top and bottom ends have circular ridges that enable the can to expand and contract as the pressure of the air surrounding it decreases or increases. A lever mechanism translates the small changes in the volume of the sealed enclosure into movement of a pointer against a circular dial. In this example, the dial is calibrated in hPa. The mechanism is designed to be slightly stiff, so that the pointer moves more readily when the barometer is tapped and it is easier to see whether the pressure is increasing or decreasing. See also Mercury barometer, MI009. |
| code | MI010 |
| Section | Meteorological instruments |
| Subsection | Surface-based instruments |
| image credit | courtesy Royal Meteorological Society |
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| image title | Electronic weather stations |
| caption | Electronic thermometers and hygrometers similar to those carried by radiosondes, powered by solar cells with a radio link to a base station, are used in automatic instrument stations like these. Both of them also have a wind vane to record wind direction and an anemometer to record wind speed. Left: the Radcliffe Observatory in Oxford. Right: the Abell Observatory at the Open University. |
| code | MI016 |
| Section | Meteorological instruments |
| Subsection | Surface-based instruments |
| image credit | © The Open University |
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| image title | Inside a Stevenson screen |
| caption | Inside the Radcliffe Observatory's Stevenson screen are four thermometers. The dry bulb thermometer (vertical left) measures the still air temperature. The wet bulb thermometer (vertical right) has a fabric sleeve surrounding the bulb, connected by a wick to a reservoir of distilled water. The two horizontal thermometers register the maximum (upper) and minimum (lower) temperatures. See Wet and dry bulb thermometers, MI006, maximum and minimum thermometers, MI007. |
| code | MI005 |
| Section | Meteorological instruments |
| Subsection | Surface-based instruments |
| image credit | © The Open University |
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| image title | Jar rain gauge |
| caption | Precipitation is measured as the depth of water (rain or melted hail or snow) that would result over any area from which the precipitation was unable to drain or soak away. The usual rain gauge collects precipitation using a funnel, typically 100 mm or 127 mm (= 5 inches) in diameter, that feeds the precipitation into a narrow-necked jar. This is protected from direct sunlight to avoid evaporation. The measurement of the depth of water in the jar is converted into an equivalent depth for a receptacle of the same diameter as the funnel. See also Tipping rain gauge, MI015. |
| code | MI014 |
| Section | Meteorological instruments |
| Subsection | Surface-based instruments |
| image credit | © The Open University |
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| image title | Maximum thermometer and minimum thermometer |
| caption | Inside the Radcliffe Observatory's Stevenson screen are four thermometers. The maximum thermometer (horizontal upper) is a mercury-in-glass, with a narrow constriction close to the bulb that allows the mercury to expand horizontally into the capillary but breaks the thread when the mercury in the bulb contracts, leaving mercury in the capillary indicating the maximum temperature. The minimum thermometer (horizontal lower) uses a spirit (typically clear alcohol) as the temperature sensor. When the temperature falls the spirit contracts into the bulb dragging a small light pin inside the tube that is just dipping into the spirit in the capillary. As the temperature rises the spirit squeezes past the pin leaving the end closest to the bulb marking the minimum temperature. See also Six's Maximum and minimum thermometer, MI008. |
| code | MI007 |
| Section | Meteorological instruments |
| Subsection | Surface-based instruments |
| image credit | © The Open University |
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| image title | Mercury barometer |
| caption | This barometer uses a mercury-filled tube to indicate the amount of pressure that the atmosphere exerts on a surface. The tube is sealed at its upper end and is initially set up completely filled with mercury when horizontal. It is raised to the vertical while keeping the open end submerged in mercury. The mercury sags, creating a vacuum in the sealed end. The pressure of the air is responsible for supporting the column of mercury. About 760 mm of mercury can be supported in this way, several cm or so more when the pressure is higher and several cm or so less when it is lower. Barometers of the age of the one shown here are often marked in inches rather than millimetres. This one also includes markers and labels for the mid-latitude weather associated with changing pressure. A thermometer is included for good measure. See also Aneroid barometer, MI010. |
| code | MI009 |
| Section | Meteorological instruments |
| Subsection | Surface-based instruments |
| image credit | © The Open University |
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| image title | Six's maximum and minimum thermometer |
| caption | Six's maximum and minimum thermometer registers both the maximum and the minimum temperature in a single device. The temperature is sensed by a bulb filled with clear spirit, mounted at the top of one limb of a U-shaped capillary. Round the bend of the U the capillary is filled with a second, more viscous liquid (sometimes mercury, sometimes a coloured liquid) that does not mix with the spirit. Spirit in the capillary is in contact with this second liquid and scales are arranged so that the expansion of the spirit in the bulb can be read at either end of the second liquid. Temperature increases downwards on the bulb side and upwards on the other. Two pins in the capillary are pushed upwards by the second liquid but their size is designed to prevent them being dragged downwards. See also Maximum thermometer and minimum thermometer, MI007. |
| code | MI008 |
| Section | Meteorological instruments |
| Subsection | Surface-based instruments |
| image credit | © The Open University |
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| image title | Stevenson screen 1 |
| caption | Air temperatures are measured at a standard height of 1.5 metres, using thermometers placed inside a ‘Stevenson screen’. This is a white painted box with an insulated floor and roof; louvered sides shade the inside from direct sunlight, but allow a gentle, free circulation of air. This particular screen is at the airfield of Cranfield University near to Milton Keynes. |
| code | MI003 |
| Section | Meteorological instruments |
| Subsection | Surface-based instruments |
| image credit | © The Open University |
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