Meteorological Thermometer

Thermometer, Meteorological

 

any of a group of specially designed liquid-filled thermometers for meteorological measurements, primarily at weather stations. Various meteorological thermometers differ in size, structure, measurement range, and scale divisions, depending on their function.

Mercury-filled meteorological thermometers in weather-station and aspiration psychrometers are used to determine the temperature and humidity of the air. They have scale divisions of 0.2°C; the measurement limits are –35° to 40°C (or –25° to 50°C). At temperatures below –35°C—near the freezing point of mercury—the readings of a mercury-filled thermometer become unreliable; therefore, low-temperature alcohol thermometers are used. These thermometers are analogous in design to psychrometric thermometers. They have scale divisions of 0.5°C and various measurement limits—the lower limit may be –75°, –65°, or –60°C, and the upper limit may be 20° or 25°C.

Mercury-filled maximum-registration meteorological thermometers are used to measure the maximum temperature over a given time period. Their scale divisions are 0.5°C, and their measurement limits are –35° to 50°C or –20° to 70°C. Their working position is almost horizontal, with the bulb slightly depressed. The readings of maximum temperature are retained because of the presence of a pin in the bulb and the evacuation of the bore above the mercury column. As the temperature rises, additional mercury is forced out of the bulb into the bore through the narrow ring-shaped opening between the pin and the walls of the bore. The mercury column remains in the bore even upon a reduction in temperature, since the bore is under a vacuum. Thus, the position of the end of the mercury column relative to the scale corresponds to the value of the maximum temperature. The thermometer is shaken to bring its readings into correspondence with the temperature at a given time.

An alcohol-filled minimum-registration meteorological thermometer is used to measure the minimum temperature over a given time interval. Thermometers of this type have scale divisions of 0.5°C and lower limits from –75° to –41°C and upper limits from 21° to 41°C; their working position is horizontal. Minimum temperature readings are retained by a barbell-shaped indicator pin within the alcohol in the bore. The bulges of the pin are smaller than the inner diameter of the bore. Thus, upon an increase in temperature, the alcohol entering the bore from the bulb flows around the pin without moving it. As the temperature decreases, the pin comes into contact with the meniscus of the alcohol column and moves with the column toward the bulb, since the forces of surface tension of the alcohol film are greater than those of friction. The pin remains at the position closest to the bulb. The position of the end of the pin closest to the alcohol meniscus indicates the minimum temperature, and the meniscus indicates the temperature at any given moment. To bring a minimum thermometer into working position, the bulb is raised and held until the pin drops to the level of the alcohol meniscus.

Mercury-filled meteorological thermometers are used to determine the temperature of the soil surface. The scale divisions of such thermometers are 0.5°C, with lower limits from –35° to –10°C and upper limits from 60° to 85°C. Measurements of soil temperature at depths of 5, 10, 15, and 20 cm are made with Savinov mercury-filled angle thermometers. Their scale divisions are 0.5°C, with measurement limits of –10° to 50°C. Savinov thermometers are bent at an angle of 135° near the bulb, and the bore is thermally insulated from the bulb to the beginning of the scale, which decreases the effect of soil layers above the bulb on the reading. Measurements of soil temperature at depths up to several meters are carried out using mercury-filled subsoil thermometers placed in special devices. Their scale divisions are 0.2°C, with various limits; the lower limit may be –20° or –10°C, and the upper limit may be 30° or 40°C. Less commonly used are mercury-thallium psychrometric meteorological thermometers, with limits of –50°to35°C.

Resistance, thermoelectric, transistor, bimetal, and optical thermometers are also used in meteorology. Resistance thermometers with copper or platinum resistors are commonly used in remote and automatic weather stations and radiosondes. Thermoelectric thermometers are used for measuring temperature gradients, and transistor thermometers (thermotransistors) are used in agrometeorology for measuring the temperature of the topsoil layer. Bimetal thermometers are used in thermographs for recording temperature. Optical thermometers are used in terrestrial, airborne, and satellite installations for measuring the temperature of cloud formations and various areas on the earth’s surface.

REFERENCE

Sternzat, M. S. Meteorologicheskie pribory i nabliudeniia. Leningrad, 1968.

M. S. STERNZAT

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