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CHAPTER 13. TEMPERATURE AND DEW POINT This chapter describes procedures for observing and reporting temperature and dew point in a METAR or SPECI observation. The temperature data obtained using the procedures and practices in this chapter are normally in terms of the Celsius scale. However, temperature may be given in both degrees Fahrenheit and Celsius since some instruments may be marked in only one scale. Dew point shall be calculated with respect to water at all temperatures. Practices and procedures for the operation of temperature and humidity instruments and related equipment are presented in Chapter 17, Operation of Equipment.
A hygrometer is an instrument which measures the water-vapor content of the atmosphere. A hygrothermograph is a recording instrument that combines the thermograph with a humidity sensor for providing a continuous chart record of both temperature and humidity data. A hygrothermometer is an instrument system usually with readouts inside the weather office or observer's building for obtaining ambient temperature and dew point from remote sensors. An instrument shelter is a box-like structure designed to protect temperature measuring instruments from exposure to direct sunshine, precipitation, and condensation, while at the same time providing adequate ventilation. A maximum thermometer is a mercury-in-glass thermometer made with a constriction in the bore between the bulb and the graduated portion of the stem. Rising temperature forces a portion of the mercury into the graduated section. This mercury is retained and affords a reading, until reset, of the highest temperature reached. A minimum thermometer is an alcohol-in-glass thermometer with a freely moving, small, dark-colored glass index placed in the bore. As the temperature falls, the retreating upper end (meniscus) of the alcohol column moves the index toward the bulb. When the temperature rises, the index remains at the lowest point until reset. A psychrometer is an instrument used for measuring the water-vapor content of the air. It consists of two ordinary glass thermometers. The bulb of one thermometer (left thermometer) is covered with a clean muslin wick which is saturated with water prior to an observation (the wet-bulb). When the bulbs are properly ventilated, they indicate the wet- and dry-bulb temperatures of the atmosphere. 13-10. PSYCHROMETRIC CALCULATOR A psychrometric calculator is a circular slide rule used to compute dew point and relative humidity from known values of dry- and wet-bulb temperature and the normal station atmospheric pressure. Instructions for the use of this calculator are printed on it. Psychrometric tables are tables prepared from a psychrometric formula and used to obtain dew point and relative humidity from known values of dry- and wet-bulb temperature. Relative humidity is the ratio, expressed as a percentage, of the actual vapor pressure of the air to the saturation vapor pressure. A sling psychrometer is a device for determining psychrometric data consisting of two matched thermometers mounted on a common back. One thermometer (left thermometer) is covered with a muslin wick which is saturated with water prior to an observation (the wet-bulb). Ventilation is achieved by whirling the thermometers with a handle and a swivel link until the maximum wet-bulb depression has been obtained. A townsend support is a metal instrument support for the shelter-mounting of maximum and minimum thermometers. This device is so designed as to facilitate resetting the thermometers and holding them in fixed positions between observations of maximum and minimum temperature extremes. A thermograph is a self-recording thermometer which provides a continuous record of temperature on a chart mounted upon a clock-driven cylindrical drum. Wet-bulb depression is the difference between the dry- and wet-bulb temperatures. Examples:
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