Air temperature

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The temperature of the atmosphere represents the average kinetic energy of the molecular motion in a small region, defined in terms of a standard or calibrated thermometer in thermal equilibrium with the air. Many different types of thermometer are used for the measurement of air temperature, the most common depending on the expansion of mercury with temperature, the variation of electrical resistance with temperature, or the thermoelectric effect (thermocouple).

The temperature of a given small mass of air varies with time because of heat added or subtracted from it, and also because of work done during changes of volume.

The rate at which the temperature changes at a particular point, that is, as measured by a fixed thermometer, depends on the movement of air as well as physical processes such as absorption and emission of radiation, heat conduction, and changes of phase of water involving latent heat of condensation and freezing. The large changes of air temperature from day to day are mainly due to the horizontal movement of air, bringing relatively cold or warm air masses to a particular point, as the large-scale pressure-wind systems move across the weather map. See also Air mass; Air pressure.

Temperatures near the surface are read at one or more fixed times daily, and the day's extremes are obtained from special maximum and minimum thermometers, or from the trace (thermogram) of a continuously recording instrument (thermograph). The average of these two extremes, technically the midrange, is considered in the United States to be the day's average temperature. The true daily mean, obtained from a thermogram, is closely approximated by the mean of 24 hourly readings, but may differ from the midrange by 1 or 2°F (0.6 or 1°C), on the average. In many countries temperatures are read daily at three or four fixed times, so that their weighted mean closely approximates the true daily mean.

Averages of daily maximum and minimum temperature for a single month for many years give mean daily maximum and minimum temperatures for that month. The average of these values is the mean monthly temperature, while their difference is the mean daily range for that month. Monthly means, averaged through the year, give the mean annual temperature; the mean annual range is the difference between the hottest and coldest mean monthly values. The hottest and coldest temperatures in a month are the monthly extremes; their averages over a period of years give the mean monthly maximum and minimum (used extensively in Canada), while the absolute extremes for the month (or year) are the hottest and coldest temperatures ever observed. The interdiurnal range or variability for a month is the average of the successive differences, regardless of sign, in daily temperatures.

Over the oceans the mean daily, interdiurnal, and annual ranges are slight, because water absorbs the insolation and distributes the heat through a thick layer. In tropical regions the interdiurnal and annual ranges over the land are small also, because the annual variation in insolation is relatively small. The daily range also is small in humid tropical regions, but may be large (up to 40°F or 22°C) in deserts. Interdiurnal and annual ranges increase generally with latitude, and also with distance from the ocean; the mean annual range defines continentality. The daily range depends on aridity, altitude, and noon Sun elevation. See also Atmosphere; Insolation; Radiation.

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The various types of temperatures include:
i. Indicated air temperature (IAT). The uncorrected reading from the free air temperature gauge.
ii. Basic air temperature (BAT). The indicated air temperature corrected for instrument error.
iii. True air temperature (TAT). The basic air temperature corrected for the temperature rise resulting from the heat of compression and skin friction. Also known as outside air temperature (OAT).
iv. Corrected mean temperature. The temperature of the static air; this is the same as ambient temperature and true air temperature.
v. Ram air temperature (RAT). The static air temperature plus temperature rise because of compressibility. It is usually 80% of true air temperature.
vi. Total air temperature (TAT). The temperature actually sensed by the temperature probe (i.e., BAT or basic air temperature). It is ambient temperature plus rise resulting from compressibility plus rise from skin friction.

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