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00140lt IlLl III. AIR TEMPERATURE Effects of location and installation of measuring equipment . Many hydro-meteorological studies involve free-air temperature, but this is difficult to measure accurately owing to radiative heat transfers that will be considered. At best, the ordinary records of temperature serve only as an index to the true free-air temperature. The dependability of a temperature measurement as an index for the surrounding area is determined in large part by the exposure of measuring equipment. Variations in temperature as measured under different exposures, although cODDl1ollly small in comparison to those for some other meteorologipal factors, may be highly significant. The equipment must be protected against direct solar radiation. Ideally, it should be sited in a flat open area that assures adequate ventilation of the instrument shelter by free movement of the air. It should not be exposed ona steep slope, on the roof of a building, or in a sm.ll depression. At official climatological stations, the standliI'd height of temperature instruments is approximately 5 feet above the land surface. During stormy periods, air temperature measured at land surface in all open valley commonly differs radically from that of the main air milss aloft. In cOllBequence, these temperatures at land surface may be misleading in regard to the altitude at which rainfall changes to snowfall. Usually, estimates Qf such altitude can be made more satisfactorily from region-wide average te~erature than from the temperatures at individual stations. Upper-air temperatures are measured. by means of radiosonde instruments. These measurements by radiosondes should represent the temperature of free air as it Jiloves over the higher mountains; they may not represent the temperatures that would be measured. near the land surface. . ~es of temperature records and equipment At virtually all temperature stations, daily maxima and lIUm.ma are recorded, using mercury-in-glass thermometers to measure the maxima and alcohol-in-glass thermometers to measure the minima. At most stations such records are for the 24-hour period ending between 5:00 and 8:00 p.m., local standard time. At a few stations, however, the maxima and minima . are observed by calendar days; at others, by the 24-hour period ending at 8:00 a.m. In procedures of the U.S. Weather Bureau, "mean daily temperature" is computed by" averaging the daily extremes--that is, maximuln..plus miIlinnlm divided by" two. "Mean (average) daily maximum temperature" is derived by adding the daily maxima for a month and dividing by the number of days in the month; "mean daily minimum temperature" is derived likewise. "Mean temperature" for a month is taken to be the average of the mean monthly maximum and mean monthly minimum temperatures. ''Mean annual temperature" is taken to be the average of the monthly means for the particular year.