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" -14- AIR TEMPERATURE Effects of location and installation of measuring equipment Many hydro-meteorological studies involve free-air temperature, which is difficult to measure accurately owing to the radiative heat transfers oonsidered in the preceding ohapter. 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 expo- sures, although commonly small in comparison to those for some other meteorological 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 on a steep slope, on the roof of a building, or, in a small depression. At official climatological stations, the standard height of temperature instruments is approximately, feet above the land surface. Upper-air temperatures are measured by means of radiosonde instru- ments. These measurements by radiosondes, when adjusted for lapse rate, should represent the temperature of free air as it moves over the higher mountains. TYPes of temperature records and equipment At virtually all temperature stations, daily maxima and minima 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 ,: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, maximum plus minimum 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 like- wise. "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. At some stations, a continuous trace of temperature is obtained from a thermograph. Most such instruments are cloclt..driven and will operate up to eight days without servicing. They are not as accurate as the official mercury-in-glass or alcohol-in-glass thermometers, but records from them are adjusted to readings from those official thermometers.