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-15- Ou1249 Hourly air temperatures are taken at approximately 300 first-order Weather Bureau stations in the United states, also at some Federal Aviation Agency stations. Upper-air temperatures are measured at selected Weather Bureau stations, by radiosonde flights every 12 hours up to an altitude of about 100,000 feet. Dependability of observations Effects of exposure Temperatures measured at stations on steep south- or west-facing slopes are likely to average too high, owing to the angle of the sun's rays in daytime and to drainage of cold air away from the slope at night. Thermometers exposed on a roof top usually indicate higher minimum temper- atures than those at the standard level, owing to the usual night-time ,temperature "inversion" near the land surface. Temperature observations taken in a hollow usually will show average minima much lower than in the surrounding area, owing to night-time drainage of cold air. Although the effect is small, average temperatures in urban areas tend to be higher than in adjacent rural areas. Generally, the average temperature at a city station has been found to increase as the city expands. These effects should be considered if the records are to be used for disclosing climatic trends or for detailed correlations. Accumulation of snow around a temperature station may bring the point of obserVation too close to the effective land surface and so within a non-representative layer of air. In such instances, rough adjustments may be made according to a general- ized vertical temperature profile. Hour of observations Since June 1948, maximum and minimum daily temperatures have been recorded and published as of the day on which the observations were made. Thus, at a station where the daily observations are taken at 8:00 a.m., the maximum temperature as recorded nearly always occurred on the previous day. The hour of observation at each station is published in the station index of the monthly climatological bulletins. Monthly means are affected by different hours of observation. Thus, the nearer the hour of observa- tion is to the hour at which maximum temperature occurs ordinarily the higher the mean will be, especially if the observations are made earlier than 5:00 p.m. Observations late in the evening are the most consistent. other sources of error Possible errors in temperature measurements include those due to reading a maximum thermometer in the horizontal position; in a minimum thermometer, the indicator displaced by vibration; separation of the alcohol or mercury column in the thermometer; and, "retreater" maximum thermometers. In such instances of instrumental error, also in errors of technique, commonly there is no rational basis for adjusting the published records of temperature.