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The two primary methods for averaging daily near-surface air temperature are twice-daily averaging (i.e., the sum of the maximum and minimum divided by two) and hourly averaging (i.e., using the 24 hourly values of a day). To quantify the influence of synoptic (air mass) climatology on the numerical difference between the two types of daily temperature averaging, a synoptic climatology for 215 weather stations across the Contiguous United States (CONUS) was developed for the period 1981-2010 and then correlated with those difference values. A significant correlation is found between air mass frequency and the temperature difference, especially during the non-transition seasons (i.e., summer and winter). This pattern is due to the influence of air mass type on the shape of the daily temperature curve. The correlation results also show spatial variability, as air masses that are more frequent in the northern CONUS (e.g., Dry Polar) influence the diurnal temperature curve differently than air masses more common in the southern CONUS (e.g., Moist Tropical). Future work can capitalize on these results by investigating different geographic regions and utilizing this correlation-based approach to analyze how anticipated future changes in the synoptic climatology of air masses influence the diurnal temperature curve.