Daily mean temperature estimate at the US SURFRAD stations as an average of the maximum and minimum temperatures

At thousands of stations worldwide, the mean daily surface air temperature is estimated as a mean of the daily maximum (T-max) and minimum (T-min) temperatures. We use the NOAA Surface Radiation Budget Network (SURFRAD) of seven US stations with surface air temperature recorded each minute to assess the accuracy of the mean daily temperature estimate as an average of the daily maximum and minimum temperatures and to investigate how the accuracy of the estimate increases with an increasing number of daily temperature observations. We find the average difference between the estimate based on an average of the maximum and minimum temperatures and the average of 1440 1-min daily observations to be -0.05 +/- 1.56 degrees C, based on analyses of a sample of 238days of temperature observations. Considering determination of the daily mean temperature based on 3, 4, 6, 12, or 24 daily temperature observations, we find that 2, 4, or 6 daily observations do not reduce significantly the uncertainty of the daily mean temperature. The bias reduction in a statistically significant manner (95% confidence level) occurs only with 12 or 24 daily observations. The daily mean temperature determination based on 24 hourly observations reduces the sample daily temperature uncertainty to -0.01 +/- 0.20 degrees C. Estimating the parameters of population of all SURFRAD observations, the 95% confidence intervals based on 24 hourly measurements is from -0.025 to 0.004 degrees C, compared to a confidence interval from -0.15 to 0.05 degrees C based on the mean of T-max and T-min.