Searching for the Daily Mean Temperature

The true daily mean is the sum of the twenty-four hourly temperature observations divided by twenty-four. However, most of the individual observers in the nineteenth and twentieth century climate record could not make observations every hour of every day. Climate networks prescribed convenient times that approximated the true daily mean to an acceptable accuracy. The Signal Service and the Weather Bureau adopted the homonymous observation times of 8 a.m. and 8 p.m. and divided the sum of the paired values by two to produce the daily mean. The Surgeon General and the Smithsonian Institution used the sum of the observations at 7 a.m., 2 p.m., and the doubled value at 9 p.m., and divided the sum by four. Self-registering thermometers required only one observation each day to record the daily maximum and minimum temperatures. In 1926, the Weather Bureau adopted the mean derived from dividing their sum by two. The National Weather Service continues that choice. Automated observations allow the true daily mean to be used instead of other surrogate means. For this paper, one year of hourly data from a station in the Kentucky Mesonet were used to produce the true daily mean for each day and replicate the calculation of twenty-two different surrogate methods used during the past two centuries. The surrogate means were compared to the true daily mean. The results of those comparisons and the effects on monthly and annual means will be presented in tables and graphs.