Errors in daily minimum and maximum air temperature induced by observation time

Cooperative weather observers record daily minimum and maximum temperature once daily, generally either near-sunrise (0700 or 0800 LST) or near-sunset (1700 or 1800 LST). Differing times of daily observation, both across networks and over time, may introduced inhomogeneities and discontinuities in air temperature records. To account for these artificial changes, minimum and maximum air temperature are standardized to the calendar day (midnight observations). Standardization has been routinely performed on monthly minimum and maximum air temperature but not for daily values. Standardization requires knowledge of observation times and errors or offsets associated with observation times. Observation times are available in station metadata or through objective procedures (DeGaetano 1999). Offsets are calculated by simulating once-daily readings of hourly air temperature records. Errors associated with non-midnight observations of daily minimum and maximum air temperature at Cooperative weather stations are estimated from hourly air temperature recorded at nearby First Order weather stations. Daily minimum and maximum temperature are computed over each 24-hour period from 0100 LST to 2400 LST. With 2400 LST used as the standard observation time, estimates of the offset for any other observation time can be obtained.

The annual cycle of daily offsets for near-sunrise and near-sunset observation times follow the annual cycle of monthly departures for these observation times. Near-sunset observations, on average, overestimate daily minimum and maximum temperature (RMSE ≤2°C), while near-sunrise observations generally underestimate daily minimum and maximum temperature in the winter (3°C≤RMSE≤5°C) and overestimate in the summer (1°C≤RMSE≤3°C). When individual days are in question, however, unsystematic offsets exceeding |5°C| can be observed for near-sunrise observations in all seasons. With the exception of near-sunrise observations of maximum temperature, daily offsets of near-sunrise and near-sunset observations are frequently (>85% of days) small (i.e., <|0.2°C|). Although monthly average minimum and maximum air temperature are routinely adjusted for observation time errors, this paper cautions against applying systematic corrections to daily minimum and maximum air temperature.