Historically, daily maximum and minimum temperatures were recorded by human observers using true maximum (MaxTr) and true minimum (MinTr) thermometers.  Widespread use of automated weather stations has improved our ability to monitor weather conditions at higher frequency than daily.  However, MaxTr and MinTr are not always recorded at these stations.  In some cases, the highest and the lowest hourly average air temperature values (Max1h and Min1h, respectively) are used instead as the daily high and low rather than MaxTr and MinTr.  Max1h will be lower than MaxTr and Min1h will be higher than MinTr.  In addition, the daily temperature range recorded using true maximum and minimum thermometers (RgTr) will be larger than that recorded using highest and lowest average 1-hour temperatures (Rg1h).  Methods for estimating reference evapotranspiration  (ET₀) have been developed and tested using mainly  MaxTr and MinTr.    The use of Max1h and Min1h as input to these methods will impact the ET₀ values calculated.  A multi-location field study in western Canada that included 6 site-years at four different locations in 2 different years found that Max1h underestimated MaxTr by an average 0.75^oC and Min1h overestimated MinTr by an average 0.53^oC.  The use of Max1h and Min1h, compared to MaxTr and MinTr, resulted in a reduction of 2% to 7% in estimated ET₀ using five different methods ranging from empirical equations to a coupled soil-plant-atmosphere model.  A simple linear correction to Max1h and Min1h reduced the difference in ET₀ values to <1% compared to those obtained using MaxTr and MinTr for all five methods.