What’s behind an average high?

Dear Tom,
When you report the average high temperature on a given day, are you reporting the actual average of all highs on that date?
— PJ Perry
Dear PJ,
No. The high temperature on a given day is the highest temperature recorded in the 24-hour period, midnight to midnight, standard time, rounded to the nearest degree. The average high temperature on a given day is the average of all highs recorded on that day in the 30-year period, 1981-2010, smoothed so that the value is either the same from one day to the next or changes by only 1 degree (upward from its winter minimum to its summer peak or downward from its summer peak to its winter minimum). Similarly for low temperatures.
The averages at a location are recalculated every 10 years; the next calculation will be for the years 1991-2020.