During the recent cold spell, I noticed many references to the “average temperature.” How is that number calculated?

Dear Tom,
During the recent cold spell, I noticed many references to the “average temperature.” How is that number calculated? Is it the average of each of the day’s 24-hour temperatures?
— Richard Paul Vernon Hills
Dear Richard,
It is not based on the average of the hourly temperatures. By international convention, a meteorological day is defined as the 24-hour period midnight-to-midnight, local standard time. To calculate the day’s average temperature, add the day’s maximum and minimum temperatures and divide by two, then round to the nearest degree, if necessary. If the temperature for a day was continuously 60 degrees from midnight to 11 p.m., then quickly fell to 40 degrees for the last hour of the day, the day’s average temperature would still be 50 degrees, the arithmetic average of the day’s high and low.