As an example here is my data for 2014. Generally the mean calculated by taking the average of the maximum and minimum gives a higher figure than the mean of all data points during the day. That is not always the case - especially in winter -e.g. if you have a sudden clearance of cloud just before dawn which pushes the minimum right down. Then the simple max / min average will be lower.
Often the differences are actually very small. Sometimes though the differences can be substantial as evidenced by March and September in 2014.
The size of the difference does not appear to be impacted by the absolute diurnal variation in temperature. It will be down generally to more subtle factors in most cases which are hard to pin down to specifics.
Thanks for the detailed response GW, funny that the two largest differences last year were in the equinox months.
My own short experiment, working with hourly data from 9th to 14th August 2015, has returned mean values as follows:
Back Yard: 16.63 from Max/Min, 16.38 from Hourly
Heathrow: 18.95 from Max/Min, 18.55 from Hourly
That gives a difference between the hourly and max/min means of -0.28*C for my location - which happens to be identical to your value for the whole of August 2014 - while for Heathrow the difference is a much larger -0.47*C, but this was strongly affected by yesterday, for which the max/min mean was a remarkable 1.64*C higher than the hourly data mean. Take that day out, and the difference for 9th-13th is -0.24*C which is actually lower than that for my location.
A good example as to why it's important to have large datasets to minimise the impact of unusual events. With data by the minute covering 10 years, I daresay you have that covered GW!
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