When is an average or not an average?

First, I have not followed up on the reference in this Instapundit blog post, so I do not know whether this is really the way the British Met Office really computes average temperatures:

In fact, the Met still asserts we are in the midst of an unusually warm winter — as one of its staffers sniffily protested in an internet posting to a newspaper last week: “This will be the warmest winter in living memory, the data has already been recorded. For your information, we take the highest 15 readings between November and March and then produce an average. As November was a very seasonally warm month, then all the data will come from those readings.”

But suppose for a minute that an average for a whole season (year) really is fifteen days, and that the highest fifteen all (most) came from November. Would that really represent the average for the winter? Can you think about why they would not use every day's reading from November through March? Should they use just the high for the day, or the high and the low for each day? And where should the reading come from? How many locations around England would be enough for what would seem like a good average calculation? What if averages in the past were computed differently? Could you make comparisons?

Average seems like such an easy concept - but it often is quite tricky. To say nothing about trying to understand the variability of temperatures (do they compute standard deviations?)

For example, the fifteen day record could occur in a season where the other one hundred and thirty five days were pretty cold...