If you have 1,ooo stations and a 100 year record – you would expect 10 stations per year to have their hottest month ever. (There are other considerations for the time of year when record heat can occur, but they cancel out of the equation and are not shown below.)
1,000 / 100 = 10
It looks like the WaPo is playing fast and loose with the Heat Index measurement:
http://www.washingtonpost.com/local/july-heat-in-washington-was-unprecedented/2011/07/29/gIQA0uTzlI_print.html
The Chicken Little claims are what is unprecedented and WaPOO shares top honors.
The warming is undoubtedly human induced but GHGs have little to nothing to do with it.
Yes, we have had a persistent jet stream pattern with trough out west (cool) and ridge (warm) from the Southern Plains to the Mid-Atlantic. And plenty of storms along the frontal boundary in between. That is no proof of gloabal warming, any more than the 14 out of 15 below normal Januarys for DC (from 1956 thru 1971) were a sign of an impending ice age. All it proves is that weather can be streaky. Same goes for the 6 severe orange freezes in Florida between 1977 and 1989, far more than the rest of the last 100 years combined.
Wait, I’m not sure I understand the logic here. Why, in any given year, should we expect any station to have its hottest year ever?
I’m frankly not good at statistical thinking, but let me flail around and try to explain my understanding.
Particularly after a hundred years, it seems possible to me that most stations have already experienced their hottest year.
Of course, that may not be true. One hundred years may not be a large enough sample. There may still be many stations left which have not experienced their highest possible reading. Over time, though, I’d expect new records to become rarer and rarer, if there is no trend.
Perhaps if we also looked at stations that had their coldest year ever, we’d have a better chance of seeing a trend. If the world was truly warming (or cooling), wouldn’t we expect to see a rise in the number of stations reporting one extreme at the expense of the other?
All that said, I attach no significance to any one station reporting an extreme.
In a Gaussian distribution of 1000 stations and a 100 year record, we would expect about 10 of them to have their hottest month ever in the current year.
Ah, OK, I think I see now.
You’re saying that you need roughly a thousand year record to ensure that most of a thousand stations have seen their record highs and lows. (Assuming there is no trend.)
Am I nevertheless right in thinking that if there was a trend, you would see an increasing number of extremes at one end, and a decreasing number at the other?
If the year was hotter than the mean, we would expect to see more than ten. If it were cooler than the mean we might expect to see fewer than ten.
OK, that explains things. Thanks for your patience.
Two last questions: are there in fact a thousand stations with a hundred year record?
And if so, is there a trend in the number of extremes?
Yes, the USHCN record is more than one hundred years and has more than one thousand stations.