Hottest summers and coldest winters from 1900
"Summer" is defined meteorologically as June, July, and August; winter is defined as December, January, adn February. Of course, other things make a good summer (e.g. sunshine, amount of rain) or a good winter (e.g. amount of snow).
The hottest summers
The hottest summer of the twentieth century was 1976, by a considerable margin. Here are the means for the top ten:
Since then 2003 has shot into number 3, with a mean of 17.3C, and 2006 makes the Top Ten too, with a mean of 17.2C.
Are "summers not as good as we remember them"? No: they're better. Consider this table of the distribution of the number of hot summer months (with CET equal to or greater than 17.5C) across the decades of the twentieth century:
Here are the hot months (more than 17.5C):
July 1900, July 1901, July 1911, August 1911, July 1921, July 1923, July 1933, August 1933, July 1934, August 1947, July 1955, August 1955, August 1975, July 1976, August 1976, July 1983, August 1984, July 1989, August 1990, July 1994, July 1995, August 1995, August 1997, July 1999, July 2003, August 2003, August 2004, July 2006, July 2013, July 2014, July 2018.
And here are the very hot months (more than 18.5C):
July 1921, August 1947, August 1975, July 1976, July 1983, July 1995, August 1995, August 1997, July 2006, July 2018.
So the only really outstanding British summers, with two consecutive hot months, have been 1911, 1933, 1955, 1976, 1995, and 2003. So much for the great British summer! Note also the scarcity and pretty random distribution of these hot months. I have written about our nostalgia for non-existent weather of yore elsewhere (Harley, 2003, "Nice weather for the time of year" - see www.trevorharley.com for download.).
Before the twentieth century, they were worse. the only very hot month before 1921 were July 1783, with a mean of 18.8C, and 1852, with a mean of 18.7C from when records began in 1659).
The coldest winters
Which are the coldest winters of the twentieth century? Here are the top ten.
Are winters not as good as they used to be? Not lately they haven't, but it isn't clear that there is any trend until very recently.
Let's consider the distribution of severe winters, defined by two criteria.
1. Severe months with average temperature less than 2.0C. In the first half of the century, there were 14 severe months, and two occasions (Jan-Feb 1917 and 1929) when there were consecutive severe months. In the second half, there were actually a few more - 17 severe months, with two occasions (1963 and 1979) when there were two consecutive severe months, and only one occasion with three consecutive severe months (1916-1917). This difference however will be statistically insignificant.
2. Very severe months with average temperature beneath freezing. In the first half of the century, there were two very severe months (January 1940 and February 1947. In the second half, there were five (February 1956, January 1963, February 1963, January 1979, and February 1986 - and December 1981 came very close).
I think two things are happening. First, people's memories are distorted by the exceptionally severe winters of 1947 and 1962-63, in particular. Second, I think there is a recency effect where there haven't been many severe winter episodes in the last few years. Consider this table of the distribution of the number of severe winter months across the decades of the twentieth century:
Older people might be affected by the bump in the middle of the century (particularly the war years, which were exceptional) and compare that with the 90s. There is no overall trend, however: note the scarcity of severe months in the 0s and thirties. It is also possible that central heating means that we are less sensitive to cold weather when it happens.
Here are the severe months (mean less than 2.0C):
Feb 1902; Dec 1916, Jan 1917, Feb 1917, Feb 1919, Jan 1929, Feb 1929, Dec 1933, Jan 1940, Jan 1941, Jan 1942, Feb 1942, Jan 1945, Feb 1947, Dec 1950, Feb 1955, Feb 1956, Jan 1959, Dec 1962, Jan 1963, Feb 1963, Feb 1968, Feb 1969, Jan 1979, Feb 1979, Dec 1981, Feb 1983, Jan 1985, Feb 1986, Jan 1987, Feb 1991, Jan 2010, Dec 2010.
Here again are the very severe months (mean less than 0C):
January 1940, February 1947, February 1956, January 1963, February 1963, January 1979, February 1986, December 2010.
It is true that although we have some recent hot months, until 2010 you have to go back to February 1991 for a "severe" cold month. This difference is a clear suggestion of change.
Before the twentieth centuries, winters were better (if you like then cold and snowy). Remember the Little Ice Age? In the seventeenth and eighteenth centuries, winter knew how to be winter. Take the coldest winter of them all, the "Lorna Doone" winter of 1683-84, with a CET of -1.2. Or 1739-40, with a CET of -0.4. Now that is chilly.
We have at last had a cold winter, with 2009-2010 giving an average of 2.4, which would have made it 8th in the above list, and the coldest since 1978-79. January 2010 counts as a "severe" month (at 1.4C).
And of course now December 2010 at last gave us a very severe month, with a CET mean of -0.7, the first such month with a negative average since February 1986.