Looking at the CET list, September has been warmer than June in the following years since 1900 (27 times) It probably happened several more times before as well. 1903, 1907, 1909, 1916, 1926, 1929, 1946, 1948, 1949, 1951, 1955, 1956, 1958, 1961, 1964, 1971, 1977, 1978, 1980, 1981, 1985, 1987, 1989, 1991, 1997, 1998, 1999.