Is Warm Weather Hitting What It Used To Be?

The table below shows OPS by months for each of the last two years in both leagues.

2014	AL OPS	NL OPS
April/March	0.716	0.695
May	0.713	0.710
June	0.715	0.682
July	0.711	0.693
August	0.690	0.696
Sept/Oct	0.705	0.696
2013	AL OPS	NL OPS
April/March	0.733	0.704
May	0.737	0.706
June	0.717	0.711
July	0.713	0.699
August	0.729	0.704
Sept/Oct	0.717	0.695

I always assumed that June, July and August would normally be higher than the rest of the year. But the last two years don't look that way. So I wondered what the long term trends were.

The table below shows how different these three months were in percentage terms compared to the entire season in OPS for the AL. These are 10 year averages, with 1974-1983 being a nine year average not using 1981 due to the strike.

Year	June	July	August
1914-1923	1.36%	0.65%	-0.13%
1924-1933	2.13%	2.62%	-2.10%
1934-1943	1.70%	2.28%	-0.37%
1944-1953	0.37%	1.51%	0.29%
1954-1963	1.05%	1.61%	-0.67%
1964-1973	1.26%	1.51%	-0.14%
1974-1983*	0.50%	1.09%	-0.15%
1984-1993	0.90%	1.03%	0.61%
1994-2003	0.70%	0.33%	0.09%
2004-2013	0.58%	0.38%	1.02%

From 1924-1933, OPS in July was 2.62% higher than the entire season OPS, on average. Notice neither of the last two periods are even .5% higher. Why any of these changes have happened, I don't know. Now the NL.

Year	June	July	August
1914-1923	1.09%	1.18%	-2.35%
1924-1933	3.30%	1.46%	-2.37%
1934-1943	1.63%	1.37%	-0.22%
1944-1953	1.21%	1.50%	-0.71%
1954-1963	0.71%	-0.05%	-0.25%
1964-1973	1.05%	1.88%	0.84%
1974-1983*	1.06%	0.95%	-0.66%
1984-1993	1.42%	0.13%	-0.64%
1994-2003	1.03%	0.06%	-0.36%
2004-2013	-0.59%	0.96%	1.91%