You hear a lot of people say players hit more home runs in warm air than in cool air, and more home runs in humid air than in dry air. Since I have four years of data to compare against, I decided to compare the number of home runs hit per month to the average temperature and humidity. I couldn’t find what I felt were reliable humidity data for Citi Field, so I went with precipitation data instead, thinking that precipitation would roughly correlate with humidity. The data base I am using hasn’t published information for August, and September isn’t even over yet, so I can only compare the first four months of the season.
I have created three images, containing a total of five charts.
The first image:
The first chart shows the temperature data. The middle line depicts the historic average temperature for Flushing, and each bar represents the average temperature for the month in each respective year above and below the historical average.
The second chart shows the number of home runs per month. The middle line represents the average number of home runs for each month, and each bar shows the number of home runs hit in that particular month in each respective year.
The third chart shows the precipitation. The middle line is the historic average of precipitation in each month, and each bar shows the actual precipitation of each respective month.
The second image:
A scatter plot of Home Runs as a function of Temperature. As the trending line shows, there is an insignificant negative correlation between temperatures and the number of home runs in any given month.
The third image:
A scatter plot of Home Runs as a function of Precipitation. The trending line shows a slight, albeit marginally significant, correlation between home runs and the amount of precipitation in any given month.
Going back to the first image, you can see that, roughly speaking, in any given month, the year with the most rain has the most home runs. For April, 2011 had the most rain and was the only month with above an average number of home runs. 2009 had slightly less rain, and the second most number of home runs. The only glaring exception to this general rule (and this is a limited data set) is July, where the month with the second most amount of rain had far and away the most home runs.
Again, this is only data on precipitation, not strictly humidity.
I left out August data in this chart, as I have already explained, but in August 2011 there was a huge, huge, huge amount of rainfall. Way above average. August also posted the most number of home runs out of any single month in 2011, more than June and July combined, both of which had below average rainfall.
So what can we make of this? Well, for one, temperature doesn’t seem to matter in Citi Field. At least not for the first four months of the season (which happens to include the second coolest and warmest months). Precipitation, which I am using to approximate humidity, does seem to have a slight effect. But, measuring strictly from precipitation as opposed to purely humidity data, it is a minor effect at best.