Turning Up the Heat

You may have noticed, especially if you’re a Baltimore Orioles fan, that run-scoring has been trending up as the season has progressed. This isn’t new, or particularly surprising, as the coming of summer turns up the heat, enabling fly balls to carry farther.

Let’s not just take this trend for granted; let’s bust out some detailed batted-ball data to get a better feel for production changes on all types of batted balls.

I measured, as of May 20, production on all exit-speed/launch-angle buckets, so that I could prepare my first interim park factors for the 2017 season. (Update coming in next couple weeks.) I did so again at the All-Star break, enabling me to isolate production during the interim period. The results are below:

Any batted ball with a launch angle of 50 degrees and up was considered a pop up, between 20 and 50 degrees a fly ball, between 5 and 20 degrees a line drive, and from 5 degrees and downward a ground ball.

Overall, you’ll note that overall production on balls in play jumped from .324 AVG-.535 SLG from Opening Day through May 20, to .336 AVG-.567 SLG between May 21 and the All-Star break. Frequency of each batted-ball type fluctuated very little between those two periods, with the MLB liner and pop-up rates increasing fractionally, and grounder rates creeping slightly downward.

You can’t pin the increasing production on higher exit speeds. Every bucket of 100 mph and up, across all major BIP types, went down in frequency from the pre-5/20 to 5/21-7/9 periods.

You will note that the production within all of those 100-plus mph buckets went sharply upward from the earlier to the later period. Interestingly, the reasons for these increases differ depending on the BIP category.

With fly balls, the reason for the increase is obvious and intuitive. The temperature goes up, the ball carries farther, and it goes over the fence more often. The table below shows the details, and allows us to segue into the line drives, where the story is a little different.

Hit a fly ball at 105 mph or greater, and there’s a very high certainty that it’s headed over the wall. Between our two key time periods, that homer percentage increased from 82.7% to 85.3%. Considering the limited number of batted balls (6.3% of all fly balls) in that bucket, this isn’t the source of the bulk of our offensive surge.

Things start getting serious in the next two buckets, the 100-104 mph and 95-99 mph flies. Of all fly balls, 32.7% fit into that range. Production on the higher of those two buckets surged from .667 AVG-2.321 SLG through 5/20 to .738 AVG-2.636 SLG from 5/21-7/9, and the homer percentage spiked from 48.8% to 57.6%. In the lower bucket, production increased from .290 AVG-.891 SLG to .354 AVG-1.080 SLG, with the homer rate rising from 15.0% to 17.5%.

Even in the humble 90-94 mph bucket, right on the edge of the “donut hole” where fly balls go to die, production went up from .102 AVG-.243 SLG before 5/20 to .138 AVG-.356 SLG between 5/21-7/9, almost completely due to a near-doubling of the homer rate within that bucket from 2.1% to 3.8%.

So that’s the fly balls. Look at the liners, however. The homer rates in the 100-plus mph buckets actually trended slightly down from the earlier to the later period. What gives here?

Well, the double percentage in the 105-109 mph bucket shot upward from 29.7% through 5/20 to 33.3% between 5/21-7/9. The single percentage in the 75-79, 85-89 and 90-94 mph buckets all went up fairly significantly, mitigated to an extent by a decline in the 80-84 mph bucket. Fewer line drives are being caught as the season progresses, despite the fact that they aren’t being hit as hard. Weather conditions like temperature and wind don’t affect liners to nearly the same extent as fly balls. My gut is that a combination of fielder fatigue and playing-surface conditions is the culprit.

That point would appear to be buttressed by the bottom section of the first table above, focusing on ground balls. Overall grounder production increased from .222 AVG-.240 SLG through 5/20 to .225 AVG-.244 SLG between 5/21-7/9. As in the other BIP groups, the 100-plus mph buckets declined in frequency from the earlier to the later period, so exit speed isn’t the reason for the increase. The decrease in the percentage of 100-plus mph batted balls within each BIP type would also point toward increased player fatigue.

Production on grounders in the two highest exit-speed buckets rocketed upward from the earlier to the later period; from .465 AVG-.497 SLG to .599 AVG-.678 SLG on 110-plus mph grounders, and from .452 AVG-.474 SLG to .484 AVG-.528 SLG on 105-109 mph grounders. This again would be indicative of both somewhat fatigued fielders and/or harder, faster playing surfaces. The doubles percentage increased sharply in both of those high-velocity buckets.

As an aside, you will notice the extremely high volume of ground-ball singles in the 90-94 mph bucket, and the high volume of ground ball outs in the 80-84 mph bucket. These are largely composed of grounders that didn’t generate an exit-speed reading; beginning this year, they are being arbitrarily classified in those buckets. More detail can be found at the invaluable Baseball Savant site.

So, run scoring does increase as the weather warms up. This isn’t exactly a groundbreaking concept, but some of the reasons for the increase might be a little counterintuitive. It’s not just the temperature and the wind, it’s also the effects of the elements on the playing surface — and of the game’s grinding calendar on the players themselves — that impact the product on the field.