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Evan Horowitz

Could weather have played a part in Deflategate?

At around 4:15 p.m., the tarp covering the field at Gillette Stadium was removed under cloudy skies. Jim Davis/Globe Staff

Did the New England Patriots intentionally deflate their footballs in order to soften the feel and gain an advantage on offense?

It’s increasingly clear that the balls really were underinflated. A letter from the National Football League states that 11 of the 12 game balls were low.

Now the question is: whodunit? Was it a team decision, a conspiracy of insiders, a rogue individual, or even — and this is not as far-fetched as it might sound — just the weather? Conditions on Sunday were unseasonably warm, with steady wind and rain. Could that have made the difference?

The facts

Footballs are supposed to be filled to a specific pressure, 12.5 to 13.5 pounds per square inch (psi). ESPN is reporting that when the balls were retested, they were found to be underinflated by about 2 psi.


The temperature

A change in temperature can indeed cause a change in pressure. It’s one of those basic laws you learned about in High School chemistry. When the temperature goes down, the pressure does too.

In this case, what matters is that the balls are set up and tested inside, where it’s warm, and then taken out onto the field, where it’s colder.

If we make a few basic assumptions, we can estimate how much that drop in temperature might have affected the balls last Sunday.

We know that the temperature at kickoff was 51 degrees. Let’s assume that the temperature inside — where the balls were filled — was 71, and that they were already at the low end of allowable pressure: 12.5 psi.

As the balls cooled from 71 degree to 51 degrees, their pressure would actually drop from 12.5 pst to 11.5 psi. That’s quite a lot, though not enough to explain the full 2 psi decline.

For the pressure inside a football to drop 2 psi, the balls would have to get close to the freezing point, which didn’t happen Sunday, though it’s possible it has happened in some of those legendary cold-weather games — like the 1967 ice-bowl, when the temperature hit 15 below.


Barometric pressure

It’s tempting to assume that outside air pressure might also play a big role, but that’s probably not the case. For one thing, while the field is often much colder than the locker room, there’s rarely a big difference in pressure.

Plus, if anything, the atmospheric pressure on Sunday was actually low (which is what generally happens when it rains). And low pressure would make the balls seem more inflated, not less.

Slow leak

Footballs aren’t perfectly sealed. Even the best-designed balls will let some air molecules escape. And the fact that 300-pound men pile atop these footballs on a regular basis surely forces a bit more air out.

It’s hard to know exactly how much of an effect this would have, but we’re probably talking about tiny amounts — not enough to account for a substantial drop in pressure.

So did the weather do it?

Probably not. Temperature may have played a role, but the field wasn’t cold enough to explain the full 2 psi drop. More likely, the weather had an accomplice.

Evan Horowitz digs through data to find information that illuminates the policy issues facing Massachusetts and the U.S. He can be reached at evan.horowitz@globe.com. Follow him on Twitter @GlobeHorowitz