If your last run suddenly felt effortless this week, the thermometer may deserve some of the credit.

The first real fall snap actually has a part to play in how your body responds to running longer distances.

The sweet spot of temperatures

A large study analyzed the results of nearly 1.8 million marathon finishers from six major marathons in Paris, London, Berlin, Boston, Chicago and New York over a span of 10 years. They concluded that air temperature was a primary factor for running speed.

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Of course, factors like humidity and pollution mattered, too, but air temperature had the largest effect. Running speed peaked when temperatures ranged between 39 and 50 degrees Fahrenheit, depending on the runner.

This lines up nicely with the temperatures you start to see around the first real fall cold snap.

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I personally figured my training paid off, which I am sure it does. But, it also helps explain why so many runners feel like they've suddenly found another gear once summer heat fades.

What about outside those temps?

That range of temperatures is very specific. So, what happens on either side of it? Well, the study showed that runners' speeds slowed down at an increasing rate. The mechanism is simple. In warm conditions the body sends blood toward the skin to shed heat, leaving less for working muscles. Pace pays the difference.

Running pace also slowed when it was below the perfect range of temperatures, but the data didn’t go too far below this range because you are getting into temperatures that are just too cold. 

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These temperatures also influenced how many runners dropped out before the finish. The number of runners who withdrew increased greatly once the temperatures climbed far above or below the sweet spot.

This could be why there are so many fall marathons on the calendar. Take the TCS New York City Marathon on Nov. 1, for example. Morning temperatures around that date can drop into this sweet spot, leading to some solid times. 

It ultimately depends on the runner

“Ideal” weather isn't one size fits all. Certain conditions can be favorable for certain runners because of their body types and the location where they spend their time training. I personally don’t like to run in cold temperatures, so I would rather be on the upper end of this range or even warmer. 

Interestingly, the fastest women in the study may also agree with me. They peaked at the warm end of that range, around 50 degrees.

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Plus, there are other factors like humidity, wind and pollution. When a cold wind cuts through the air, it can slow performance, especially if you are running into it. Those sensitive to pollution could also see their times drop. Ground-level ozone in particular was shown in the study to impact performance.

What if you don’t run marathons?

It honestly doesn’t matter if you are running a marathon or not. The key is to get out and enjoy the crisp fall air and be happy the oppressive summer temperatures are coming to an end. 

If you've been chasing a personal best, this season could be your moment. Just resist two common mistakes runners make on that first crisp morning: overdressing (since most people still overheat once they get moving) and going out too fast, which can leave you fading hard by the second half.

Rob Shackelford is a meteorologist and climate scientist at weather.com. He received his undergraduate and master’s degrees from the University of Georgia studying meteorology and experimenting with alternative hurricane forecasting tools.