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Weather a key variable for Boston marathon runners

Posted by Andrew Mooney  March 28, 2013 09:38 AM

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The saying "Everybody complains about the weather, but nobody does anything about it," probably wasn't coined by a runner, but it should have been. Part of every runner's preparation for the Boston Marathon is a prayer to the weather gods, asking for favorable conditions. The weather has a major impact on the results of any road race, and Boston weather always presents a multitude of possibilities to marathoners on Patriots' Day, many of them unpleasant.

The right weather can help speed runners on their way along the 26.2 mile course from Hopkinton to Boston. But no matter how fast you are or how hard you've trained, when Mother Nature tosses out a day like she did last year -- the sun was bright, the air was still, and temperatures were near 90 -- everybody's performance suffers significantly.

Historically, Boston’s typical April weather is actually pretty good for marathon running. According to one study, elite runners run fastest when the temperature is about 39 degrees Fahrenheit, while the slowest runners in the study performed best at about 50 degrees. Another way of looking at the effect of temperature is to examine the frequency at which runners drop out of races. The fewest runners drop out when the temperature is about 53 degrees.

The average temperature in Boston in mid-April is not too far from that ideal, ranging from about 47 degrees at 9 am, when the first runners leave Hopkinton, to a high of 53 degrees by 4 pm, when the last runners are crossing the line on Boylston St.

The average temperature may be conducive to fast running, but as Mark Twain once said, "There is a sumptuous variety about the New England weather that compels the stranger's admiration -- and regret." The link between higher temperatures and slower finish times is very strong. Over the last ten years, the correlation coefficient is 0.8, which represents a very strong positive correlation. A mid-pack runner's time slows about 18 percent when race temperatures go up 20 degrees. Someone capable of running a 3:28 under “ideal” conditions would be projected to run about 4:05 when the temperature on race day hits 70 degrees.

Slower runners are affected more by an increase in temperature, perhaps in part because they're out on the course longer. Since most races start in the morning, temperatures are usually lowest for the start and then rise during the race. Thus, the average race temperature for most races is higher for slower runners than for faster runners. This is especially true for Boston, where the slower runners start well after the elites.

The wind can also have a major effect on marathon runners. Wind matters more in Boston than at other races. If a course loops around so the finish is near the start, the effect of the wind is more likely to even out, pushing runners ahead at some points and back at others. But since Boston is a point-to-point course, the wind can blow at runners from one direction for the entire race.

All else being equal, the drag on a runner created by air resistance varies according to the square of the runner's velocity through the air. That means that the performance hit from a 10 mph headwind is four times greater than that from a 5 mph wind.

A tailwind helps runners, but not quite as much as a headwind hurts them. One estimate says that, when running at a six minute-per-mile pace, a 10 mph tailwind would increase one’s performance by about 6 seconds per mile, while the equivalent headwind (six-minute miles into a 10 mph wind) slows one down by about 12 seconds per mile. That's a possible swing of almost eight minutes over the course of a marathon.

According to Windfinder.com, April is Boston's windiest month, averaging about 14 mph. The chance that the wind is 13 mph or more is 55 percent. Most often, April's wind is from a little south of east -- a headwind for the race -- but tailwinds are also fairly common.

Unlike heat, the wind affects faster runners more than slower runners. The additional resistance from running into a steady wind at a 5:40 mile pace is twice that encountered when running into the same wind at a 8:00 mile pace. Unfortunately, that's not much of a consolation for the slower runner, who still has to spend an extra hour fighting against the wind.

Runners can counteract much of the negative effect of a headwind by "drafting." A marathoner can reduce wind resistance by as much as 80 percent by running one meter behind another runner. The benefits of drafting drop off quickly as the distance between runners increases, so a marathoner needs to choose carefully when picking a runner to follow.

Wind isn't always bad. A stiff breeze on a hot day can help counteract the negative effect of the temperature. On the other hand, on a cold day the wind can make the last few miles miserable for tired, sweaty runners. In 2007, when the race was held during a nor'easter, 20-30 mph headwinds combined with temperatures in the 40's left a number of runners in the medical tent getting treatment for hypothermia.

Temperature and wind have the greatest effect on runners, but precipitation also makes a difference. In April, there's almost a 50 percent chance of some form of precipitation at some point during the day in Boston. The last time there was any snow on Patriot's Day was in 1961, but rain is common. Rain on a hot day helps runners stay cool enough to keep going, but there are few hot, rainy days in April. Marathoners have to use more energy when running in weather that is cold and wet.

It's true that "if you don't like the weather in New England now, just wait a few minutes." Unfortunately, marathoners standing at the line in Hopkinton can't wait for perfect weather. They hope for the best but have to ready to deal with the worst. For their sake, let's hope that in 2013, Zeus, Amadioha, Feng Po, Zephyrus, Amm, and all the other weather gods cooperate and bring cool breezes on Patriots’ Day to help speed all runners on their way to Boston.

This year, Ray Charbonneau is running the Boston Marathon (his fifth) as a sighted guide for a vision-impaired runner. 50 percent of the proceeds from sales of his latest book, Overthinking the Marathon, go to benefit the Mass. Association for the Blind. Ray's articles on running have appeared in the Boston Globe, Ultrarunning, Marathon & Beyond, Level Renner, Cool Running and other publications. Find out more at y42k.com.

This blog is not written or edited by Boston.com or the Boston Globe.
The author is solely responsible for the content.

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Stats Driven is powered by David Sabino, who over the last two decades has been a source of statistical analysis on the pages of Sports Illustrated, New York Times, and Chicago Tribune. David has written about all seven recent Boston-area championships for Sports Illustrated Presents commemorative issues, was the creator of such long time features as SI’s Player Value Ranking, NBA Player Rating and long running fantasy football and baseball columns.

He has also authored or made contributions to many books, including the Sports Illustrated’s 100 Fenway: A Fascinating First Century.

Now living in Marblehead, he’s focusing his attention on the Boston sports scene, specifically delving into the numbers affecting the Red Sox, Patriots, Celtics and Bruins, with the goal of informing and entertaining real fans. You can follow him on Twitter at @SabinoSports.

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