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Pick sunrise or sunset. Your best bet on a hot day is to head out in the early morning or evening, when your shadow is twice as long as you are tall. According to the National Weather Service, exposure to direct sunlight can increase how hot it feels by as many as 15 degrees.
Mind the 90-degree line. "When the mercury is above 90 -- the temperature of the surface of your skin -- you'll gain heat from the air around you, and your body heat will have nowhere to go," Cheuvront warns. At that tipping point, you'll sweat more and your body temperature will rise rapidly, making you more susceptible to heat-related illness. Go easy or go inside.
Bottom's up! Cheuvront advises following the recommendations of the American College of Sports Medicine: Stay well hydrated throughout the day by drinking at least eight cups of water, then make sure to have eight to 12 ounces about 15 minutes prior to your run. Sip three to eight ounces every 15 or 20 minutes as you run, and don't forget to drink after your workout.
Field the Heat
Rule number one before you run: Check the heat index -- a combination of air temperature and humidity -- rather than your thermometer to get a better idea of the real feel outside. (At 70 percent humidity, an 84-degree day can feel as if it's 90.) "Also, the more humid it becomes, the less your sweat evaporates from your skin, meaning your body's key cooling mechanism is disabled," says Michael Bergeron, PhD, the executive director of the National Youth Sports Health & Safety Institute. To run sun smart, determine the day's heat index (see the National Weather Service's guide), then follow the guidelines below.
Higher than 104 degrees:
No-brainer: Move it indoors, because you're at severe risk of heat-related illness, including heatstroke. (A 100-degree day with just 40 percent humidity will feel like 109 degrees.)
Between 91 and 103 degrees:
Exercise early in the morning, when it's coolest, and keep your workout superlight, suggests Brian Rosetti, a running coach and the founder of the Run S.M.A.R.T. Project.
Between 80 and 90 degrees:
Keep workouts shorter than usual and moderate, like this routine from Rosetti.
79 degrees or lower:
Save your long or most challenging runs for these milder days.
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