Heat + Humidity. How Hot Is It…Really?

It’s HOT!

I’m training for two upcoming marathons and that means lots of outdoor running. A few Saturdays ago, I went on my usual 6am run but I could tell it was going to be unusually hard just because it felt so humid, not to mention this is the hottest August (on avg.) we’ve had in Houston. When you take those hot temps and add humidity, it’s extra hard on the body and can make it feel like it’s 10-15 degrees hotter. I wanted to share this chart with you so that you can see what exactly your body is dealing with. If you live in Houston or another humid city, you need to take extra precautions to keep your body cool during outdoor workouts. Even as the Fall approaches, temps + humidity can continue to take their toll.

This chart represents the Heat Index, which is the temperature your body feels when the heat and humidity are combined. *Keep in mind that the chart is actually based on partial shade and with light wind. Exposure to direct sunlight can increase the Heat Index by up to 15°F!!

Relative Humidity (%)
40 45 50 55 60 65 70 75 80 85 90 95 100
110 136
108 130 137
106 124 130 137
104 119 124 131 137
102 114 119 124 130 137
100 109 114 118 124 129 136
98 105 109 113 117 123 128 134
96 101 104 108 112 116 121 126 132
94 97 100 102 106 110 114 119 124 129 135
92 94 96 99 101 105 108 112 116 121 126 131
90 91 93 95 97 100 103 106 109 113 117 122 127 132
88 88 89 91 93 95 98 100 103 106 110 113 117 121
86 85 87 88 89 91 93 95 97 100 102 105 108 112
84 83 84 85 86 88 89 90 92 94 96 98 100 103
82 81 82 83 84 84 85 86 88 89 90 91 93 95
80 80 80 81 81 82 82 83 84 84 85 86 86 87
With Prolonged Exposure and/or Physical Activity:
Caution:
Fatigue possible
Extreme Caution:
Sunstroke, muscle cramps, and/or heat exhaustion possible
Danger:
Sunstroke, muscle cramps, and/or heat exhaustion likely
Extreme Danger:
Heat Stroke or Sunstroke likely

*Source: National Weather Service

Approximately 400 people die each year from exposure to heat, according to the Centers for Disease Control and Prevention. All that gross sweat is actually serving a purpose and is a good thing.  Your body uses sweat to cool itself through evaporation and once your sweat starts to evaporate, your body temp drops. This why it’s more dangerous in humid conditions: sweat won’t evaporate as much and thus, your body stays hotter.

Tips to remember:

  • Limit your outdoor activity to morning and evening hours.
  • Cut down on exercise. Your normal 4 mile run may be really hard…so either scale back the distance or pace, or both!
  • Stay hydrated! Start hydrating your body the day before if you can and then you will need fluids before, during and after your runs. You will most likely need  a sports drink with some electrolytes, sodium and potassium especially if you are sweating a lot or working out more than 30-60minutes. See Sweat Rate Below…
  • Don’t forget sunscreen, sunglasses or a hat. I always run with a hat.

Calculate Your Sweat Rate (Good Rule for Runners):

Weigh yourself before and after your workout. 1lb weight loss=1pint fluid loss. For example, if you lose 2 pounds during an hour run, that’s 2 pints or 32 ounces (your “sweat rate”). Thus, you need 8 ounces of water or sports beverage every 15 minutes to replenish your sweat.

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One comment

  • Jen September 14, 2010  

    I live in South Florida so this is super helpful. I’ve never seen the sweat rate formula before. I’ll try that next time I run.

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