Running in Humidity, How to Stay Strong and Safe!

Have you noticed that running in humidity is MUCH harder? That’s not your imagination (nor mine). Here’s why running in humidity is such a beast, and then, what to do about it!.

WHY Running in Humidity is Tough

1) Unfortunately, our muscles aren’t very efficient! Only ~20% of the energy our muscles produce moves us (forward hopefully). The balance is given off as HEAT, HEAT! Great news for those winter, and chilly runs. Not so much for hot and humid runs.

2) The way our bodies cope with heat when exercising/running is through sweating, or more precisely, through the EVAPORATION of sweat. The evaporation takes the heat with it as it leaves the body.

3) Here’s the kicker: when it’s humid, sweat doesn’t evaporate well, or at all! It clings to your skin and drips off, taking very little heat with it Therefore, at that point, intensity during a run feels quite tough because heat is dissipating so poorly.

George Chiampas, D.O., the medical director of the Chicago Marathon states “Of all the climate measurements we take to assess heat risk for our runners, humidity is the biggest factor.”

WHY Your Body Feels the Strain

Physiologically, here’s what happens when running in high humidity (and temperatures). As your body diverts more blood to your skin in the attempt to sweat, and divert heat:

Your heart rate increases. At 60-75˚F, heart rate increases two-four BPM; at 75-90˚F, heart rate increases up to 10 BPM. Add high humidity, the numbers are higher.
Decreased blood flow to muscles. Muscles have less blood, therefore less oxygen, less energy to use for running. That corresponds to the above point, where your heart (and lungs) work harder to compensate for the loss in oxygen.
Increased risk and rate of dehydration. If hydration is not well managed, a loss of just 2% of body weight (through sweat) leads to ~4-6% drop in performance.

In other words, for the same effort level, you run slower. Or, put another way, for a certain pace, your body is working harder to maintain the same pace on a cool day.

As a matter of fact, temperature/humidity need to be fairly low for optimal running performance. A study of a number of marathons completed around the world found that the optimal temperatures for best performance are between ~39-49˚F (or ~4-10˚C)!

Below we will dive into what you can do to cope with running in heat and humidity. But first, a tip on how to know which weather metric to pay attention to for your effort on a given day.

WEATHER METRICS TO MONITOR

Interestingly enough, it’s not the temperature, nor the humidity. It’s the dew point. This is the meteorological measure that best indicates how easy, or tough, that summer run will be.

The dew point means the temperature at which water condenses. And that definition doesn’t mean too much to me, but, this table below (thank you, Runner’s World) does.

Dew Point – Your Guide to Degree of Discomfort

DEW POINT (°F)	RUNNER’S PERCEPTION	HOW TO HANDLE
50–54	Very comfortable	PR conditions
55–59	Comfortable	Hard efforts likely not affected
60–64	Uncomfortable for some people	Expect race times to be slower than in optimal conditions
65–69	Uncomfortable for most people	Easy training runs might feel OK but difficult to race well or do hard efforts
70–74	Very humid and uncomfortable	Expect pace to suffer greatly
75 or greater	Extremely oppressive	Skip it or dramatically alter goal

Here’s a handy dew point calculator to determine the dew point (although the Weather.com app shows dew point as one of its metrics).

I prefer dew point to the heat index, as the heat index only accounts for temperatures (F) of 80˚ or greater. That doesn’t help much with runs at 7 a.m. when the temperature is 75˚, and the humidity is 90%+. Since most of my runs are done before the temperature hits 80˚, the heat index is irrelevant.

The heat index metric, however, does become helpful, obviously with temperatures ≥80˚. This can be helpful for longer races, such as marathons, that typically start with lower temperatures, but can increase significantly over the course of the race (average run time of a marathon for females is 4:42).

HOW to COPE with Running in Humidity:

With all that said, what to do when running in unsavory weather conditions – high dew point, high heat index, high humidity, high temperature?!

Give yourself time to acclimate to the sultry conditions. Different resources say it can take up to two weeks before your body (and mind) can adjust to the tougher conditions. Note, this does not mean that after two weeks, you’ll be hitting your cooler weather paces, but it will be a bit easier to manage the workout.
During this acclimation period, choose shorter/lighter sessions, and don’t push too hard as your body is adjusting.
Adjust your pace. Moderating your pace (ie 10-20% slower than your normal pace) for dew points at 65˚ or higher (note chart above). Note, this applies to those runs or races after you’ve acclimated, too!
Focus more on your heart rate/perceived exertion versus pace. This is saying the above, just a bit differently! Focus less on pace, and more on the target heart rates you normally strive for during a workout, or an appropriate exertion level (ie conversation pace runs should still be at conversation pace).
Properly hydrate. Ensure you’re hydrated before you run! Regularly drink (in ounces) half of your body weight (in lbs) to maintain proper hydration. Sorry, beer/alcohol/soda don’t count, but essentially everything else does!
Remember, sorry to be graphic, your pee should be a pale color of lemonade throughout the day. If it’s dark, you’re not drinking enough. And if it’s almost clear, essentially, you’re drinking too much.

THERE’S MORE…

Salt/electrolytes in your daily diet, plus include them on your long, hard runs. More detail here on hydration, and electrolytes. Salt and electrolytes are critical in helping your body regulate its fluid balance with exertion.
Adjust your mindset. Don’t beat yourself up because you are running slower in less than ideal running conditions. Remember that it’s your effort that is important, and slogging through tough conditions will put you in a better position once the mercury begins to drop come fall.
If for some reason, the below tips are forgotten, please know the warning signs for dehydration, and heat exhaustion/stroke. A very likely scenario while running in high humidity and temperatures if not managed well: fatigue, nausea, dizziness, headaches, tingly skin, and confusion.

At that point, stop if you are feeling/noticing any of these symptoms, even if you haven’t finished your run, or reached the finish line yet.