Summer Running

Summer is officially here and with it comes those dreaded salty, sweaty, fatigue inducing runs.  You may feel drained more easily these days.  You may find you can’t hit the same paces you could a month ago.  You may begin to wonder why you’re running in the first place when you could instead be sitting with a cold drink in your hand, perhaps one with a cute little umbrella.  Particularly in these first few days, you may find yourself confused by what’s going on with your runs.  Have no fear, all this is perfectly normal and to be expected.  

Let’s start out by diving in with a few facts.  There are 3 main physiological processes at odds when it comes to your running performance in summer like conditions.  

The first, physical exertion like running, raises your body’s core temperature.  When conditions outside are already warm, your body struggles to keep your core temperature in check.  In order to cool itself, your body diverts blood to your skin which induces sweat.  Sweat evaporating off your body has a cooling effect and helps to regulate your core temperature, but this brings about a second conflict.  

In diverting blood to your skin, you have less available to carry oxygen to your working muscles.  Your muscles use oxygen to break down glucose and convert it into adenosine triphosphate or ATP, which is the body’s energy source.  If your working muscles do not have enough oxygen or it is not delivered quickly enough via your red blood cells, they will convert the available glucose into lactic acid and your muscles will be forced to slow down.  This is similar to what happens at altitude.  

That brings us to the third process at play, sweat causes fluid loss which leads to dehydration.  Lack of hydration causes your heart to work harder at pumping blood, because as you dehydrate your blood actually becomes thicker.  This is called cardiac drift.  More specifically that is when your heart rate increases while the intensity stays the same.  You can counter some effects of cardiac drift simply by matching your fluid intake to your sweat rate. Extreme dehydration by contrast can lead to serious conditions such as heat stroke, whereby your body stops sweating completely and you can no longer regulate your core body temperature.

So in short, a high core body temperature, less blood available for your muscles and dehydration, are three of the physiological challenges brought about in summer like conditions that cause a decrease in running performance.

These facts should give some important insights to consider when you think about summer running.  It’s not just the air temperature that plays a role in how you feel.  Simply checking the weather forecast for the current temperature gives a good starting point for the effects on your performance but not the full picture.

For instance, studies have shown that your performance could be slowed by up to twice as much in hot humid conditions versus hot dry conditions even with the same heat index value.  A dry heat, that is when humidity levels are below 40%, is easier to run in due to the efficiency of the body’s cooling system.  In high humidity this system is stalled.  Remember sweat evaporating is how your body is cooled, but when the air is already moist, it’s difficult for your sweat to be absorbed into the atmosphere.  This in turn makes it feel even hotter over time and leads to a further increase in sweat rate, which then prompts greater dehydration.  It’s a vicious cycle in hot humid air and one that is familiar to runners who live in the eastern United States.

The other thing you want to keep in mind as summer conditions progress and increase in severity, is that your body’s response is non-linear.  Meaning you can’t assess a 10 degree difference in temperature or a 5% difference in humidity in the same way across the board.  Both become exponentially more serious the higher the readouts go.  In terms of what the minimum threshold is for slowing down, studies have shown that in marathoning it begins at a mere 60 degrees F (15.6 deg C).

There is a bright side to all this however.  Your body not only acclimates to running in the heat, it actually adapts over time on a physiological level.  Studies show that with 2 weeks of consistent summer running your body adapts, and 75-80% of those adaptations come within the first 4 to 7 days. Those adaptations pay dividends come fall.

“Research has found that there are many positive adaptations that the body makes in response to the heat, including increased sweat rate, increased blood plasma volume, reduced overall core temperature, increased oxygen delivery to the muscles, reduced blood lactate and increased skeletal muscle force,” explains running coach Angie Spencer, RN, owner at Marathon Training Academy. “All of these adaptations work together to make your body more efficient at cooling, which can pay off when temperatures start cooling down.”

In the meantime, it is important throughout these summer months to make the necessary adjustments and train wisely.  The risks associated with not taking proper precautions can include potentially serious conditions, such as: severe dehydration, heat cramps, heat exhaustion and heat stroke.  In order to stay healthy and avoid any truly unpleasant physical effects, you want to take a few steps before even stepping out the door.

  • Run by feel rather than by a set number on your watch.  A hard effort in the summer months will not match the same pace and distance as a cool fall day.  For more details on running by feel, please visit this detailed explainer.
  • Hydrate early, often and always after.  Pre-hydration is key because it is difficult to keep up with fluid loss once you start, especially in humid conditions.  So you want to start hydrating the day before and also in the morning before you lace up.  If you’re planning to be out for longer than 45-60 minutes, you will want to hydrate during your run as well.  Then once you finish be certain to continue replenishing those fluids.  You’ll need a balance of electrolytes and water to compensate for all that’s lost in your sweat.  If you’d like to calculate your personal hydration rate, which is unique to each individual, you can do so by weighing yourself prior to your run and then immediately after.  You should be drinking 16 ounces of fluid for every pound lost.
  • Stay in the shade when possible and go out either early in the day or late in the day.  Direct sunlight will further sap your energy so if there’s areas with cover, it’s best to head there to run.  The middle of the day, when the sun is highest in the sky, will always be the hottest period of any day so it’s best to avoid the hours between roughly 12 and 4 pm.
  • Wear light colored breathable clothing.  Synthetic fabrics, not cotton, will keep the sweat from being absorbed by your clothes, trapped against your skin and weighing you down.  Look for technical fabrics that wick the sweat away. Light colors reflect more light and will therefore, be cooler than black or other dark colors.

A few other little tricks that can help are pre-cooling, pouring cool water on your head while you run, and drinking cold icy drinks during strenuous exercise.  Precooling can be done in a number of ways either with a cooling vest or by putting ice on your neck and head or even a cold shower before you go out.  Occasionally in a hot race you might see sprinklers or volunteers handing out ice, make sure you take advantage of these opportunities.  All these tricks are based on the same principle, which is helping to keep your core body temperature low for as long as possible and/or lower your core body temperature, which alleviates some of the strain on your heart.

If you’d like to know in advance the precise difference in pace depending on certain conditions, there are various calculators that do just that.  You can also use these calculators after the fact, as a guide to determine just how much your race performance was impacted by race day conditions.

The Jack Daniel’s vDOT calculator uses just the air temperature as a gauge.  As we’ve discussed above, humidity plays a big role in how hot your body actually feels and performs, but this calculator will tell you exactly what the temperature effect is per mile. 

The Runner’s Connect calculator uses dew point and air temperature.  Dew point is often considered a more accurate way of measuring the effects of humidity when combined with hot temperatures.

The calculator Luke Humphrey Running uses includes temperature, humidity and even wind speed.

For an overview assessment of how your run performance is affected, there are charts that provide a basis.  I’ve included three below. The first chart uses just dew point, the second chart combines dew point and air temperature, and the third chart combines relative humidity and air temperature to show when conditions are potentially dangerous. If you find yourself faced with dangerous conditions outside, I recommend taking that day off – or if your situation allows running indoors on a treadmill.

Knowing what to expect and preparing yourself to best tackle each day’s challenges will make all the difference these next few months. So don’t skip your run for that cold umbrella drink, just save it for after!


Dew Point deg F (Celsius)AdjustmentEasy RunningHard Running
Below 55 °F (12°C)0%UnaffectedUnaffected
55°F (13°C)-60 °F (15°C)1%UnaffectedSlightly difficult
60°F (16°C)-65 °F (18°C)2 – 3%Slightly difficultDifficult
65°F (18°C)-70°F (21°C)3 – 5 %DifficultVery difficult
70°F (21°C)-75°F (23°C)5 – 8 %DifficultVery difficult
75°F (23°C)-80°F (25°C)12 – 15%Very difficultNot recommended
Above 80°F (25°C)Just runJust runNot recommended

Add together air temperature and dew point and see where the combined number places you:
100 or less: no pace adjustment
101 to 110: 0% to 0.5% pace adjustment
111 to 120: 0.5% to 1% pace adjustment
121 to 130: 1% to 2% pace adjustment
131 to 140: 2% to 3% pace adjustment
141 to 150: 3% to 4.5% pace adjustment
151 to 160: 4.5% to 6% pace adjustment
161 to 170: 6% to 8% pace adjustment
171 to 180: 8% to 10% pace adjustment
Above 180: hard running not recommended

The above are the pace adjustment percentages to use for continuous runs.  For repeat workouts such as 400’s 800’s, or mile repeats, I recommend using half of the continuous run adjustment as the body has a chance to cool somewhat during the recovery between repeats.


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