Summer Runs: How to Acclimate to the Heat

With summer weather comes summer running, and with it a period during which your body needs to adjust to the higher temperatures and, in many regions of the country, drastically increased humidity. Knowing how these atmospheric changes affect you physiologically can help you prepare and ensure that you race safely and with minimized detriment to your endurance performance.

The body is better at handling external cold than external heat, and exercise raises internal temperatures in addition to the outside weather. The two problems with external heat are increased body temperature, which immediately affects performance, and dehydration, which imposes a more gradual drag on your ability to function in hot conditions.

Performance starts to suffer at just three degrees over normal temperature, and in a race, runners are not inclined to back off to meet this unsettling fact. This is where acclimatization comes in to help; a properly acclimatized body makes adaptations to allow for optimal performance when it recognizes that the challenge of extreme heat is present.

The Basic Timeframe

Runners respond differently in a battle with heat. One study Jack Daniels conducted found that some runners perspire twice as much as others—in identical heat conditions and with matching body composition, weight, and running speeds. It takes about two weeks of training in warm conditions to acclimate properly, and it is important to know when to train in these conditions.

You must start this two-or-more-week process with runs early in the morning or late in the evening. These are the coolest times, though morning is the most humid and evening generally hotter than the cooler morning. Eventually, you need to put in runs at the time of day and under the warm conditions in which the race will be occurring.

What to Expect

Cooling occurs when sweat evaporates off the body. To achieve this, the body diverts blood to the skin to cause sweating—this means less blood is carrying oxygen to the exercising muscles. In this way, the body reduces the amount of blood available to enhance performance.

When conditions are above 69 degrees Fahrenheit, even well-acclimated runners should expect slower race times. The following chart appears in Daniels' Running Formula and will help you determine just how you'll be compromised for longer race distances.



Racing for 2 hours 10 minutes

Racing for 2 1/2 hours

Racing for
3 hours

Racing for 4 hours


+2 minutes

+2.5 minutes

+3 minutes

+4 minutes


+4 minutes

+4.5 minutes

+5.5 minutes

+7.5 minutes


+6 minutes

+7 minutes

+8.5 minutes

+11.5 minutes


+8 minutes

+10 minutes

+12.5 minutes

+17.5 minutes

After calculating what pace you think you can manage at a given temperature, run about 2/3 of the race at that pace and then if you are feeling good, you may increase it a bit.

Minimizing the Effect of Heat

Fluid loss is influenced more by time spent running than distance run. You can compensate by weighing yourself without clothes on before and after runs in various conditions over a set period of time. Doing this will give you an idea of how much fluid you ought to be replacing to stay safe at, say, a 2 percent net fluid loss. Longer times out in the heat simply become a multiple of this calculated replacement amount.

Dress for heat by wearing as little clothing as possible, and keeping it loose fitting. Porous fabrics are best. Keep in mind that a sun-protecting brim may be helpful, but a cap can make your head much hotter, so a visor is preferable.

Dry vs. Humid Climates

Failure to replace fluids becomes more of a problem in dry climes. Negative effects on performance begin to occur with a loss of 3 percent of body weight due to fluid loss. At 5 percent, expect to be severely affected. In dry heat, you may fail to notice that you are sweating because, as with high altitude, fluid does not drip off the body in the amounts noticeable in more humid environments. Be especially mindful of fluid replacement in a dry climate, and remember in general that your perceived desire for fluid does not keep up with the body's needs.

It's important to also understand the effects of humidity. When sweat evaporation cools the skin, circulating blood is also cooled. This process is the key to maintaining a reasonable body temperature. In humid climates, sweat evaporation, and therefore cooling, can slow to a standstill. The warm weather heats your body, exercise adds to an increase in body temperature, and the humidity keeps you from cooling.

By staying aware of these effects and starting the acclimatization process, your body will adapt more readily to higher temperatures, making it safe and more pleasant for you to continue your regimen and to race, even if compromises in race times and in training intensity are inevitable on the hottest days.

Daniels' Running Formula by Jack Daniels, PhD, 1998, Human Kinetics, Champaign, IL, pp. 186, 190-198

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