Running in high temperatures and high humidity can prove challenging to the fittest athletes. A myriad of factors likely contribute to summer-time struggles and below I'll break it down:
Humidity is the biggest factor that contributes to poor performance and potential danger during your run. 85° Temps with less than 45% relative humidity will have a real feel of 85°. But 80° Temps with a relative humidity of over 70% puts that close to a real feel of 100°!
When running, your body releases sweat that evaporates from the surface of your skin to cool itself down. The problem is that humidity (excessive moisture in the air) doesn't allow the sweat to evaporate thus rendering your body inefficient at cooling it's core temperature. When this happens, your body goes into a “survival mode” which sends excess blood flow to organs and to aid other vital process within the body. As a result, your heart rate rises causing effort to increase. Your digestion almost completely ceases making it really challenging to absorb calories or fluids and can result naseau. Your risk for muscle cramps and side stitches increases dramatically. Finally, your brain temperature rises and this can impact the body's ability to assess and regulate your body's core temperature; this is the main contributing factor to heat related illnesses.
What can you do?
Your body adapts to the demands of heat and humidity and learns be be more efficient, but running will still be a challenge and potentially even dangerous. My athletes come to me with this question, and I wish there was a magical answer, but there isn't one.
First and foremost: Prepare for success, the old adage that if you fail to prepare, you're prepared to fail has never been truer. Make sure you're hydrated. I personally prefer Spring Energy's Electroride mixed with 16-20oz of water. Make friends with your foam roller.
Slow Down! Understand that performance and paces will be impacted in the high heat. Run by effort, not pace. Pay attention to your heart rate if you have the technology to do so. This part is hard for many people.
Take your training indoors. Gyms and fitness centers are, or should be, climate regulated. This may help you salvage some quality in your training but also will be much safer.
Fatigue, headaches, nausea, tingly skin and dizziness are all symptoms of heat related illness such as a heat stroke. Call it quits in this case - always.
It's not all bad though, as several studies suggest that the stressors of high humidity and heat can mimic the effects of altitude training. Training smart and properly during the brutal summer months will prime you for PR season in the fall when temps and humidity drop off.