It's the Heat AND the Humidity

We knew it was coming.  We knew St. Louis summer would arrive in full force at some point.  So far, our summer months seem to have gone: April, May, May, May, October.  You knew we couldn't escape without a run of July and August thrown in.  It was fun while it lasted, but now we must turn our attention to dealing with an actual Midwest summer scorcher.  Although the last couple of summers should have taught us how to deal with heat and humidity, it's always good to sit through a refresher course.
First of all, let's discuss why heat and humidity affect runners so much.  When we run our body temperature is raised, so we must cool ourselves to keep from overheating.  The way that occurs is that some of our blood flow is diverted from our running muscles to our skin, causing our sweat glands to perspire.  As our perspiration evaporates, heat is taken away - and we cool off.  It's a pretty cool way to keep us running efficiently.  The hotter it is out, the more blood flow is needed to the skin to try to keep our engine from overheating.  If there is more blood going to the skin, however, there is less going to the muscles we use to run - and we have to run harder to go the same speed.  What does this mean in hard numbers?  Temperatures between 60 and 75 degrees will cause our heart rate to climb by around 2 to 4 beats per minute.  When the temps reach 75 to 90 degrees, you're looking at around a 10 beat per minute increase.   That's significant.
How does humidity play its role in this whole thing?  Well, we've all heard the saying, "It's not the heat, its the humidity."  I guess we kinda shot a hole in that one already.  Actually, it's the heat AND the humidity.  Humidity is the percent of water vapor in the air (as related to the amount that could be in the air).  The higher the humidity, the harder it is for perspiration to evaporate.  When our sweat doesn't evaporate, we don't cool, so our body sends more blood to our skin, which takes more blood from our working muscles.  This is why proper hydration is a topic that is much talked about this time of year.  If we are constantly sweating, we are losing valuable water and electrolytes.  The less water you have in your system, the thicker your blood and the worse it does to keep you cool, transport nutrients, clear cellular waste, etc. 
Does that mean we have to hang the running shoes up at this time of year? Absolutely not. We just have to adapt and overcome. Our bodies are amazingly adaptable and resilient. We can get through this and continue to train towards our goals.  We just need to make some accommodations to continue training and racing at an optimal level.
Don't be a Slave to the Garmin 
As we said, the heat alone causes our heart rate to increase compared to optimal weather.  Add in the humidity and you're likely to be in a completely different Heart Rate Zone when running at your cool-weather training and racing paces.  This is a great time to practice running by feel.  I often recommend running for minutes and by feel... especially on our easy/recovery runs.  Your body doesn't know how far or how fast you ran.  It simply knows you ran at this effort level for this amount of time.  Go exploring, freestyle your run to places (hopefully shaded) you haven't run before.  Just run your typical effort level for the amount of time your goal mileage run would've taken. 
If you do have to race, then use the follow graph from Runner's World as a guide on what to expect.
50–54 Very comfortable PR conditions
55–59 Comfortable Hard efforts likely not affected
60–64 Uncomfortable for some Expect race times to be slower than in optimal conditions
65–69 Uncomfortable for most Easy training runs feel OK but difficult to do hard efforts
70–74 Very uncomfortable Expect pace to suffer greatly
75+ Oppressive Skip it or dramatically alter goal
(Editor's Note: As a point of reference, on 8/27/13 the high temperature in St. Louis was 96 degrees.  The Heat Index high was 99 degrees - and the Dew Point maxed out at 71 degrees.  Dew Point is available on most online weather websites and smart phone apps.)

Bottoms Up
Hydrate, Hydrate, Hydrate.  Hydrating doesn't just refer to the time while running.  It's about consistently replacing and topping off our water levels.  This is an all day event.  Water is not the only thing lost while sweating.  We need to replace the electrolytes and salts we lose.  While running, I recommend adding a bottle or two to your water belt - or investing in one if you don't use one.  Make one of those bottles a concentrated electrolyte mixture to take swigs of along with water.  Since it's easier to find a drinking fountain than an electrolyte fountain, carrying your electrolytes with you is a good idea.
So that's what Sunrise/Sunset looks like...
Look for the times of day that have the best dew point.  Reschedule that 5 o'clock post-work run to a 5 am pre-work run.  Hitting the roads when the sun's not up is one of the best ways to continue to get our training in.  This also means we may need to search for our headlamps and reflective material.
Cool Clothes
Cotton is rotten... especially when the weather gets to percolating.  Light-colored, synthetic fabrics will help your body breathe and wick moisture from your skin to aid in the cooling process.   A hat or visor will help keep the sun from beating down mercilessly.  Also, if you do have to get your run in during the sunny part of the day, waterproof sunscreen is the way to roll. 
It's Always Cool in the AC
If the weather is just too much to handle, don't throw in the towel... bring it to the gym.  There are tons of ways to cross train to help get through a particularly nasty stretch of weather.  Treadmills, pools, spin classes, etc.; you can find plenty of ways to keep your fitness level up 'til Mother Nature welcomes you back outside with more comfortable weather.
Hot weather is almost a summer right of passage for those with big fall aspirations.  Be smart about your training, and you will come through this heatwave a smarter, stronger athlete.  Follow these few tips, and you can keep your training rolling right along. 
Good luck and happy racing!
Coach Cary

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