After a frigid winter and a fairly cool spring, and just as we welcome the first day of summer, temperatures are finally starting to ramp up in Wisconsin (93 ℉ today!). While I’m generally enjoying the warm weather, our marathon training is now in full swing, bringing with it much longer runs than I’ve ever had to do in the summer heat. So, I’ve been doing some research on the effects of soaring temperatures on running, and how to deal with them, and wanted to share some of my favorite facts, figures, and tips that I’ve come across with you!
1. You can easily determine how much fluid you are losing and how much you need to take in by weighing yourself before and after a run. The amount of weight you lose (converted to ounces; 1 lb = 16 oz) during an hour-long run (plus the amount of water/other fluids you drank in ounces) is equal to the amount of fluids you sweat out per hour. It is suggested that you drink every 15 minutes during a run, so if you divide your hourly sweat loss by 4, you can determine how many ounces you should be taking in each time you drink. This is a great way to quantitatively determine if you are getting enough fluids, but also to avoid over-hydrating, which can result in dangerously low electrolyte levels. Of course, keep in mind that the results of this test will change based on the temperature, the terrain, your pace, etc., so test in different conditions and adjust accordingly! For a more detailed procedure of this process, as well as examples of results based on weight and temperature, head over to Runner’s World.
2. As it gets hotter, you get slower. It is estimated that your finishing times (in a race) will increase by 1.5-3% for every 10° above 55 ℉. This may not sound like much, but it can account for almost an extra 2 minutes per hour at just 65 ℉, and an extra 5 minutes at 85 ℉. I should note, however, that these estimates are conservative. Jeff Galloway, based on his own experiences, suggests that degradation in performance happens much faster, and that you might expect to run up to 20% slower when temperatures get above 80 ℉. For training purposes, some recommend that you slow down 2 seconds per mile for every degree above 65 ℉, meaning that if it is 85 ℉, you should be running 40 seconds per mile slower than you would normally.
You will also slow down with increasing humidity, and, more importantly, dew point, which is the temperature at which water condenses. The higher the dew point (and the closer it is to the temperature of the air), the more saturated the air is with water, and the harder it is for sweat to evaporate off of your skin to cool you off. A dew point of 50-59 ℉ is the most conducive to running, but as soon as it rises above ~60 ℉, your running performance will begin to degrade. At dew points of >75 ℉, you should probably skip your run or find a treadmill in a nice air-conditioned room.
3. As temperatures rise, your heart will begin to pump faster, and you will feel like you are working much harder. This is because your body is attempting to cool itself through radiation, i.e., heat from your body is released to the cooler air around you (so this only works when temps are below 98.6 ℉). To accomplish this, more blood flow is directed towards your skin so that its heat can be radiated to the air, and, on hot days, this can require your heart to to circulate up to 2-4 times as much blood than on cooler ones. When you run in hotter weather, you can expect your heart rate to increase by 2-4 beats per minute (BPM) when temperatures are in the 60-75 ℉ range, and up to 10 BPM in the 75-90 ℉ range. As with temperature, these are conservative estimates, and some indicate that your heart rate may increase by 20 BPM or more, depending on the type of workout you are doing. In addition, because more blood flow is being directed towards your skin instead of your muscles, less oxygen is delivered to your muscles, giving you less energy to run.
4. Your body takes >2 weeks to adapt to running in hot weather. If your first few runs in hot weather feel more difficult, you are not alone, and, as it turns out, there’s a scientific explanation for it! Blood levels of a marker called heat shock protein 70 (Hsp70) are correlated with heat adaptation in endurance athletes. In a 2008 study, Sandstrom et al. took daily measurements of Hsp70 in the blood of an ultramarathoner during a 15-day, pre-race heat adaptation period. The researchers found that Hsp70 levels continued to rise through the end of the 15-day study period, simply suggesting that heat acclimation takes longer than 15 days. So, just like a new habit, building up your heat tolerance is going to take a few weeks, but it will get better!
5. Learn to recognize the symptoms of heat exhaustion and heat stroke in yourself and others. Signs of heat exhaustion include nausea or vomiting, fatigue, headache, disorientation/confusion, dizziness, fainting, pale skin, profuse sweating, rapid heartbeat, dark-colored urine, and muscle cramps/twitches. Signs of the more deadly heat stroke include most of the above, as well as a core body temperature >105 ℉, a cessation of sweating, red/hot/dry skin, muscle weakness, rapid/shallow breathing, and seizures. For heat exhaustion, you or the affected person should immediately get out of the heat, if possible to an air conditioned room, to rest and get hydrated (see more tips here). If things don’t improve within 30 minutes, you should contact a doctor. If you suspect you or someone else is suffering from heat stroke, call 911 immediately and begin first aid to try to lower core body temperature (see cooling tips here).
Meagan is a geochemistry research lab manager, runner, Netflix binge-watcher, and Mom to a rescue dog, a bunny, and a human child. She started running in May 2011 and ran her first half marathon in October 2012, followed by her first marathon in October 2013. In July 2018, she joined the triathlon world and completed an Olympic-distance race. After an extended break (pregnancy/maternity leave), she is making a long-overdue return to running and is preparing for a high-elevation half marathon at Crater Lake National Park in August 2020.