I live in Colorado and the forecast high is 90F (32.2 C) with 17 percent humidity. Colorado is high desert and Denver’s annual total precipitation from rain and snow is 14.3 inches. On days like today we say, “At least it’s a dry heat.” And in the winter when it’s in the teens, “At least it’s a dry cold.”
John Marsh suggested a column, “Explaining the effect of humidity on heat tolerance. I know a lot of people who aren’t used to riding in humidity are really affected by it when they go elsewhere to do rides and tours. You really need to acclimate to it as you might with altitude.” John lives in Atlanta, GA where the forecast high is 90F (32.2 C) with humidity of 50 percent.
You know that riding when you are hot is harder—harder to maintain a target pace, harder to put out more power to climb, and harder to ride farther. And the higher the humidity, the harder it is to ride.
Interestingly, it doesn’t have to be very hot for performance to decline. In one experiment, trained cyclists rode at 70 percent of VO2 max in lab temperatures of 4, 10, 20 and 30C, (39, 50, 68, 86F). They were able to maintain that effort for the longest time at 10C (50F), and performance declined progressively after that. Thus, overheating can be an issue even if you ride in temperate conditions that most of us would consider ideal for cycling!
Why Do You Get Hot?
Seems obvious, right? It’s hot outside! Actually, you don’t overheat just because the ambient temperature is high. You overheat in several different ways and understanding these will help you perform better in the heat:
- Energy production. The human body is only 20 to 40 percent efficient, which means that only 20 to 40 percent of the energy you get from eating is translated into forward motion. If you are a 160 lb. (73 kg) rider pedaling at 14 mph (22.5 km/h) for an hour, you are burning about 500 calories. Only about 100 to 200 calories are producing forward motion, and the remaining 300 to 400 calories are producing heat! You have to dissipate this heat or your core temperature will rise. Energy production and heat dissipation are the primary factors in overheating, whether you are riding on a 90F (32.2 C) day or climbing hard when it’s only 60 F (15 C).
- Radiation. When the sun is out, you gain heat from direct radiation, as well as from radiation reflected from the pavement. You may also gain heat from radiation through diffuse clouds. Your body also radiates heat even when it’s hot outside. The higher the sun is in the sky the more radiation heats you up.
- Respiration. You may dissipate heat when you exhale if your breath is warmer than the environment or if it’s really hot out you may actually gain heat through breathing. Hot air feels harder to breathe.
- Conduction. You also gain heat through hot parts of your bike. On one Race Across AMerica I sat on a black gel-filled seat cushion all day, and through conduction I developed second-degree burns on my butt, which caused me to drop out of RAAM.
Sweating—Your Body’s Primary Cooling Mechanism
About 85 percent of your body’s cooling comes through the evaporation of sweat into the air from the skin, and directly from your skin, which radiates heat to the air. The harder you exercise, the more sweating is the dominant cooling mechanism. Because the primary source of heat is your working muscles, internal cooling to keep your core organs cool is much more important than external cooling. Think of your body like a car’s engine. Most of the cooling is provided internally by coolant, which flows through the engine and then through the radiator. Only a little cooling comes from heat given off directly to the air around the engine. Your blood is the coolant, your muscles are the engine and your skin is the radiator.
As you become more acclimated to riding when you are hot, you start to sweat at a lower core temperature, and you sweat more, which keeps your core temperature lower. Your total blood supply increases so that you can sweat more while still maintaining blood flow to the muscles.
Why Humidity Is a Problem.
Relative humidity indicates how saturated the air is with water. The mixture of air and water is usually expressed as a percentage; a higher percentage means that the proportion of water is higher, i.e., it is more humid. When air is described as being 0 percent humid, it’s totally dry. If your body is trying to evaporate moisture into the air, conditions are great.
If the humidity is 100 percent (i.e. the air is as saturated as it can possibly be with water—think of a steam room), it’s impossible for sweat to evaporate, so it merely pools on the skin and drips off, taking very little heat with it in the process.
Relative Humidity and Heat Index
At 13 percent relative humidity like here in Colorado sweat evaporates so fast I don’t even know I’m sweating. With John Marsh’s 50 percent relative humidity in Atlanta sweat evaporates much less readily. If conditions are hot and the relative humidity is much above 40 percent, sweat evaporates less effectively and any significant level of exertion feels hard because heat dissipation is poor.
Because of the effect that humidity has on how hot it feels, the Heat Index combines temperature and humidity to indicate a “feels like” temperature. You can use this “feel like” temperature to make decisions like what clothing to wear and whether it makes sense to exercise.
For example, according to the Heat Index, if the air temperature is 80F (26.6C) and the relative humidity is 35 percent, then the Heat Index temperature is the same as the actual air temperature i.e. 80 F (26.6C). In Atlanta at 90F (32.2C) and 50 percent relative humidity the heat index is equivalent to 95F (35C). In Colorado at 90F (32.2C) and 17 percent relative humidity the heat index is only 86F (30C).
Here’s where to calculate the heat index.
How hard you can ride is a function of both how readily sweating is keeping you relatively cool and also of psychology. In an experiment a group of trained cyclists were asked to ride at the same perceived effort they would expend if riding a 20 to 40K time trial. They repeated the trial rides at 15, 25 and 35C (59, 77, 95F). Based solely on perceived effort, their power output and speed declined as the temperature rose.
Effect of Acclimatization
Because sweating is the primary cooling mechanism as you adapt to the heat your body improves how you sweat:
- Your sweat rate increases (maybe more so in high humidity)
- Your blood plasma volume increases (to assist with cooling and blood supply to the skin and muscles)
- You start sweating earlier (i.e. after a smaller increase in core temperature)
How to Acclimatize
You can bring about these changes through long endurance rides in hot conditions. However, few of us have the time to do many long rides
Research shows that you can also adapt if you ride so that your core temperature rises 1 to 2C (1.8 to 3.6F) for a period of 60 to 90 minutes. The fitter you are, the faster you will acclimatize. Fortunately, you don’t have to train with a thermometer to acclimatize! Simply ride hard enough to sweat heavily for 60 to 90 minutes. If you are already very fit, you can achieve significant acclimatization by riding this hard on four successive days. If you aren’t quite as fit, you may need as many as eight sessions with a rest day in between each session. If you live in an area where the summer isn’t hot, but will be traveling to a hot place to ride, you can acclimatize by riding with enough clothing so that you sweat heavily, which is particularly easy if you ride a trainer in the sun without a fan for cooling.
You may be tempted to spend most of your time in an air-conditioned environment and just ride for a few hours at a time in the heat. However, you don’t acclimate fully unless you spend a lot of time in a hot environment, i.e., acclimate passively.
Always be cautious with heat acclimatization training. It’s easy to overcook it and heat illnesses are potentially life-threatening if you’re not careful. According to the Mayo Clinic the symptoms of heat exhaustion in addition to heavy sweating include:
- Cool, moist skin with goose bumps when in the heat
- Heavy sweating
- Weak, rapid pulse
- Low blood pressure upon standing
- Muscle cramps
If you start to develop any of these symptoms get off the bike, get into a cool environment and drink cool (not ice cold) beverages to start to lower your core temperature. If you don’t start to cool down then take a cold shower. Dealing with heat exhaustion is critical – if untreated you can progress to heat stroke, which can be fatal.
Whether you live in a humid area or will be traveling to a humid area the higher the heat index the harder it is to ride. You can adapt to riding in a humid area by acclimating just as you would acclimate to ride in a less humid but hot environment.
Riding in a humid environment accept that it feels harder. As noted above riding at the same level of perceived exertion performance declines as the temperature / heat index rises. You can’t go as fast or put out as much power as you can in cooler conditions. Because of this ride by perceived exertion instead of by speed, heart rate or power.
Riding in a humid environment follow the same rule of thumb on hydration: drink to satisfy your thirst but don’t force yourself to drink.
- Cycling in the Heat 101
- 12 Myths About Cycling and Hydration
- Learning from the Pros: Heat and Hydration
- Heat Tolerance and Aging
- Heat Training Improves Bicycling Performance
- How to Ride Safely in the Summer Heat
Cycling in the Heat bundle
My two-part eArticle Cycling in the Heat Parts 1 and 2 explains more about the science of riding in the heat, and managing your fluids and electrolytes, in my two-part eArticle series:
- Cycling in the Heat, Part 1: Ride Management is19 pages and covers how to acclimate to hot conditions, how to train in hot months, what to wear, eat and drink, how to cool down if you overheat, and how to deal with heat-related problems.
- Cycling in the Heat, Part 2: Hydration Management is 21 pages and covers how to determine how much you should drink depending on your physiology and sweat rate, how best to replace your fluids and electrolytes, the contents of different sports drinks, how to make your own electrolyte replacement drinks, how to rehydrate after a ride, and how to deal with hydration-related problems.
The cost-saving bundled eArticles Cycling in the Heat Parts 1 and 2 totaling 40 pages are just $8.98 (a 10% savings).
Coach John Hughes earned coaching certifications from USA Cycling and the National Strength and Conditioning Association. John’s cycling career includes course records in the Boston-Montreal-Boston 1200-km randonnée and the Furnace Creek 508, a Race Across AMerica (RAAM) qualifier. He has ridden solo RAAM twice and is a 5-time finisher of the 1200-km Paris-Brest-Paris. He has written nearly 30 eBooks and eArticles on cycling training and nutrition, available in RBR’s eBookstore at Coach John Hughes. Click to read John's full bio.