QUESTION: How hot is too hot to be out riding my bike? I came home the other day from an afternoon ride and felt a little sick for the rest of the afternoon, with no appetite. – Arnold W
RBR’S STAN PURDUM REPLIES: If you’re looking for a number, start with a feels-like temperature of 90 degrees (Fahrenheit), which is about where many forecasters start warning about dangerous heat, but several factors can move that limit up or down.
Feels-like temperature takes into account not only the ambient air temperature, but also the relative humidity and wind speed to determine how weather conditions feel to bare skin. As such, it’s a better measure of what effect the weather is likely to have on you.
For example, here are the projected air and feels-like temps in my area for this June day, according to weather.com:
| Time of day | Air temp | Feels-like temp |
| 7 a.m. | 68 | 68 |
| 8 a.m. | 72 | 72 |
| 10 a.m. | 80 | 86 |
| 11 a.m. | 83 | 90 |
| 2 p.m. | 90 | 97 |
| 4 p.m. | 90 | 97 |
| 5 p.m. | 88 | 96 |
| 8 p.m. | 85 | 93 |
Clearly, if I want to ride today, it would be better to ride in the morning than in mid afternoon or evening.
Bear in mind, however, that these numbers don’t account for the fact that road cyclists are usually riding on blacktop, which on high-temp days can get hot enough to fry food, and which radiates heat, raising the temperature of the air immediately above it. So road riders can assume their feels-like temperature (as well as the actual temperature) is a bit higher.
The factors that can move the “too hot” limit up or down include:
- Fitness. This refers not only to your general health, but also to your overall athletic condition. The better your fitness, the greater the likelihood you can handle higher temps when riding.
- Heat you are used to. People who regularly work out in higher temperatures adjust to it to some extent and can usually tolerate more heat than those who aren’t accustomed to such efforts.
- Shade. You may have some routes that have more shade than others, and on a blazing hot day, you can choose one of those. For example, near me is a paved trail that is in shade most of its distance. And there’s enough leafy canopy that it’s sheltered from the sun almost all day. The temperature on that trail can be as much as 10 degrees lower than it is in direct sunlight. On very hot days, I can still ride that trail.
- Terrain. Pumping up hills raises your body temp a lot, and the downhill cruise that follows usually isn’t long enough to dissipate all the heat. So choose a flatter route on hot days. If, however, you need to ride hills, be prepared to stop frequently — even on the way up — to let your body temp return to normal.
- Flexible distance rides. A friend of mine has a “hot day” loop route of about 10 miles that includes two parallel mostly flat country roads with some connecting roads that make for easy shortening of the ride. He drives to the loop and is never more than a few miles from his car. If he’s handling the heat well, he pedals the whole loop two or three times. If the heat becomes too intense, he shortcuts on a connecting road and gets back to his car quickly.
- Using an ebike. While you still get plenty of exercise riding an ebike, you can keep your exertion level lower by using the assist function, thus, keeping your body temp down. What’s more, since the assist enables you to ride faster than you might pedaling a regular bike, you’ll make more breeze as you move. (It will still be as hot when you stop, however, so don’t let the assist lull you into thinking you don’t need to drink as often.) And, finally, if you the heat starts to get you, you can bump up to a higher assist level and get home quicker.
While “feels-like” is a useful number for deciding when it’s too hot to ride, here are a few other weather-measuring scales that you may find helpful as well:
Heat Index. A chart that uses both air temperature and humidity to arrive at a number that gives a better indication of how the weather will affect you. For example, if the temperature is 90 but the humidity is 70%, the heat index number is 106 degrees. But if the humidity is only 5%, the heat index is 84 degrees. The heat index is calculated in the shade, however, which limits its accuracy for riding on the road.
RealFeel. This is AccuWeather.com’s proprietary “feels-like” temperature that considers humidity, cloud cover, winds, sun intensity and angle of the sun when arriving at a number. The latter measurement is useful in determining what time of day to ride, since the angle of sun at different times of day determines how intensely its heat is felt.
Wet Bulb Globe Temperature. A parameter that estimates the effect of air temperature, relative humidity, wind and solar radiation. Often used by the governments, military, OSHA and athletic organizations to manage workload of individuals in direct sunlight. See more here.
If you are riding in hot weather often, it’s a good idea to keep a chart with the air temp and feels-like temps and notes on how you felt and performed under those conditions. The chart will help you on subsequent days when deciding whether it’s too hot to ride.
Stan Purdum has ridden several long-distance bike trips, including an across-America ride recounted in his book Roll Around Heaven All Day, and a trek on U.S. 62, from Niagara Falls, New York, to El Paso, Texas, the subject of his book Playing in Traffic. Stan, a freelance writer and editor, lives in Ohio. See more at www.StanPurdum.com.
When I was a mere child, in my early 60’s, I’d ride in any temperature. At age 66 or so, on a hot hilly ride in KY, I started to feel a bit queezy and blamed a rest stop snack. NOPE! I did not learn until after the crash that mild nausea is an early symptom of heat exhaustion. (I finished the ride with a badly bruised scapula on one side and bruised ribs on the other. No, I slept very little the following 6 weeks or so!) As one ages, one does lose heat tolerance. I’ll do rides of 15 miles or less in 90+ degree weather (mostly “commuting”), but my days of 30-70 mile rides are over.
Richard, heat tolerance is one of the main reasons I bought a ebike. Not just the heat but combined with Arkansas high humidity I was (unknowingly to me) dealing with some heat related problems that was becoming detrimental to my health. In my younger days I rode mtn. bikes year round until my early fifties and then started rode biking during the hotter summer months. I finally realized at 66 that I couldn’t continue riding like that and started looking at ebikes. For me it was a game changer. Having the different levels of assist I can control my output thereby controlling my heart rate. I usually ride with least amount of assist but as it gets hotter and heart rate raises especially on the hills where you slow down and very little or no wind cooling affect I adjust the assist level up lowering my heart rate thereby keeping me from over heating and prolonging my rides. I’ve ridden many 60 to 80 miles on it of which I would not have been able to do without it. I’m really enjoying this bike for my longer and faster group (usually 5-15 yrs. younger than I) rides. I still ride my Roubaix for recovery rides and shorter less hilly group rides. Anyways, just an idea you might think about.
Left out of this discussion is that cycling creates its own breeze and cyclists are covered in sweat, nature’s best air-conditioning. I’ve ridden Sanderson to Del Rio in 100 degrees. That’s over 110 miles.
I think the cutoff temp for cycling is determined by the user, within reason.
I ride first thing in the morning. Not only is it typically cooler, but there’s also typically less breeze/wind to tackle, at least in Central New York State where there are vastly open farm fields that are typically unforgiving in windy conditions.
I’m an above knee amputee of 32 years and exert beyond what most can’t even fathom. My prosthetic does not for any perspiration, making me warmer even more quickly. Other people with medical conditions may contend with variables that determine when they ride. And ample hydration is a must.
Interesting….here in the Phoenix area where I live, during most summer days, the low temp is above 90 degrees. Yet, we and others often ride 30-50 miles without incident. Hydrate and necessary electrolytes..and we have riders over 80 years of age.
So, it can be done…with care….and acclimation…
Walt, your “we and others” are exceptions to an life-threatening rule. Our club in Phoenix has several people like you. Infinite heat tolerance and NO understanding that this is not the norm. They’ve led group rides in the late morning where riders have almost DIED, and learned nothing from the experience. They continue to mock the rest of us who do very short rides and are off the bike before 8 AM on hot mornings, or prudently cancel rides if the overnight low is above about 85.
Please get it through your head that you are putting yourselves in serious danger. If not you specifically, then others who may ride with you and are unaware.
You need to recognize that relative humidity has a HUGE effect on this. In dry conditions, your sweat evaporating removes over 500 calories of heat per kg of water. In humid conditions, your sweat just drips off and much of it doesn’t evaporate so you get no cooling effect. The old phrase “It’s a dry heat” is really meaningful. People who ride in dry conditions (like Phoenix) have no idea what it’s like to ride in hot weather and high humidity.
It’s not the heat – it’s the humidity. Sorry to bring up this cliche but it is very true. Here in humid SE Virginia, riding around the Dismal Swamp, it often times feels cooler riding in the late afternoon or evening even if the temperature is higher than in the morning. Morning temps of high 70’s are typically right at the dewpoint so no sweat evaporates from your skin when riding. 10-20 degrees hotter and the RH is lower so the sweat will evaporate and keep you cooler.
This cooling breeze when the RH is a little lower helps a lot to keep you cool. I also run a lot and can say that the 20 mph constant breeze of the bike makes things a lot cooler than a 7-8 mph breeze while running. Around here running in the dead of summer with high dewpoints is very difficult and riding the bike is much more comfortable. I’ve been running in Vegas in the 100’s and it is much cooler than riding here at the dewpoint temperature.
I live in Dallas TX and the temperatures during the summer months can be quite hot. Riding in the morning helps. My rule is home by noon. However, humidity tends to be higher in the morning but feels like temperatures are definitely cooler in the morning. If the noon predicted temperature is approaching 98 F I will ride without a helmet. Your body heat is regulated best by allowing the heat escape from the top of your head. It would be nice if some manufacture would develop a better helmet for summer riding, until then I will take my chances without a helmet for a couple weeks during the year to avoid heat exhaustion.
I don’t understand Fahrenheit, please include Celsius.