Cycling in Hot Weather

Check the heat index rather than the temperature, start shorter than you think, and carry two bottles. Those three things cover most of what goes wrong on hot rides. The heat index is the National Weather Service’s measure of “how hot it really feels when relative humidity is factored in with the actual air temperature”, and their own worked example is the reason it matters: 96°F at 65 percent humidity produces a heat index of 121°F.

That is not a detail. Your body sheds heat by evaporating sweat, and humid air cannot accept much more water — so the cooling system you are relying on quietly stops working while the effort stays the same. A 96-degree ride in dry air and a 96-degree ride in humid air are different events.

The first hot week is the dangerous one

Nobody publishes a heat acclimatization protocol for cyclists. OSHA publish one for people new to working in heat, and the principle transfers cleanly, so we would rather borrow a good rule than invent one:

• Start at about 20 percent of your normal duration on the first hot day — OSHA’s own wording is that new workers “should work only 20 percent of the normal duration on their first day”.
• Add about 20 percent a day until you are back to a normal schedule, which takes roughly a week.
• Reduce duration, not intensity. OSHA are specific about this: the body adapts by doing realistic work for shorter periods, not easy work for the usual length of time.

Their reason for the rule is the part worth sitting with. OSHA report that almost half of heat-related deaths at work happen on a worker’s first day, and over 70 percent in the first week. The risk is not the hottest day of the year — it is the first hot day, before you have adapted, which in cycling terms is the morning you ride your usual loop at your usual effort in a heat wave.

The warning signs, from OSHA’s own list

These are the six heat illnesses OSHA name, with their published symptoms. We have added what to do on a bike; the symptom lists are theirs, not ours.

One rule covers all of them: confusion is the emergency signal. Heat stroke is the condition on that list that kills, and OSHA list confusion, slurred speech and loss of consciousness among its symptoms. A rider who has stopped making sense needs 911 and active cooling, not encouragement to finish the ride.

How to ride a hot day

1. Go early.Air temperature peaks in the afternoon and road surfaces hold heat afterwards, so early morning is both cooler and — usually — less humid, which is what decides whether sweat can evaporate.
2. Plan the route around water. Two bottles is roughly an hour of drinking in real heat. A loop that passes a store, a park tap or your own front door beats a point-to-point that does not.
3. Shorten it, and keep the effort honest. Shorter at normal effort beats long and easy, both for adaptation and for the simple reason that you are closer to home when it goes wrong.
4. Use shade deliberately. A tree-lined road at 2pm is a different temperature from open blacktop. On a hot day the shaded route is worth the extra distance.
5. Tell someone, or carry a phone.Heat illness impairs judgement, which is exactly the condition in which being alone and unreachable stops being a minor detail. The League of American Bicyclists’ ride-smart guidance on riding predictably applies doubly when you are tired and hot.

The two mechanical things heat changes

Tire pressure rises. Air expands as it warms, so a tire inflated to the sidewall maximum in a cool garage sits above that after an hour on hot asphalt. The effect is modest at sensible pressures and is one more argument for setting pressure from your weight and tire width instead: road pressure and gravel pressure.

Sweat corrodes.It is salty, it drips onto the stem, headset and bolt heads, and it works on them for the rest of the summer. A wipe-down after hot rides is a thirty-second job that bike owners discover the value of about two years too late — washing a bike properly covers the rest of it, including the places water should not go.

What this guide will not tell you

We do not publish hydration volumes, electrolyte advice or a temperature at which you should not ride, because the sources we trust do not publish them for cycling and your medical history is not something a bike site knows. What is here is what the National Weather Service and OSHA actually publish, applied to riding a bike, plus the kit that makes a hot ride more comfortable.

If the real problem is that summer has made your rides unpleasant rather than risky, the gear page is the shorter answer: seven pieces of summer kit and what each listing publishes. And the same logic in reverse — dress for the conditions, start shorter than you think — is how the cold-weather page handles the other end of the year.