Why Your First Hot Run Feels So Hard
In extremely hot and humid weather, your body's ability to cool itself is challenged.[4] Humid air makes it worse: the heat index measures how hot it feels when humidity is added to the air temperature, and full sunshine can add up to 15°F to how the heat feels.[3]
So the same run that felt fine last week asks more of you now. Cooling is competing with the run itself, and that is the whole reason it feels harder, and the reason to expect less from yourself on those days. It is not lost fitness.
That is also why the first hot spell of the season hits hardest. Your body has been running in cooler air, and it meets the heat unprepared. A hot day later in the summer is the same weather on an adjusted body.
It gets easier. Bodies adjust with repeated heat exposure, which is why getting used to heat starts with the first hot day.[3] Give it a couple of weeks of regular hot runs and the same day feels noticeably different.
How to Change a Hot Run
Work through this order until the run feels manageable:
- Run the cooler hours. Schedule runs earlier or later in the day when the temperature is cooler.[1] Our guide to the best time to run today helps you compare windows.
- Start slow. Pace the activity and pick up speed gradually.[1] On hot days, slower is the honest pace.
- Take walk breaks. Taking breaks when you can is part of running in heat.[2] Walk breaks count as part of the run, not as quitting.
- Use the shade. Stay in the shade as much as possible, and choose outdoor activity in the coolest parts of the day or evening.[2]
- Drink more water than usual, and don't wait until you're thirsty. Muscle cramping may be an early sign of heat-related illness.[1] On a hot day, carrying a bottle on a short run makes sense.
- Dress light. Wear loose, lightweight, light-colored clothing.[1] Our guide to what to wear running today covers clothes by temperature.
Most of these changes stack: slower plus shade plus walk breaks is how a hot run stays comfortable. If the whole day is hot, a shorter run beats skipping it.
Let RunOutNow Check Your First Hot Spell
When recent local temperatures suggest this is the first sudden heat, RunOutNow assumes your body has not adjusted yet and rates the heat more cautiously, and it says why. A familiar-looking hot day may get a stricter warning early in the season.
You can compare start times across the next 24 hours for your full run window. During an unusual hot spell, the app's idea of a mild window becomes more cautious too, and the outfit follows the same window.
Current weather and outfit advice work without an account.
When Heat Should Change Your Plan
There is no single temperature that is too hot for everyone. What makes a hot day hard is the jump from recent weather, the humidity, and how your body answers it. A first hot day deserves more care than the same day later in the season, and the same rule works in reverse: a hot day after hot days finds your body already partway there.
If you feel faint or weak, stop and get to a cool place.[1] Heavy sweating, dizziness, nausea, or a headache are signs to stop and cool off.[4]
In the app, serious heat gets an explicit warning or a suggestion to change or postpone the run. It never blocks a run. Shortening or skipping a hot run is a smart adjustment, not a failure.
RunOutNow does not guarantee safety. Check the tips in the app before you go out, then use your own judgment and common sense.
Common Questions
Is 70 degrees too hot to run?
No. What matters is the jump from recent weather: the first 70-degree day feels harder than the same 70 after weeks of warm runs.
Can you run in 90 degree weather?
You can, with the changes above: cooler hours, shade, a slower pace, walk breaks, and more water. If you feel faint or weak, stop.
Why does humidity make running harder?
Humid air stops sweat from cooling you as it should, so your body sheds heat less efficiently. The heat index shows how much hotter it feels.[3]
What should I wear running in the heat?
Loose, lightweight, light-colored clothing.[1] Our guide to what to wear running today covers it by temperature.
Your next step: On your next hot run, pick the cooler end of your window, start slower than usual, and let RunOutNow check whether this is a first hot spell before you head out.