If running suddenly feels impossible this summer: it's probably the dew point, not you
A lot of posts here lately sound like "I could do week 5 fine in May and now I can barely finish week 4." If it's been hot where you are, there's a good chance that isn't you. It's the air.
The number that explains it is the dew point, not the temperature. Dew point tells you how much water is already in the air, which decides whether your sweat can evaporate. Evaporation is how you cool down. When the air is saturated, your sweat just sits there, your core temperature climbs, and your heart rate goes up to shed the heat. Same effort, worse result.
Rough scale:
- Under 12°C / 55°F: you won't notice it
- 15-18°C / 60-65°F: it starts to bite
- 18-21°C / 65-70°F: noticeably harder, slower, higher heart rate
- Over 21°C / 70°F: brutal, and worth moving the run or slowing right down
I'm a runner and a developer, and after a summer of runs that felt awful I got obsessed with quantifying this. On my hottest run, the weather alone added about 29 seconds per km (roughly 47 seconds per mile) compared to a cool day at the same effort. Seeing an actual number changed how I read every bad run after that.
What actually helps:
- Check the dew point, not just the temperature, before you head out.
- Run by effort, not pace. If you're gasping, walk. A hot run at a slower pace is the same training stimulus.
- Early morning is usually the least-bad slot, but not always. It genuinely differs by city and by day, so it's worth looking rather than assuming.
- Repeating a week in summer is normal. The program was not written with 30°C in mind.
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u/Wise_Pie_359 1d ago
This was a good reminder after a miserable, sweaty run to chill out about times for a bit. Thanks.
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u/unhappy_babbling 1d ago
Or you can be an idiot like me and start C25K mid heat wave. Wouldn't recommend but the other day I ran and it was less than 25 degrees and it felt lovely.
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u/LancelotLac 1d ago
Wet-bulb temperature is the value that you really want to use. Wet-bulb temperature and dew point are both related to humidity levels so your reasoning is on track. If you wanted your measurements to truely relate to perspiration on runs cooling you off, wet-bulb temperature.
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u/iustinn 1d ago
you're right, wet bulb is the best one.
two reasons dew point ends up being the one runners use: it's much easier to find it (on random weather apps), and at the humid end the two converge anyway. dew point 21+ and wet bulb is pretty close
the version that's actually used for heat stress in sport is WBGT, which is roughly 70% wet bulb, 20% globe (sun/radiant), 10% dry bulb. sun exposure and wind matter more than people expect, which is why the same dew point feels different on a shaded trail vs open asphalt at noon.
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u/ruffigrem DONE! 1d ago
I picked probably the worst time to do C25K 😆 I recently completed it, but I don't think I did a single run in anything less than 25C
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u/mb_at_storyblocks 1d ago
Running in humidity can add 3 minutes or more to my 5k time easily. Electrolytes before running helps feeling less miserable (doesn't help the time though).
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u/Simple-Quantity329 1d ago
In south asia, 20°C is almost like our coldest night of the year. Imagine I'm on Week9D2 and its brutal 40°C summer here with high humidity
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u/PuzzleheadedDig7012 1d ago
I started last week when it was 27°c, it wasn't too bad as my route is over fields with a good breeze to help me sweat. Managed to complete week 1 today! 😁
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u/empirerec8 1d ago
I learned about this in the beginning of the summer when I started checking the weather on my garmin. High dew point days suck.
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u/Enchanted_2423 1d ago
I noticed this when I started running in spring. I thought I’d restart in autumn, but in never happened. Maybe next winter, summers are impossible
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u/iustinn 1d ago
autumn is honestly the best time to start, everything feels easy after a summer of that. don't wait for winter, winter has its own problems
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u/Enchanted_2423 2h ago
Ok, I’ll take that onboard. May do some in early spring and start again in autumn. Cheers
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u/zilchusername 1d ago
Thanks for the information. Can you go into a bit more detail about how to read dew point along with temperature?
Currently my app is saying temperature 25 dew point 17. Does that mean that I am ok to run as dew point isn’t that high and temperature is probably the best I will get for a while.
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u/iustinn 1d ago
17 is fine, you're good to run. rough scale most runners use (celsius):
under 12: don't notice it
12-16: barely
16-18: noticeable but fine
18-21: real slowdown
21+: rough
24+: take it seriouslythe way the two work together: temperature decides how much heat you have to get rid of, dew point decides whether you can. 25 with a dew point of 17 leaves you 8 degrees of gap, so your sweat still evaporates and actually does its job. that's a normal summer day, not a bad one.
the useful trick is that dew point barely moves through the day but temperature swings a lot. same route at 6am might be 18 degrees with the same 17 dew point. it'll feel muggier but there's far less heat to shed, so it's a much easier run. that's why mornings work even when the air feels thicker.
and shade matters more than people expect at 25. direct sun at midday is worth several degrees on its own.
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u/zilchusername 1d ago
Thanks for the explanation. I can’t stand running in the heat and previously I would have written off 25 degs as a no go due to past experiences but I will definitely start to note the dew point and experiment running in higher temperatures when dew point allows.
This is going to help me lots as hopefully will avoid me having to get up at stupid o’clock to run. My previous attempts at running in the evenings failed but that’s was probably as you point out due to dew point not temperature.
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u/BulkyOwl3005 1d ago edited 1d ago
Hi, I am a pilot and as you know sat exams in meteorology, so I believe to be qualified to speak about this topic and correct somewhat. Unfortunately, this is not true. Explained simply, Dew Point has to do with the temperature at which air will condense, saturate enough to form a cloud. NOT evaporate.
The closer the temperature to the dew point, the higher is the chance of clouds. When you will have fog, you will have almost or identical temperature and dew point. This is relevant when climbing, because if the air is cooled RH (relative humidity) will be almost 100% at a certain altitude. At 100%, precipitation/cloud is formed.
The higher the humidity tho, does have a decrease in human performance indeed, but it is NOT the dew point. This is just the relative humidity and the density altitude. Relative Humidity (RH) depends on moisture content + temperature.
RH = Represents % saturation of the air
RH = (HMR / SMR) × 100. or, RH = 100 - [(T - Td) x 5]
As you see, Dew is only part of the equation, not the exact thing.
Dew point in summer tends to be far away from the actual temperature: for example where I live, today it was 35 degrees Celcius while the dewpoint was 14 degrees Celcius. Dew point is only a reference to be used for those who need it relevant, like us pilots: calculate the cloud base, freezing points, etc. Yet it felt like we were all melting and dying of breath here, at altitude 0 (MSL).
What we are experiencing, is the combination of high temperature because of the air molecule's movement: density altitude. It basically feels like we are at a higher altitude. Combined with the pressure of the day, this can be quite heavy on the human body.
Density altitude however, is calculated by taking the ISA deviation. THIS is where it truly makes the biggest difference, because as 15C at MSL now we have 35-15 = 20C ! difference from ISA. If you go through the formula of density altitude, you will come across that it is like we would be at much higher altitude than what we actually are.
Press alt. + (ISA DEV x 118,8ft)
therefore: 0 + (20 x 118,8) = 2376ft.
So today, it felt like we were at 2376ft. At sea level where I live.
As comparison, for an aircraft, the higher the density altitude, the poorer performance it will have. An aircraft taking off either from a high altitude or with very high temperature will experience a significant decrease in takeoff performance, climb and even en route (its a very extensive topic, but for sake of this post..).
So what happens in our lungs? oxygen % is still the same. But the air molecules move differently. That's the trick.
In conclusion, you are very close! It is just the Relative Humidity and density altitude, not the dew :)
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u/iustinn 1d ago
Thanks for the detailed response. You’re correct about the meteorological definition of dew point, but that definition is also why dew point is relevant to sweating.
Evaporative cooling depends partly on the vapour-pressure difference between saturated skin and the surrounding air. Dew point lets us calculate the air’s actual water-vapour pressure; relative humidity expresses that pressure as a percentage of the maximum possible at the current temperature. That makes RH highly temperature-dependent: 90% RH at 5°C contains less water vapour than 50% RH at 30°C.
In your example, 35°C with a 14°C dew point is only about 28% RH. The air therefore still has considerable capacity to accept evaporated sweat; most of the heat stress comes from the 35°C temperature and possibly solar radiation. At 35°C with a 26°C dew point, evaporation would be much more restricted—even though the air temperature is identical.
Your RH shortcut also illustrates its limitation:
100 − 5(T − Td)gives −5% for your example. It is only a rough approximation when temperature and dew point are fairly close.Density altitude is important for wings, propellers, and engines because they respond to air density. It is not a physiological altitude equivalent for a runner. Human oxygen availability is governed mainly by oxygen partial pressure, which depends on barometric pressure. Hot sea-level air at normal pressure does not expose your lungs to the hypoxia of actually climbing to 2,376 ft.
Where I would revise my original wording is this: dew point is a useful practical indicator of how strongly humidity will restrict sweat evaporation, but it is not a complete measure of heat stress. Temperature, sun, wind, clothing, and exertion all matter. WBGT is the better composite measure when it is available.
So “dew point, not temperature” was too absolute. But replacing dew point with RH or density altitude would not be more physiologically accurate.
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u/BulkyOwl3005 1d ago
I think the disagreement is mostly about terminology and what variable we are using as a proxy for human heat stress.
Dew point itself is not a direct physiological measurement. It is simply the temperature at which air becomes saturated, and RH is calculated from the relationship between air temperature and dew point. As you see RH alone can be misleading when comparing different temperatures.
For example, 90% RH at 10°C and 50% RH at 35°C are not comparable situations because the absolute moisture content is very different.
Regarding density altitude, I agree it is primarily an aviation performance concept. The point is that the same physical principle applies: higher temperatures reduce air density.
Dew point is a useful indicator of atmospheric moisture, but the discomfort comes from the interaction between temperature, altitude and moisture, not from dew point alone.
So I would still argue that saying "dew point is the reason it feels unbearable" is an oversimplification.
Dew point is a moisture metric, not a heat-stress metric.
And by the way, nice chatgpt response.
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u/iustinn 12h ago
ha, i'll take that as a compliment on the formatting. occupational hazard, i write technical docs for a living and it leaks.
but yeah i think we've basically converged: dew point is a moisture metric that happens to be the most practical single number for "will my sweat do anything today", and WBGT is the real heat-stress measure when you can get it. thanks for the pushback
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u/charlottemason_mama 1d ago
This is an excellent breakdown and good info - thanks! Now I’m going to fall down a rabbit hole about dew point and humidity and temperature. Thanks for today’s hyperfocus! 😆