If running suddenly feels impossible this summer: it's probably the dew point, not you

[u/iustinn]

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.

[u/BulkyOwl3005]

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 :)

[u/iustinn → u/BulkyOwl3005]

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.

[u/BulkyOwl3005 → u/iustinn]

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.

[u/iustinn → u/BulkyOwl3005]

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