Dew Point Pacing Guide: How to Adjust Marathon Pace When It’s Humid

Dew point 101 (why it matters for runners)

Dew point is a measure of atmospheric moisture: it is the temperature air must be cooled to reach saturation. Higher dew point means more water vapour in the air. Meteorologists often use dew point because it reflects moisture content more directly than relative humidity.

For runners, the big issue is evaporative cooling. When moisture content is high, sweat evaporates more slowly, so your body must work harder to shed heat. That means a higher heart rate, a faster rise in perceived effort, and a greater chance of late-race slowdown.

Important nuance: dew point is not the whole weather picture. Sun, wind, and air temperature matter too. But dew point is one of the fastest, most practical humidity-stress signals you can use on race morning.

This is also where controlled training helps. Runners using Norwegian Singles for marathon training often benefit because better control of threshold and marathon effort makes it easier to switch from pace to RPE without panicking.

The dew point pacing table (simple)

This table is a runner-friendly way to decide whether you should follow goal pace or switch to effort-based pacing. Use it alongside your Race-Day Pace Adjuster for a fuller estimate.

When to switch to effort (the rule)

If you want one rule you can use without overthinking:

Extra “yes, switch now” triggers

• Air temperature ≥ 20°C and dew point ≥ 18°C
• Full sun with low wind
• Your heart rate is unusually high in the first 3–5K at what should be easy marathon effort
• You are already sweating heavily before 10K

If any of these are true, your best race is almost always the one with an intentionally boring first half.

If you want the decision tree for that moment in-race, use When to Adjust Marathon Pace Mid-Race and Should I Slow Down If My Heart Rate Is High?.

Race-day pacing script (HR + RPE + pace)

Use this as a practical script for humid days. It is designed to prevent early debt and survive heart-rate drift. For the pace-first version too, pair it with How to pace the first 10K.

How to use your tools on race morning

1. Check forecast dew point in your weather app
2. Plug conditions into Race-Day Pace Adjuster
3. Print pace band for the adjusted plan
4. Run the first 10K on an effort cap, not on ego pace

How to practise humid pacing in training

Workout 1: MP blocks with notes

• Long run 28–34 km, or scaled to your level
• Include 2 × 5–8 km at marathon pace with easy jog between
• Record average HR, RPE at start and finish, and whether pace cost rises late

Workout 2: RPE progression long run

• Start easy (RPE 4)
• Move to steady (RPE 5)
• Finish controlled (RPE 6), optionally near MP if the day is good

Workout 3: Easy drift check

• 90–120 minutes easy
• Observe: does HR rise sharply at the same pace? does pace drop at the same effort?
• This is a useful base-fitness + hydration-habit check

If you are comparing controlled threshold systems with more traditional steady work, see Norwegian Singles vs Tempo Runs.

Common mistakes (and fixes)

1. Using relative humidity instead of dew point.
   Fix: Use dew point for moisture stress; RH is temperature-dependent and often misleading.
2. Trying to force splits at dew point ≥ 18°C.
   Fix: Switch to effort pacing; let pace float while protecting early effort.
3. Overreacting to HR drift late.
   Fix: Drift is expected in long, warm efforts. Use RPE + mechanics to decide whether to push or stabilise.
4. Fueling too late.
   Fix: Start early and stay consistent; humidity increases strain and makes late saves harder.
5. Ignoring sun and wind.
   Fix: Dew point is a humidity signal, not the whole environment. Use the conditions adjuster for a fuller estimate.

If you repeatedly turn controlled sessions into grind sessions, that usually makes humid racing decisions worse, not better. In that case, read Who Should Not Do Norwegian Singles.

FAQ

Why is dew point better than humidity for pacing?

Relative humidity is relative to temperature and can be misleading. Dew point reflects moisture content more directly and is a more practical measure of how muggy it will feel once you are sweating.

At what dew point should I stop chasing goal pace?

A practical rule: at ~18°C (65°F) and above, switch to effort-based pacing from the start. At ~21°C (70°F) and above, many runners need to adjust time goals unless conditions are otherwise favourable.

Can I use a dew point table to predict exact pace adjustments?

Use the table to choose strategy. Exact seconds-per-km depend on temperature, sun, wind, acclimation, and fitness. For a tailored estimate, use the Race-Day Pace Adjuster.

Does dew point matter if the air temperature is cool?

It still affects evaporation, but overall strain depends on temperature, sun, and wind too. Dew point is a strong humidity signal, not the whole heat picture.

What is the best simple pacing plan for humid marathons?

Conservative first 10K with an HR/RPE cap, steady mid-race pacing by effort, then race the last 12K by RPE and mechanics if fueling is still working.

References

1. National Weather Service glossary: Dew point definition. NWS
2. NWS explainer: dew point vs relative humidity. NWS
3. NWS humidity discussion: dew point as a practical moisture indicator. NWS
4. Jenkins EJ, et al. (2023). Impacts of air temperature and humidity; absolute humidity governs evaporative rate. PubMed
5. Mantzios K, et al. (2021). Weather parameters and endurance running performance. PMC
6. Souissi A, et al. (2021). Cardiovascular drift during prolonged exercise. ScienceDirect
7. Bright FM, et al. (2025). Elevated humidity impairs evaporative heat loss and self-paced performance in the heat. PMC

Educational content only. Adjust pacing for your history, acclimation, and race-day conditions.