The idea

Heat and humidity make a given pace cost more effort. Running at your cool-weather goal pace on a hot day pushes you into a harder zone than the workout intends, which is how easy runs turn into slogs and long runs fall apart late. Pheidi keeps the effort the same and slows the pace instead, so a hot-day easy run is still an easy run.

What drives the adjustment

  • Temperature. Below 15°C (59°F) there's no penalty. Above it, your target pace eases progressively.
  • Dew point. Humidity is the bigger story, so we use dew point rather than relative humidity. A high dew point adds an extra penalty on top of the temperature one, and the two compound when both are high. (In manual mode you enter a humidity percentage and we convert it to a dew point for you.)
  • Race distance. The marathon is hit hardest because you're in the heat for hours; a 5K tempo at the same temperature gets a smaller adjustment than a marathon long run.
  • Age. Older runners are more affected by heat, so the adjustment scales up modestly with age.

The model is the same one behind our public heat-adjusted pace calculator, calibrated against large marathon datasets. Adjustments are capped at 20%. Beyond that, conditions are about safety, not splits. Very small adjustments (under about 1%) aren't shown at all, since the pace change would be invisible.

Where you'll see it

  • Day view. A note explains the conditions (for example, "Hot and humid at 32°C. Treat the adjusted pace as your ceiling"). If you use VDOT paces, the pace range shown is already heat-adjusted.
  • Week view. A small flag marks any run that's been adjusted, with the approximate slowdown.
  • Morning email. If you get the 5 AM reminder, the heat note rides along with it.

Adjustments only appear for runs in roughly the next week. In automatic mode that's as far as a forecast is reliable; in manual mode it keeps the standing conditions you set from spreading across your whole plan. Your RPE and effort target never change. Only the clock does.

Choosing how it works

Open Settings and pick one of three modes under Weather Pace Adjustment:

  • Off. The default. Paces are never adjusted for weather. Pick this if you train indoors, race in a steady climate, or just don't want it.
  • Automatic. Set your location (search by city or ZIP code, or tap "Use my location") and upcoming runs pick up your local forecast automatically.
  • Manual. Enter a typical high temperature and, optionally, the humidity. No location needed. Pheidi applies those conditions to your upcoming runs until you change them, which is handy when you already know it's a hot stretch or your forecast source isn't your race location.

Automatic forecasts come from Open-Meteo. Your coordinates are only used to look up the weather; if the lookup ever fails, plans simply show their normal paces.

Weather pace adjustment lives inside the Pheidi app — sign in to enable it.