What does the weather cost mean?

Reading it

In Map your run, the big number under the run’s name is the weather cost, such as +3 min 25 s, with “weather cost at your pace” beneath it.

• Ideal: the weather costs under 30 seconds, and RunFast says “The weather costs you nothing today”.
• A minus, such as −1 min 10 s: the wind is behind you, and the weather makes you faster.
• On the right: your start and finish times at your pace, and “about 09:47 with the weather”, your finish time with the cost added.

What goes into it

• Heat and humidity, together as heat stress (WBGT). Marathon times are fastest at a WBGT of about 7.5 to 15 °C (46 to 59 °F). Above that, runners slow by about 0.4% for every degree Celsius, and below it by about 0.3%. Slower runners lose more, up to about twice the rate. See heat stress levels.
• The wind: a headwind costs you the extra effort of pushing through the air at your speed. A tailwind gives some back, at most 5%.

Rain isn’t in the cost. It has its own tile, and it can make the run Fair or Tough.

The hills aren’t in the cost either. They’re already in your split times. See setting your pace.

Stretch by stretch

• The route’s colours show where the weather bites. See why the route is coloured.
• Drag along the profile to see the cost on any stretch, such as “+8 s/km (heat)”.
• Kilometre by kilometre: the WEATHER column gives the seconds each kilometre (or mile) costs you.

The rating beside it

Each run is also rated Good, Fair or Tough, with the reason underneath, such as “Heat stress High from km 7”. The cost tells you how much; the reason tells you why.

Where else you’ll see it

On your runs and races, on club pages, when you follow someone live, and in the evening-before email.

It’s a guide, not medical advice: race flags and medical teams come first.