Running pace, translated for the day you're actually in.

Each ring is a separate cost, sized by what it is taking from you right now. Change anything above and watch them move.

Nothing about the runner changes between the top of this and the bottom of it. Same legs, same fitness, same intention. The only thing moving is the air, and the number moves with it.

A modelled summer day for the city you picked above: the temperature swings on a curve through the afternoon, the dew point barely moves — which is the whole problem. Every pace on it is the real engine.

Same legs, same fitness, same intention, four days that actually happen somewhere every year. The pace on each is what Meridian says that air, that ground and that altitude will support — computed here, in your browser, by the engine that runs in the app.

Four sets of climate normals, one threshold pace, and the same model the app uses. Nothing here is an illustration.

The pace you can hold at a given effort is not one number, it is a curve. This is your effort from the calculator above, priced across a whole thermometer — and the reason it bends rather than tilts is that heat costs almost nothing until the body stops being able to dump it, and then it costs a great deal.

A three-hour marathoner does not have a three-hour marathon everywhere. This is the same runner, in the same shape, on a typical summer day in each of these cities — and the honest finishing time each one supports.

• Today's paceYour planned session, translated into the conditions you're actually running in. Temperature, dew point, wind, elevation, altitude.
• Anywhere on earthCompare today against ideal conditions, your goal race, or any city you're travelling to.
• Race dayThe pace you want against the pace the course and forecast actually support — with mile-by-mile splits and fuelling that scales with the heat.
• It learns youConnect Strava and Meridian works out how you respond to heat, not how the average runner does. Until it knows, it says so.

Nobody has ever stood in a start pen and converted their body mass into a sodium target. Meridian works in rates internally and then rounds hard, because your sweat rate could be four times another runner’s and it has not measured yours. What comes out is what to eat, roughly when, and how much to drink — and it moves with the forecast, because heat slows your stomach down at exactly the moment you are asking more of it.

It also tells you when the answer is nothing. An easy hour before breakfast does not need a plan, and the recovery window is mostly marketing unless you are running twice. See a full race-day plan →

Two runners of identical fitness can lose two minutes or six over the same hot marathon. An average can't tell you which one you are.

Your own history can. That's the one thing a web form will never have.

I spent a summer watching my paces fall apart, unable to tell whether I was losing fitness or just running in soup. Nothing I found answered that honestly.

So I started building the thing I wanted. First real test is New York, 1 November. — Max

It tells you which conditions are driving the adjustment, and it says modeled until your history has earned it the right to say personalized.

Not a training plan. Not a social feed. Not another dashboard. One number, one sentence, out the door.