Weather by ETA: why the time you pass each point changes everything
Meteorology
4 h
On a long ride, the wind at the real time of passage (ETA) can be the opposite of the wind at the start.
Planning with start-time weather introduces a systematic error into the power plan.
Most pacing tools treat weather as a still photo: they take the wind at the start time and apply it to the whole route. On a long event, that is simply false.
The wind changes while you ride
A 4-hour ride starts at 8:00 and finishes around noon. The 8:00 wind at the start has nothing to do with what will blow at 11:30 at km 110. Wind speed usually builds through the morning and its direction rotates. Planning with the start-line snapshot is planning for a race you are not going to ride.
What matters is not the wind at the start, but the wind at the moment and place you pass each segment.
What weather-by-ETA means
The idea is simple and powerful: first estimate what time you will pass each segment(your ETA, which depends on your power, your CdA and the profile), then pull the forecast for that specific hour at that point. So every segment of the plan uses its real wind, temperature and rain probability.
And it is far from irrelevant. Wind enters the power equation through speed relative to the air: a 20 km/h headwind on the wrong segment can cost you more time than a hill. The component that really matters is the effective wind projected onto your direction of travel (head/tail), not raw wind speed.
Why it improves the plan
- Smarter watt split: push into headwind segments and ease with a tailwind, just like climbs and descents.
- Realistic final time: if the wind picks up mid-morning, your ETA recalculates and the plan stops promising an impossible time.
- Air density: temperature and altitude change ρ, and ρ multiplies all your aerodynamic drag. Cold, dense air = more watts for the same speed.
- Rolling in the rain: when wet, Crr rises. Knowing which segments you will hit in rain sharpens the times.
How AthletePro does it
AthletePro Super Record computes your ETA per segment and queries the forecast (Open-Meteo) for that hour and that point, with multi-point sampling on long routes. You will see it in the plan as wind arrows on the map, a rider-relative wind rose, a yaw histogram and a segment-by-segment weather table. All of it feeds the power split and the estimated time.
Planning with your ETA weather is not a luxury: it is refusing to optimise for an imaginary race.