What Headwind Costs: Watts, Seconds and Expectations
Wind is not a symmetric variable. Attempting a PR into 15 km/h of headwind spends a training day with no realistic outcome.
The asymmetry of wind
Tailwind shortens the time it acts on you; headwind lengthens it. So headwind works on you for longer and costs more than the same tailwind gives back. On an out-and-back course you are always slower in wind than in still air.
How many watts offset headwind?
To hold your still-air speed into 12 km/h of headwind on flat ground you need roughly 60 to 100 watts more, depending on CdA and baseline speed. For most riders that is simply not available.
- 5 km/h headwind: around 25 to 40 watts more
- 10 km/h: around 50 to 80 watts more
- 20 km/h: 120 watts and up — not a PR day
Use headwind properly
Headwind days are excellent training days: steady resistance, no coasting, very consistent threshold stimulus. Use them for intervals and save your mental energy for the day the direction lines up.
In short
Headwind days are training days. Save peak form for the hours when wind direction matches the segment.
Frequently asked questions
Can a better position offset headwind?
Partly. A more aerodynamic position lowers CdA by maybe 5 to 10 percent, which is worth only a few km/h of wind.
Is an attempt worth it in light headwind?
Below about 3 km/h of effective headwind a PR is still possible in good form. Above that the probability drops fast.