Sprint performances are recorded alongside a wind reading, and marks assisted beyond a defined limit cannot be ratified. The rule exists because air resistance is a substantial part of the task.
Air resistance scales sharply with speed
The force of air opposing a runner rises with the square of the relative speed between the runner and the air, so it grows quickly at sprinting velocity.
A following wind reduces that relative speed, which reduces the resistance and allows the same physical effort to produce a faster time.
The effect is large enough to alter results meaningfully, which is why a limit was needed rather than a note on the result sheet.
The measurement is narrow by necessity
Wind is measured for a defined period during the race, at a set height and position beside the track, using a gauge that records the component along the running direction.
That single reading represents conditions across the whole race, though wind in a stadium is turbulent and varies between lanes and along the straight.
A legal reading is therefore an approximation, and two athletes in the same race can experience genuinely different conditions while sharing one recorded figure.
Stadium architecture shapes the air
Stands, roofs and gaps in the bowl channel wind into patterns that bear little relation to the conditions outside the venue.
Some stadiums are known for swirling conditions and others for producing consistent following winds along the home straight, which affects where records tend to be set.
Because the gauge sits in one location, these local variations are not captured, and the reading can understate or overstate what the athletes actually encountered.
The rule applies unevenly across events
Wind readings apply to sprints, hurdles and horizontal jumps, where the athlete moves in a consistent direction and the assistance is straightforward.
Events run around a bend for their full distance are exempt, on the reasoning that a wind helping on one side of the track hinders on the other.
That reasoning is imperfect, since the hindrance and the assistance do not cancel exactly, but no better rule has proved workable.
Assisted marks still count for placings
A wind-assisted performance wins the race and counts for qualification, selection and prize money. Only the record and ranking status is affected.
This means an athlete can produce the best performance of a career and have it excluded from the record books while keeping the medal.
Altitude assistance is treated separately and is not excluded at all, which is an inconsistency the sport has never resolved.
