Sprint marks are accompanied by a wind reading, and a time can be excellent yet ineligible for records. The measurement exists because air resistance is a large part of the work.
Why wind matters so much
A sprinter at full speed spends a substantial share of total effort simply displacing air, and the cost rises sharply with velocity.
A following wind reduces the relative speed between runner and air, cutting that cost and allowing the same physical effort to produce a faster time.
The effect is large enough to matter at the margins that separate performances, which is why it cannot be ignored in events decided by fractions of a second.
How the reading is taken
A gauge positioned beside the track measures wind along the direction of running for a defined period covering the race.
The figure reported is an average over that window rather than an instantaneous reading, so a gust can be partly offset by a lull.
Only the component along the track is counted, which means a strong crosswind can register as almost nothing even though runners clearly felt it.
The threshold and why it exists
Records require the assisting wind to sit at or below a fixed limit, and a mark set above it is recorded as wind-assisted rather than disallowed.
The limit is a compromise, set low enough to prevent obviously aided performances and high enough that records remain achievable in ordinary conditions.
A headwind carries no restriction at all, since running into wind can only make a time slower and therefore cannot flatter anyone.
Which events are affected
Wind readings apply to sprints up to two hundred metres, the sprint hurdles, and the horizontal jumps, where the run-up speed determines the result.
Longer races are exempt because the athletes run in both directions around a track, so an assisting wind on one straight becomes a headwind on the other.
Straight-line events have no such cancellation, which is precisely why they need the measurement in the first place.
The limits of the measurement
A single gauge cannot capture conditions across an entire track, and swirling wind inside a stadium bowl can differ between lanes.
Stadium architecture influences this considerably, with some venues channelling air along the home straight and others sheltering it almost entirely.
The rule is therefore a practical approximation rather than a precise correction, and it is accepted as such because a better one would be difficult to apply consistently.
