A sprint time is not accepted as a record unless the wind behind the runner falls within a permitted range. The rule exists because air resistance is a large part of the task.
How much resistance a sprinter overcomes
At sprint speed, a substantial share of the runner's energy goes into pushing air aside rather than into forward motion. Drag rises steeply as velocity increases.
A following wind reduces the relative speed between the runner and the surrounding air, which lowers that drag directly. Less energy is spent on the same forward progress.
The benefit is therefore mechanical rather than psychological. Two identical performances in different wind conditions produce different times.
Why a threshold exists rather than a ban
Wind is never precisely zero outdoors, so requiring still air would make records essentially unachievable. A tolerance is unavoidable.
The permitted following wind is set at a level where the advantage is considered small enough to disregard. Beyond it, the assistance is treated as material.
The limit is a convention rather than a physical boundary, chosen so that legal conditions occur regularly at ordinary meetings. It balances rigour against practicality.
How the measurement is taken
A gauge is positioned beside the track near the finish, at a specified height and distance from the lane. It records the component of wind along the running direction.
The reading is averaged over a defined period corresponding to the race rather than sampled instantaneously. Gusts are therefore smoothed into a single figure.
Because a single gauge cannot capture conditions along the whole straight, the measurement is a representative sample. It is accepted as the standard rather than as a complete description.
Which events the rule applies to
Wind readings govern sprints up to a defined distance and the horizontal jumps, where a run-up directly determines the result. Longer races are exempt.
In events run partly around a bend, the wind changes direction relative to the athlete during the race. A single reading cannot represent it, so the rule is applied differently.
Combined events use a separate standard, since athletes cannot repeat a discipline to secure a legal reading. The rules accommodate the format.
Why altitude is treated separately
Thinner air at elevation reduces drag in the same way a tailwind does, and the effect is larger over a full race. Times at high altitude are consistently faster.
Records set at elevation are nonetheless ratified, though they are often annotated. The sport chose to accept altitude while regulating wind.
The inconsistency is acknowledged and defended on practical grounds, since altitude cannot be measured away race by race. Wind can be, and so it is.
