Wind is like uphills/downhills in that a tailwind never gives back everything a headwind of equal magnitude takes away.thehog said:
But for timekeeping, the worst case is when there's a crosswind out and back. Then you lose going both ways.
If you ride a 40 km TT @40 kph in a 10 kph 90° crosswind (whether it's out-and-back makes no difference), the resultant relative headwind for the ride is 1.2 kph. That's because the air you are moving through also is moving laterally, so by the time you've ridden the 40 km, the air where you began has been blown 10 km downwind. And the "tunnel" you've carved through the air extends from that point 10 km downwind from your start point, to your finishing point, 41.2 km away (Pythagorean theorem).
The extra 1 kph (from 40 kph) costs about 7% more energy. Although the impact of a crosswind is of a lower magnitude in relation to its full value than a headwind or tailwind, it is by no means negligible. Especially for an out-and-back, a direct crosswind is a worst-case scenario.