Bicycles: Why the wind is always against you
On a cold, grey Danish summer evening, you stumble onto your bike and set off to the bar. The fresh wind, as always is in your face as you struggle up the hill. After many hours discussing the hottest science topics, you bike home, on the same road, expecting the wind to surely be on your back, but it's not; Aeolus' winds have changed direction and are again in your face. How can life be so unfair you ask? Your misfortune rests in the difference between true and apparent wind. Apparent wind is the wind we feel when we are in motion. It's a combination of the actual wind (true wind) and the wind created by moving forward. On a bike, that wind you feel on your face is apparent wind. Even if there is a slight wind at your back (the true wind) you can still feel the wind coming from in front of you (the apparent wind).
As you travel faster the apparent wind not only increases in speed but it also changes the angle of the wind. If the true wind is coming from any direction other than directly behind you, the apparent wind will always come further ahead. If the true wind is coming from either side of you, the apparent wind will feel stronger and ahead of you.
Figure 1. Relationship between true and apparent wind. Speed of movement will change the apparent wind direction and strength. Top left, true wind from behind, bottom left, true wind from the front.
The only situation in which the apparent wind will not make your journey more difficult is when the true wind is directly behind you. In the hypothetical situation in figure 2, you can see how the prevailing wind can fell against you on the way to and coming home from the pub.
Figure 2. If the wind is blowing from the North, the apparent wind will feel as if it is head on if you travel West or East.
For detailed information about the relationship between wind resistance and biking, read the paper by O. Isvan, called "Wind speed wind yaw and the aerodynamic drag acting on bicycle and rider." Published in the Journal of Science and Cycling, vol. 4, no. 1, pp. 42-50, 2015. Here's a link. You could also attach a sail to your bike and harness the power of the wind :)
Safe biking :)