Subject: Slope Wind, the Invisible Enemy
From: Jobst Brandt

Wind as well as relative wind caused by moving through still air demands most of a bicyclists effort on level ground. Most riders recognize when they are subjected to wind because it comes in gusts and these gusts can be distinguished from the more uniform wind caused by moving through still air. That's the catch. At the break of dawn there is often no wind as such but cool air near the ground, being colder and more dense than higher air slides downslope as a laminar layer that has no turbulent gusts.

Wind in mountain valleys generally blows uphill during the heat of the day and therefore pilots of light aircraft are warned to take off uphill against the morning slope wind. Slope wind, although detectable, is not readily noticed when standing or walking because it has negligible effect and does not come in apparent gusts. The bicyclist, in contrast, is hindered by it but cannot detect it because there is always wind while riding.

Slope wind, as such, can be up to 10 mph before it starts to take on the characteristics that we expect of wind. It is doubly deceptive when it comes from behind because it gives an inflated speed that can be mistakenly attributed to great fitness that suddenly vanishes when changing course. If you live near aspen or poplars that tend to fan their leaves in any breeze, you will not be fooled.

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