This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
From: Jobst Brandt <email@example.com>
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.