jan,

I had a nice graphic that explained the range of effects on Reynolds Number from
the "fall of dew" to Mach 3. Unfortunately I haven't found it - yet.

But getting away from airplane wings for a moment,

How fast does dew fall?
How big is a droplet of water in the morning dew?

At that scale, viscosity (stickiness of the air - and yes, air is "sticky")
effects are the major forces involved.

Accumulate more water into a raindrop.
Now there is enough mass for much higher velocities.
Now the inertial forces predominate.
The wake left behind is fairly clean, but noticeable.

Freeze a bunch of that into hail stones!
Lots of mass - much higher speeds.
Now we start seeing funny things in the shape of the wake - swirling
vortices.

http://en.wikipedia.org/wiki/Von_K%C..._vortex_street
Notice how the animation shows the wake whipping back and forth.


Get enough mass and size, and altitude to fall (ie much higher speeds),
and we see the air ahead of the object starting to pile up - can't get
out of the way fast enough - resulting in super sonic shock waves.

It's all the same air.

The differences are how long (chord length) and fast something is
going through it.


Or, as another pointed out, increase the density of the fluid.