Detailed analysis of UVCS/SOHO observations has revealed that
a solar-maximum equatorial coronal hole (in 1999) exhibits 3 to 4
times lower oxygen outflow speeds than solar-minimum (1996-1997)
polar coronal holes, as well as approximately 2 times lower
perpendicular most-probable speeds.

Because the two types of holes exhibit similar wind speeds at 1 AU
(only about 15 percent different), the bulk of the wind acceleration
must occur above 3 solar radii for the equatorial hole.
Observations of coronal holes of different sizes, geometries, and
densities may be key to understanding how the high-speed solar wind
is heated and accelerated.

(Miralles et al. 2001, Astrophys. J., 549, L257)