  High speed solar wind streams (HSSs) are very efficient drivers of
geomagnetic activity at high latitudes. In this paper we use a recently
developed $\Delta{H}$ parameter of geomagnetic activity, calculated from the
night-side hourly magnetic field measurements of the Sodankyl\"a observatory,
as a proxy for solar wind (SW) speed at monthly time resolution in 1914-2014
(solar cycles 15-24). The seasonal variation in the relation between monthly
$\Delta{H}$ and solar wind speed is taken into account by calculating separate
regressions between $\Delta{H}$ and SW speed for each month. Thereby, we obtain
a homogeneous series of proxy values for monthly solar wind speed for the last
100 years. We find that the strongest HSS-active months of each solar cycle
occur in the declining phase, in years 1919, 1930, 1941, 1952, 1959, 1973,
1982, 1994 and 2003. Practically all these years are the same or adjacent to
the years of annual maximum solar wind speeds. This implies that the most
persistent coronal holes, lasting for several solar rotations and leading to
the highest annual SW speeds, are also the sources of the highest monthly SW
speeds. Accordingly, during the last 100 years, there were no coronal holes of
short duration (of about one solar rotation) that would produce faster monthly
(or solar rotation) averaged solar wind than the most long-living coronal holes
in each solar cycle produce.
