  Halo coronal mass ejections (HCMEs) originating from regions close to the
center of the Sun are likely to be geoeffective. Assuming that the shape of
HCMEs is a cone and they propagate with constant angular widths and velocities,
at least in their early phase, we have developed a technique (Michalek et al.
2003) which allowed us to obtain the space speed, width and source location. We
apply this technique to obtain the parameters of all full HCMEs observed by the
Solar and Heliospheric Observatory (SOHO) mission's Large Angle and
Spectrometric Coronagraph (LASCO) experiment until the end of 2002. Using this
data we examine which parameters determine the geoeffectiveness of HCMEs. We
show that in the considered period of time only fast halo CMEs (with the space
velocities higher than $\sim 1000{km\over s}$ and originating from the western
hemisphere close to the solar center could cause the severe geomagnetic storms.
We illustrate how the HCME parameters can be used for space weather forecast.
It is also demonstrated that the strength of a geomagnetic storm does not
depend on the determined width of HCMEs. This means that HCMEs do not have to
be very large to cause major geomagnetic storms.
