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The Pressure Gradient Force is always directed from high pressure to low pressure, perpendicular to the isobars.
The Coriolis Force is always directed 90° to the right of the motion of the air parcel (in the Northern Hemisphere).
The force of friction (if applicable) is always directed in exactly the opposite direction as the motion.
In "geostrophic balance", there is no friction. The Pressure Gradient Force and the Coriolis Force are opposite and equal. Air parcels move PARALLEL to isobars, with low pressure to their LEFT and high pressure to their RIGHT (in the Northern Hemisphere).
If friction is a factor, the winds are slower, so the Coriolis Force is weaker. The Pressure Gradient Force will always exceed the Coriolis Force under these conditions, so air parcel MOSTLY follow the isobars, but they somewhat cross the isobars TOWARDS LOW PRESSURE.
Copyright 2005 Jon Schrage