Positive ground flashes are a relatively rare occurrence in ordinary summer thunderstorms, but a higher frequency of 30% to 100% is found during winter thunderstorms in Japan and Norway, although both summer and winter thunderclouds have the same polarity of electric dipole with a net positive charge located above a net negative charge. We have computed the fraction of positive ground flashes occurring for various locations of lightning charges in a thundercloud, using the stepped leader model in which a streamer progresses in the direction of maximum potential gradient at its tip. The calculation is in good agreement with observations in that both the vertical wind shear and the height of the lightning charge contribute significantly to the occurrence of positive ground flashes.