 Since Sydney borders the Blue Mountains (part of the Great Dividing Range), it would experience a Föhn effect, which is a dry westerly wind originating from the Great Dividing Range that raises the air temperature in the lee of that mountain range and reduce atmospheric moisture.This dry wind occurs because of the partial orographic obstruction of comparatively damp low-level air and the subsiding of drier upper-level air in leeward of the mountains.As the moist air rises, it cools and it would condense, thereby creating precipitation on the upwind slopes.