Illustration showing the movement of air over the Southern Alps of New Zealand. Warm air carrying moisture blows across the Tasman Sea (left) and encounters the mountains on the South Island's west coast. It rises upwards as it hits the slopes of the mountains, cooling and causing the moisture in it to condense to form clouds. These clouds then release the moisture in the form of rain or snow and the air moves to the east. As it does so it warms and dries, forming a Foehn wind. The dry air on the other side of the mountains creates an orographic, or rain, shadow where there is little rainfall.