What You Need to Know:
Name(s): Nor’ West Arch
Common Altitudes of Formation: Varous
Signals: Moist nor’ west flow over the South Island of New Zealand
Elaborated, more comprehensive information:
As New Zealand’s prevailing winds come from the west, depressions or their associating cold fronts coming from the Tasman sea tend to result in a moist nor’ west flow over the South Island. Because the Southern Alps stand between the east and west coasts, the mountains act as a barrier, the moisture laden winds rising over the Alps, expanding and cooling resulting in precipitation. Because the Alps act as a barrier the air mass is usually depleted of its moisture by the time it passes the Alps, presenting itself as the Canterbury Nor’ Wester and the Nor’West arch.
The Nor’West arch formation begins with individual Altocumulus Lenticularis clouds (formed from the air lifting over the mountains) which build into the arch as the Tasman Sea;s oncoming front approaches. As the frontal system approaches, upper-level moisture gathers, developing into an arch (of either altocumulus, altostratus or cirrostratus genera) with a distinct western edge. Because of the upper-level moisture, the area of the arch is usually coloured white or grey whereas the are below the arch is usually blue