The Rangiora Tramping Club asks :-
Why does the North-West Arch occur?
Sam Dean, an atmospheric physicist at the University of Canterbury, responded.
The Northwest Arch is the name given to a cloud which is regularly seen on the east coast of the South Island, particularly over the Canterbury Plains. Its name derives from the distinct leading edge of the cloud which appears sharply from clear air and runs parallel to the main mountain range. The arch is a visual illusion. Because of perspective the straight line of cloud appears to be an arch.
The arch occurs when northwest wind flows over the alps, as usually occurs ahead of a cold front. The air is forced to rise up over the mountains thus cooling the air. This cooling can often be sufficient for water vapour carried in the air to condense into clouds. As the air is forced higher it may well condense enough water for rain to fall, which is one reason for the high levels of rainfall seen on the West Coast and in the mountains of the South Island. Upon reaching the other side of the mountains the air descends back down to the plains, warming as it does so and evaporating any clouds.
However under certain conditions the air will rebound back upwards from the plains. This `jump' can often be seen in rivers downstream of a rock. In the atmosphere this downstream wave is created by the presence of the mountains. As the air climbs again, it can once more cool to form a cloud. This is also why the Northwest Arch is often described on television as being a `wave cloud'. The result of these processes is that often the mountains and the west coast are covered in cloud, with a band of clear air where the descent is occurring on the eastern side before more cloud forms downstream in the wave.