The Northwest Arch

Northwest Arch seen from Tekapo

The Nor’ West Arch is one of Canterbury’s most famous cloud formations! Often associated with Foehn (‘fern’) winds, it’s accompanied by hot, dry and windy weather.

Foehn winds are the term used to describe the hot, dry air that blows from the direction of the Southern Alps (a north-westerly wind) onto the Canterbury Plains. It often brings uncomfortably warm temperatures to those on the east coast, with Christchurch recording some of its warmest temperatures under these conditions.

So, what causes the Foehn winds?

When we have a westerly wind flow across the South Island, moist air from the Tasman Sea is pushed towards the Southern Alps. The Alps act as a massive barrier to this oncoming air, and when winds are strong enough, this air is forced up and over this barrier. As this air is being forced to rise higher into the atmosphere, it cools. Colder air cannot hold as much water vapour/moisture, so clouds form along the western side of the Alps, and it begins to rain.

Once this moisture has fallen, and if the winds are strong enough, the air moves over the mountains and sinks down onto the eastern side of the Alps where the Canterbury Plains lie. It is now a lot drier than it was in the west, and with the aid of the suns heating, as it sinks it begins to warm again. Because of its lack of moisture, it warms up much faster than before and by the time it reaches the low-lying plains, it is very warm and dry, and often gusty as it speeds down the terrain.

So how does the Nor’ West Arch form?

The Arch is formed because the air that is being pushed up and over the barrier of the Alps doesn’t flow smoothly over the terrain; it forms a wave in the atmosphere, like that in the ocean. The air at the ‘top’ of the wave is much cooler than the air at the ‘bottom’ of the wave, so it condenses into a cloud at the top and evaporates at the bottom. This forms a large lenticular cloud, also known as a ‘pancake cloud’ due to its sometimes stacked look (like a plate of pancakes). The strong north-westerly winds keep the cloud in place and stretch it across Canterbury, with a distinct gap remaining over the Alps where blue skies often shine through. This creates the beautiful Nor West Arch.