The nor’wester

Associated with the heating and cooling of contracting and expanding air, is the hot north-west wind that hits the east coast areas of New Zealand, particularly Canterbury.  This ‘Foehn’ wind occurs when a moisture-laden wind comes from across the Tasman Sea (i.e. from the north-west) and over the southern alps. As it does so, a number of effects happen.

First, the moist air hits the wall that is the West Coast. Anyone who has flown from Auckland to Queenstown along the West Coast on a clear day (clear day? yeah, right) will have seen just how sharply the mountains rise up as you go back from the coast. The only place the air can go is upwards, and as it goes upwards, it cools. Cool air can hold less moisture than warm air, so it dumps it as rain. That pretty well explains why the west coast is so wet.

But now another effect happens – when water vapour condenses out of the air to form rain drops, heat is released. (Think about the reverse effect – to drive liquid water into vapour (evaporate it off) you need to put heat in from somewhere. That’s why sweating keeps you cool – the evaporating moisture takes heat from your skin. The reverse happens when rain drops form.)

So by the time the air has risen over the mountains, and lost its moisture, it isn’t as cold as you might think from its pressure alone. Now, over the other side, it falls again. Its pressure increases, and that means it gets hotter. By the time it hits Christchurch, it is warm and dry and nasty.

A nice little schematic explaining the effect is provided by the MetService.

Simplistically put, meteorology (the study of weather) is physics.

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