Foehn Wind

Have you ever wondered why the nation’s warmest temperatures tend to be in Canterbury or Hawke’s Bay? Northland is closest to the equator, so why is that not always the hottest area?

The answer has to do with wind, specifically, the Foehn wind. Also spelt Föhn but pronounced closer to fern. Foehn wind is used to talk about warm, dry air that blows from the direction of a mountain range. In Aotearoa New Zealand we usually have a westerly wind that flows across the country and a lot of our mountain ranges are orientated approximately north to south.

If the wind is blowing in from the Tasman Sea and is strong enough to be pushed up the Southern Alps, it is going to cool as it rises. When air cools to a certain point, it can no longer hold any more moisture and instead forms tiny liquid cloud droplets. And if it keeps rising, these droplets will eventually grow and fall out of the cloud as rain over the west side of the mountains.

Once the air has reached the top of the Southern Alps, it will begin to flow downwards into Canterbury or Otago. By this point, all the moisture has rained out. As the dry air moves down, it heats up more than it cooled while it was rising. This is because dry air warms up faster than high moisture air cools down, so it reaches a warmer temperature as it moves down the slopes in the east than it was when it started rising on the west side of the Southern Alps.

On 26 January 2021 there was a good example of how different the temperatures between the west and east coasts of the country could be. Hokitika on the west coast reached a high of 20.6°C, but Foehn winds helped push the temperature at Christchurch in the east up to 37.1°C, setting the January record at the time.