Each Sunday we’ll look at a weather term and explain what it is and why and how it occurs…

What are anticyclones or a ‘high’?

An anticyclone is a large atmospheric circulation system with the wind flowing counter-clockwise in this part of the world.They have been a rare sight at times this summer across the whole of the nation!
It’s associated with relatively high pressure at the surface and in the lower troposphere.
Anticyclones usually form when an airmass is cooled over a cool ocean surface (such as the Tasman sea), or over land during the autumn, winter, or spring when not too much strong sunlight is around.
The cooling causes the air mass to shrink , becoming quite a bit thinner. This shrinkage then helps surrounding air in the upper troposphere to fill up the extra space.
The added weight of the extra air causes higher pressure at the surface. The higher pressure air at the surface then tries to flow outward towards lower pressure, but as it does, the rotation of the Earth turns the wind to the left down here in the Southern hemisphere, resulting in anti-clockwise wind direction around the high pressure.
In New Zealand anticyclones don’t always deliver fine and sunny weather, as cloud or fog can be associated with them from time to time.

If you have any weather related questions, please let us know by replying to this story.