Weather Watch

Weather analyst Philip Duncan checks the forecast and the story behind the temperatures

Weather Watch: An Aucklanders' guide to cold stuff falling from sky

By Philip Duncan

Pupils from Bombay Primary School have fun with the falling snow. Photo / Brett Phibbs
Pupils from Bombay Primary School have fun with the falling snow. Photo / Brett Phibbs

This year, Christchurch residents have become used to having a new word in their language - liquefaction. This week, it was Auckland's turn to have a new word added to their vocabulary - graupel.

My dusty Collins English Dictionary doesn't have the word but it is widely used in meteorological circles - and in other countries, such as the US and Canada, where it's common in winter.

The small white, soft, pellets look like a cross between hail and snow ... and that's basically what they are. Lots of people witnessed snowflakes falling in Auckland on Monday as our historic snowstorm roared across the country - but more people would've seen graupel as it was far more widespread.

So graupel, sleet, hail, snow, freezing rain - what are all these things? Here's a quick guide:

Graupel - starts with a snowflake then as it falls to the ground super-chilled droplets of water start to freeze to it. The end result is a snowflake wrapped into a ball with ice on it - looks like hail but is light and soft.

Snow - this is water vapour that has started to freeze and stick together. Over time, as more frozen water particles stick together, it turns into a beautiful snowflake - each flake is unique and, when you look closely, they have that snowflake design.

Sleet - this is snow that partially melts as it falls to the ground. It's a raindrop that's a bit slushy with melting soft ice.

Hail - this is rain drops that are frozen. Hail can be as small as a ball bearing and - in summer with those huge thunderstorms - as big as a grapefruit (thankfully not that big here in NZ). Those balls can be deadly to humans and livestock if you get smacked on the noggin by one.

Freezing rain - I saw this term used quite a bit this week. Freezing rain isn't actually ice when it falls from the sky. It's simply rain that freezes when it hits a super-chilled surface. This happens when the ground is extremely cold but the air above is warmer. When I was in Canada a few years ago, a warm front moved in after a cold night. It was raining on the windshield of the car but turned to icicles on the metal body of the car - and roads were like skating rinks - you couldn't even stand up in some places the ice was so thick and slippery - yet it was raining.

- Herald on Sunday

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