Canterbury weather

Bob McDavitt

Bob McDavitt has been a meteorologist since 1975, forecasting for marine, aviation and the general media around New Zealand and in Fiji. Until he retired in 2012 he was the MetService Weather Ambassador. He is a veteran of two campaigns for the America’s Cup (in Perth and San Deigo). In 1998 he was awarded the Henry Hill Award for his enthusiastic approach to sharing ideas about the weather.

Canterbury has endured some big snowfalls over the years including in 2006 when thousands of homes were without power and telephones for weeks. Farms and isolated households struggled to cope as business and transportation was affected. It was one of the most severe weather events for thirty years.

Photographs of the storm and its aftermath were captured in a commemorative book.

What's happening to our weather

Weather is something everyone’s got an interest in – be it choosing what to wear, or choosing what crops to plant or how to manage stock on a farm – it affects every day in our lives. So what does the year have in store? Are our winters getting colder? We talked to well-known weather personality and MetService Ambassador Bob McDavitt.

Bob talked about factors that affect our weather. Seasonal outlooks are based on looking at trends of sea temperature covering the Pacific Ocean and we are also affected by the sunspot cycle. The low point of the sunspot cycle encourages dryness and the high point encourages storms.

Are winters becoming warmer with global warming or colder with more severe events such as the 2006 snowfall? Or is it warmer on average with more severe cold snaps?
With global warming the southerlies are about a degree warmer now than they were 100 years ago. But they’re always going to be chilling in winter, because the South Pole is always going to be a refrigerator during its six dark months of winter. Cycles in weather are many and many-faceted, so that lots and lots of combinations are possible.
We are going through an epoch (20-30 years) where we are likely to have more La Ninas than El Ninos - this is called a Pacific decadal oscillation.
We often have frosts in the winter. How is the severity of a frost measured?
A grass minimum thermometer is used and every degree below zero is counted as one degree of frost, starting at -1C
slight frosts are -1 to -3
moderate between -3 and -6
severe between -6 and -9
and if it’s more than -9 it’s very severe
Is New Zealand weather harder to predict than say, Australia’s?
Weather is a mix of pattern and chaos. There is more chaos in the weather over the southern ocean than there is in the tropics and we are closer to the southern ocean than Australia. Also Australia’s interior is naturally arid, so has less weather. So the answer’s yes.
Several areas in Canterbury claim to have micro-climates — such as Lyttelton Harbour or Horotane Valley — what is a micro-climate, and how "micro" can it be?
A microclimate is an area where its attributes modify the incoming weather in a unique way. Every building, every garden, every tree, every human being is a microclimate.
Are super-computers the best way to predict weather?
For a scientist the way to understand the world is to observe what’s happening, ponder about the processes at work, run experiments to test things, and then report the findings. This is the scientific method and computers run the mathematical models based on data from the incoming observations.
There are other ways of weather forecasting involving astrology, but for me science is best. Weather is a mix of pattern and chaos, and five minutes after the data is collected for our computer run, someone somewhere lights up a cigarette, or there’s a bush fire or something chaotic that tweaks the weather into a new pattern that hasn’t been taken into account by the computer. So computer models have limitations.
The winning formula is to get the best idea (from MetService) of what’s coming your way from over the horizon, and then tweak this using your own observations within your horizon and your own knowledge of your own micro-climate.
What did we use before super-computers?
When I started forecasting we used much the same physics as is done by the computer, but we only had time in one shift to produce one "analysis" then one "prognosis" and then write out the forecasts for tomorrow. With computers we can go days into the future based on extrapolated trends from collected data.
Libraries are a great place for finding out more about weather and the way the earth works — are there any weather related books you can recommend to readers?
Cover of The New Zealand Weather BookThe New Zealand Weather Book by Erick Brenstrum.

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