Extreme weather

Each day, people talk about the weather. It affects our jobs, our leisure and our travel, so when meteorologists (scientists who forecast the weather) predict extreme weather events, it gets our attention.

Weather versus climate

The terms ‘weather’ and ‘climate’ are often used interchangeably even though they actually describe different things. Weather is the short-term state of factors like the temperature, precipitation and wind speed in a particular area. Climate refers to the average weather conditions over 30 years or longer. Weather can change daily, whereas changes in climate tend to be gradual.

Climate is what you expect, weather is what you get.

National Oceanic and Atmospheric Administration

New Zealand’s ‘weather bombs’

New Zealand has its share of extreme weather events. Although not technically correct, the media often refer to short, sharp storms that have gale force winds and heavy, often horizontal rain as ‘weather bombs’.

The more accurate term is ‘bomb cyclone’ – an explosive low-pressure system where the air pressure drops 24 hPa (hectopascal pressure unit) in 24 hours. Bomb cyclones can be very disruptive. Strong winds can affect air, land and sea transport, damage property and bring down electrical lines. Heavy rains may cause flooding and slips. In contrast, a true weather bomb in meteorological terms is a type of extra-tropical cyclone – like Cyclone Bola.

Rights: NIWA – National Institute of Water and Atmospheric Research

New Zealand annual rainfall

Topography influences regional rainfall in New Zealand. Mountains trap moisture from the westerly airstream.

Extreme weather events like bomb lows occur on top of existing climate patterns. New Zealand is a windy place. Much of the country sits in the Roaring Forties – a belt of westerly wind that wraps around the lower part of the southern hemisphere. The funnelling effect of Cook Strait, between the North and South Islands, adds to wind speed and strength. Our mountain ranges also influence the climate. They act as a barrier for the westerly winds, often keeping the clouds on the western side of the ranges. The South Island’s west coast and the Rimutaka Range average more than 6 m of rain annually!

Extreme weather events in Australia

Across the Tasman, Australia has experienced some difficult weather events over the last few years. It had a record-breaking heatwave in 2013 – beginning the year with more than 70% of the continent reaching temperatures above 42°C. The national average temperature on 7 January 2013 was 40.3°C. Australia is known for heatwaves, but this event was unusual because the extremely hot temperatures were so widespread and persisted for a fortnight. Regional temperature records continued to be set in 2014, 2015, 2017 and Adelaide had its hottest day on record in January 2019 with 46.6°C. A heatwave occurs when the daily maximum temperature exceeds the average maximum temperature by five degrees or more for five consecutive days.

Rights: Public domain

Tropical Cyclone Yasi

A spectroradiometer on NASA’s Terra satellite captured this image on 1 February 2011. The storm extends over the Solomon Islands and grazes Papua New Guinea. Part of the Queensland coast appears in the top left corner.

Cyclone Yasi – the biggest cyclone to ever hit Australia – struck in 2011. Yasi absorbed energy from the warmer than normal waters of the Coral Sea on the northeastern coast of Australia, expanding to 800 km in diameter. That was more than double the size of the tropical cyclones that usually form in the Pacific. Fortunately, Yasi moved quickly and missed many of Queensland’s larger towns. It still managed to dump up to 200 mm of rain in just 2 hours over Melbourne and regional Victoria, causing significant flooding.

Find out how cyclones form and how Yasi got its name in the article Cyclones, typhoons and hurricanes.

Changing weather and changing climate

We expect the weather to change and are fortunate that meteorologists can track weather systems and warn us about extreme weather events. What is less certain is the severity of the impact climate change will have on weather in New Zealand, Australia and elsewhere.

New Zealand’s Ministry for the Environment predicts an increased frequency of extreme weather events – including droughts in the eastern part of the country and increased flooding due to more frequent and intense winter rainfalls. Coastal areas will also be at risk due to rising sea levels. Land-use management can help lessen the impacts of extreme weather events.

Rights: SGT Vanessa Parker/NZ Defense Force, CC BY 4.0

Bridge damage from Cyclone Gabrielle

The Esk Valley in Napier faced catastrophic consequences in the wake of Cyclone Gabrielle in February 2023. This shows extensive damge to a railway bridge over the Esk River.

Many regions in New Zealand experienced heatwaves and record temperatures during the summer months of 2019. True heatwaves were once considered rare in New Zealand. Hot air arriving from Australia is usually cooled by the Tasman Sea, but unseasonably high sea temperatures meant the air arrived warmer than usual. The hot, dry conditions helped to fuel a large bush fire in the Nelson area, burning more than 2,000 hectares, and forcing about 3,000 people to leave their homes.

The North Island was badly affected by weather events in early 2023. The Auckland Anniversary Weekend floods saw an entire summer's worth of rain fall within one day. Within days, Severe Tropical Cyclone Gabrielle caused even more devastation. The New Zealand MetService reports, "Between the 12th and 14th of February, parts of Aotearoa New Zealand recorded rainfall amounts of 300–400 mm, wind gusts of 130–140 km/h and waves were recorded as high as 11 metres along some of our coasts."

The GIF above is from NOAA and shows a 24-hour satellite animation of Tropical Cyclone Gabrielle intensifying off the coast of Australia on 8 and 9 February 2023.