Storms and cyclones
Common questions about storms and cyclones in New Zealand.
On this page
- What is a storm?
- How is a storm classified?
- How do scientists predict storms?
- What is the most effective way to inform people that a storm is coming?
- What were the worst storms to hit New Zealand in the last 50 years?
- What were the worst storms to hit New Zealand in the last 10 years?
- What are tropical cyclones?
Storms are associated with periods of strong often damaging winds, heavy flood-producing rainfall, thunder and lightning, heavy snowfall or blizzard conditions.
They are classified by how strong the winds are or by how heavy the rainfall, lightning or snow are.
To predict storms in New Zealand, scientists note the position of a storm on satellite photographs and use computers to follow and calculate its progression.
The most effective way to notify people that a serious storm is coming is with radio and TV broadcasts.
The worst storms to affect New Zealand in the last 50 years were Cyclone Bola in March 1988, causing more than 200 million dollars of damage, and the Wahine storm (Tropical Cyclone Gisele) in 1968 in which 51 people lost their lives.
In the last 10 years, the worst storms have been Cyclone Fergus in December 1996, which brought over 300 mm of rain in 24 hours to Coromandel, and Cyclone Drena in January 1997, in which one man was electrocuted when he grabbed a fallen powerline as he climbed up a bank. The early warning of Cyclone Fergus during the Christmas holiday season allowed people to evacuate from beachside campsites, preventing loss of life. These images on the right show how massive Fergus (left) and Drena (right) were.
Tropical cyclones are revolving storms that begin in the tropics. Storms of this type are called hurricanes in the North Atlantic and eastern Pacific and typhoons in South East Asia and China. They are called tropical cyclones in the southwest Pacific and Indian Ocean region. In the southern hemisphere storms rotate clockwise (see pictures above). Tropical cyclones form near the equator and gain their energy from the heat that is released when water vapour condenses into rain. They are about 500 km across and may have a central region with relatively little cloud and light winds called the eye. Tropical cyclones weaken as they move towards New Zealand because the cooler seas provide less heat. The worst cyclones tend to occur from December to April. In autumn the sea surface temperatures remain high, allowing the tropical cyclones to retain their intensity, and the chance of meeting a cold air front from the Antarctic increases as autumn progresses.
A front is where the two air masses meet. A cold front occurs when the cold air is advancing and pushing away the warm air. If the warm air is advancing, it rises over the cold air which is denser and forms a warm front. A stationary front forms if neither air mass is advancing. As the atmosphere tries to even out the pressure difference, wind is created, blowing from high pressure to low pressure. When the air reaches the centre of the low-pressure area it is forced upwards into the colder atmosphere and the water vapour in the air condenses into rain.