Tornadoes form in supercell thunderstorms, which are larger
and more severe than regular thunderstorms.
Air starts to rotate because of the winds at different altitudes blowing
at different speeds creating a wind shear. This horizontal rotating air is then
lifted off the ground by the updraft of the thunderstorm. Once the horizontal
rotating air is lifted nearly vertically, it is called a mesocyclone. The
mesocyclone is then fully developed in the updraft. This can lead to a funnel
cloud, which is right before a tornado forms. A tornado forms once the funnel
cloud comes in contact with the surface.
In the United States, tornadoes
usually travel from west to east or more specifically, southwest to northeast. This
is the dominant direction for tornadoes because wind direction in general for
the U.S is from west to east and this wind is called the Westerlies. Tornadoes
are very scarce in New Zealand, but they do happen. The average number of tornadoes observed
in the US per year is a whopping 1,000 (Source). The average number of tornadoes
observed in New Zealand per year is 8, which is extremely low and good (Source).
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| http://www.ncdc.noaa.gov/climate-information/extreme-events/us-tornado-climatology |
In the map above, you can see that
New Zealand is in the hot spots of tornado activity. Although on the graph of
tornado occurrences over the years showing an increase in annual tornado count,
it may not be true. The reason being is that a hundred years ago, not too many people
knew what tornadoes were. Technology has
now helped more people understand tornadoes then being able to report more of
them.
The three main requirements for a
hurricane is a warm ocean temperature more than 80 degrees F, deep warm ocean
layer about 200 meters in depth, and coriolis to initiate the spinning of the
air. They need to be between 5 degrees N and S. New Zealand does not satisfy
these requirements because the water is too cold being fairly far away from the
equator and they are too far south for the coriolis to be strong enough. The
main hurricane formation regions are the Atlantic, East Pacific, Indian Ocean
near Australia, off the coast of China and Indonesia. They are termed
hurricanes in the Atlantic and Pacific; tropical cyclones in Australia; and
typhoons in Indonesia. The general direction hurricanes travel is up the warm
Gulf Stream because of the west to east flow of the Westerly winds and jet
streams.
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| http://www.learnnc.org/lp/editions/nchist-recent/6248 |
Tropical cyclones (hurricanes) do
happen in New Zealand, but are very scarce! On average, ~1.75 hurricanes hit
the U.S every year (Source).
I couldn't find the average number of hurricanes to hit New Zealand per year,
but I did find that New Zealand is hit by
an average of a little over one storm of tropical origin each year. This is not
the same as a hurricane and is a weaker storm system (Source)
