Severe Weather in New Zealand

TORNADOES

Tornado hitting Hokitika, New Zealand
Photo from here

Tornado (n) : a violent and destructive storm in which powerful winds move around a central point. (Webster)

Drawing from here

How cliche am I when the first thing that comes to my mind when the word tornado shows up, I think of Dorothy and Toto? Dorothy and Toto's home was situated back in the state of Kansas. In 2009 alone, 100 tornadoes ravaged Kansas (CRH). An average of 1,253 tornadoes hit the United States every year (NCDC) and moves from southwest to northeast. That is a lot compared to the amount New Zealand gets hit with. With an annual average of 7 reported tornadoes in New Zealand (Niwa), tornadoes rarely hit New Zealand.


But how do tornadoes form?

The only way to produce a tornado is if a thunderstorm is present.

                                     

Vortex tube formed
Photo from here

The first step in creating a tornado is when the westerly winds blows and winds from the southeast blows near the surface a rotating body of air is made. This is called a vortex tube. This occurs because as the altitude increases, the speed increases which is known as vertical wind shear.

Updraft lifts tube
Photo from here

This horizontal tube of air is then lifted off the ground from the middle by the thunderstorm's updraft creating an awkward "c" shape at first. Once the horizontal tube becomes a vertical standing tube it is called a mesocyclone.

Wall cloud and tornado formed
Photo from here

Finally the still spinning tube is developed in the updraft of the thunderstorm. In the picture to the right, a wall cloud is formed bellow the thunderstorm cloud which is the black part. Then a funnel cloud is lowered from the wall cloud. As soon as the funnel cloud touches the ground, it is now a tornado.

Global Map of Tornado Occurrences
Photo from here

New Zealand is located on the southern hemisphere. North Island is situated above the 180˚ mark. Then in between the 150E and 180 is the South Island of New Zealand. It's pretty tiny compared to United States and Australia. You may not be able to see it clearly but there are a few red dots (which mean tornadoes) on the land mass. New Zealand is located to the southwest of United States and it's tornado alley.

Photo from here

In the past 30 years, the amount of tornadoes has increased. After contemplating for a couple of minutes and only thinking of global warming; I think the numbers have increased because of global warming. Thunderstorms need a warmer surface temperature to rise and cold sinking air. I guess global warming could cause warmer than normal surface temperature.





Hurricane

Tropical Cyclone Ita
Photo from here

The picture above is category 5 Tropical Cyclone Ita, expecting to hit Queensland, Australia on friday 4/11/14. New Zealand is fairly close to Australia. Luckily, New Zealand does not get hit with tropical cyclones (hurricanes) often. It has an average of one cyclone per year (Niwa). United States on the other hand has an average of seven hurricanes per year (AOML).

Hurricane Formation Regions
Photo from here

You probably noticed that I used two terms for the natural disaster - tropical cyclone and hurricane. Countries in the Indian oceans like Australia use the term cyclone. Hurricanes are used in the Atlantic and East pacific Ocean (United Sates). Lastly the term Typhoon are used near the coast of China and Indonesia.

Hurricanes need three ingredients which are 1.) warm ocean waters that are greater that 80˚F, 2.) deep warm ocean water - at least 200 m deep (because if the upper part of the water was warm and the deeper part of the water is cold, it could kill the hurricane), 3.) Coriolis (what makes the hurricane spin) so greater than 5˚ N, S because there is no Coriolis at the equator.

Hurricane Path
Picture from here

From the picture above, hurricanes start off near the north Atlantic Ocean and Gulf of Mexico where there are higher chances of warm waters above 80˚F. Then with the push of westerlies and jet streams they travel up towards the U.S. where Florida gets hit often.