1News weather expert Dan Corbett explains what’s happening to cause the tornadoes we’ve seen across New Zealand in the past few days.
What causes tornadoes?
The two main ingredients for tornadoes are instability and wind shear.
Instability is the ability to lift the clouds at the surface to great height. Because of the favourable conditions of warmer air rising through much colder air above.
Wind shear is a change in wind direction with height that can cause twisting in the clouds.
Trees were blown over on several streets, with fences and roofs lifted off homes. (Source: 1News)
What about the ones we’ve experienced recently?
These mini-tornadoes are caused by some pretty active weather.
You can think of it as a contrast of temperature, but also a change in wind direction as you go up into the clouds.
If you look at the satellite images for Monday’s Tasman tornado, you see a large swathe of cloud in the Tasman Sea, the main spoke of energy that brought the storms.
As you look closer, you can also see a little swathe moving into the upper South Island. You see the heavier downpours and, importantly, a twist in the winds.
That brought the strong, damaging winds with the tornado as it moved through.
Around 50 properties were damaged by the destructive winds. (Source: 1News)
Tell me more about the winds twisting
Looking at the Auckland tornado from Sunday, you see the same frontal boundary, with again a twist in the winds - those are the surface winds.
But the winds 1km above, they are going in different directions. So, it takes the cloud and it twists it.
That’s why you get the funnels as they drop down. Strong wind squalls, in the right conditions, twist and form eddies, like a whirlpool effect.
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