Which way do cyclones spin?
If water in a toilet, basin or a cyclone spins clockwise in the Southern Hemisphere and anticlockwise in the Northern Hemisphere, which way does it spin at the equator?
If water in a toilet, basin or a cyclone spins clockwise in the Southern Hemisphere and spins anticlockwise in the Northern Hemisphere, which way does it spin at the equator?
Meteorologists know a lot about cyclones long before they hit land.
Where a cyclone forms, the direction it spins and the general path it takes are determined by a range of factors including the Coriolis effect caused by the Earth's rotation, says Dr John McBride from the Bureau of Meteorology's Centre for Australian Weather and Climate Research.
While not entirely predictable, one thing is certain: you won't find a cyclone on the equator.
Named after French engineer and mathematician Gaspard Gustave Coriolis, the Coriolis effect describes how things move to the right in the Northern Hemisphere, and to the left in the Southern Hemisphere because of the Earth's rotation.
For example, if you travel south in a straight line in Australia, it's only a straight line on the Earth's surface. From a fixed point in space, you would see the Earth's curvature and rotation, so the journey would really take a broad south-easterly curve.
It all comes down to the fact you can't really travel in a straight line on a spinning sphere, or in this case, planet.
Weather forecasters use the Coriolis effect as part of their calculations to work out how air moves from areas of high pressure to low pressure.
"The air wants to move in a straight line from high to low, but thanks to the Coriolis effect the air is deflected to the left in the Southern Hemisphere going into the low pressure area in a clockwise direction. In the Northern Hemisphere it deflects to the right and moves into the low pressure zone counter-clockwise," explains McBride.
"The amount of deflection is related to the speed of the air and its latitude. Winds blowing slowly won't be deflected as much as strong winds. Winds at higher latitudes will be deflected more than winds blowing at the same speed, closer to the equator."
The Coriolis effect is zero at the equator, so you don't get cyclones at all — you have to be at least 5 degrees north or south of the equator for a cyclone to form, says McBride.
But the equator is not wind-free. McBride says winds still travel from areas of high pressure to low pressure.
Without a Coriolis effect, local conditions play a major role. These can include mountains, sea breezes, thunderstorms and monsoons. Large scale weather systems can also force winds across the equator from one hemisphere to the other.
Spinning the toilet myth
The Coriolis effect only really works over large distances, not on the small scale of a toilet or sink.
McBride says the idea that the Coriolis effect makes water swirl one way down the toilet in the Southern Hemisphere and the other way in the Northern Hemisphere is actually a myth.
"In fact it has much more to do with the motion of the water and the shape of the container and drain, rather than which side of the equator you're on," he says.
So if you're planning a holiday at the equator, chances are you won't get hit by a cyclone, but beware of scams 'demonstrating' the Coriolis effect in a bucket.
How are cyclones rated?Australia uses a five category system based on wind speeds to grade cyclones.
Category 1: wind speeds up to 125 kilometres per hour, causing only slight damage.
Category 2: wind speeds up to 169 kilometres per hour, causing minor damage to buildings, severe damage to signs and trees, and heavy damage to crops.
Category 3: wind speeds up to 224 kilometres per hour, very destructive, causing structural damage to roofs and bringing down power lines.
Category 4: wind speeds of 279 kilometres per hour causing significant roofing and structural damage with a lot of airborne debris and widespread power failures.
Category 5: wind speeds over 280 kilometres per hour and capable of causing almost total destruction with buildings flattened and cars overturned.
Winds are measured by official weather bureau anemometers on the ground , although good estimates can also be made from satellite data.
Dr John McBride from the Bureau of Meteorology's Centre for Australian Weather and Climate Research was interviewed by Stuart Gary.
Published 24 March 2010