Our weather often seems to have a mind of its own.
But although it may seem unpredictable, there are clear patterns to weather formation.
One of these is the global cycle of wind.
Wind direction is determined by the rotation of the Earth, through the coriolis effect.
Unfortunately, it's a little hard to explain.
"... Right... ok... the coriolis effect..."
"It's a difficult thing to understand I think..."
"... It's to the left... no is it to the right? I always get it wrong!"
Warm air travels from the equator to the poles in the form of wind, but it doesn't do this directly in a straight line...
The Earth rotates faster at the equator and slower at the poles.
This variation in speed influences the movement of the wind.
Wind movement –
Affected by variations in Earth's rotation speed
Prof Geraint Vaughan, University of Manchester – "The reason that it's complicated is because the Earth is spinning, and the spinning of the Earth means that you can't just take warm air from the equator and just move it to the poles."
Wind moves through the troposphere, the section of the planet's atmosphere which is closest to the surface.
8km high at poles
16km high over the equator
It's best to think of the troposphere as a fluid mass that moves as the Earth spins.
This effect can be demonstrated by suspending particles in water around a rotating mass.
In this example, aluminium chips represent the movement of warm air.
The spinning Earth causes winds travelling to the North Pole to veer to the right, in a clockwise motion.
And the wind travelling to the southern pole to veer to the left, in an anticlockwise motion.
Sir Brian Hoskins, Imperial College and University of Reading – "When you are on a rotating system and you start to move there's all sorts of different things that happen... And you tend to be flung off at right angles to the way you want to go and that's what the coriolis force is. It's saying 'ok you want to go that direction, I want you to go that direction!'"
The coriolis effect, the deflection of moving mass by rotation, can be observed in many other instances.
But perhaps it is most important in meteorology.
Coriolis Effect -
Deflection of moving mass by rotation
For it's the spinning of our planet, together with the depth and temperature of the atmosphere, that's responsible for the global cycles of wind, which can generate many weather conditions, from thunderstorms to hurricanes.