Weather wise

ONE of the most difficult things to get to grips when you start to study the weather is something called the general circulation, defined in Atmosphere, Weather and Climate, by Roger Barry and Richard Chorley, a key text for budding meteorologists, as large-scale patterns of wind and pressure that persist throught the year or recur seasonally.

The primary engine for global circulation is the imbalance of solar radiation between the equator and the poles.

If that was all there was to it, the global circulation pattern would be fairly simple: warm air at the equator would rise, travel north or south, and fall in a huge vertical "convection cell'', like the swirling eddies in an electric kettle full of water.

However, Earth turns at speeds that vary from 0mph at the poles, to nearly 1000mph at the equator.

A parcel of air travelling northward from the equator will therefore be given a westward "kick'' - known as the Coriolis effect - from Earth's rotation.

One model proposes three convection cells, the boundaries of each marked by pronounced westerly and easterly winds at the surface, and corresponding high-level cross-longitudinal jetstream winds in the upper atmosphere - the winds which speed jets on their way flying east.

Even this is now recognised to be a gross oversimplification. Not only does Earth turn, its surface consists of smooth oceans and bumpy land, and in the mid-latitudes horizontal convection cells, rather than vertical ones, are now thought to be the main mechanism by which heat is transferred towards the pole from the equator.