As the Walrus and the Carpenter in Alice Through The Looking Glass were ambling tearfully along the beach, "the sun was shining on the sea, shining with all its might". By doing so, the sun, as always, was providing the required energy to fuel the Earth's weather machine. The destructive force of the hurricane, the power of the waves, and the sultry warmth of a southerly breeze, are all ultimately the radiant energy of the sun appearing in a different guise.

The amount of solar energy absorbed by land or water varies greatly with latitude. Near the poles, because of the low angle of the sun in the sky, much of the incoming radiation skims off the surface and is reflected back out again to space - a tendency which is exacerbated by the high reflectivity of the snow and ice in those regions. Near the equator, on the other hand, much more solar radiation is absorbed since the sun is almost directly overhead.

But the Earth also loses heat back to space by means of longwave radiation - and as it happens, this type of radiation occurs at a more or less uniform rate over the entire surface of the planet; the tropics radiate slightly more, because of their higher temperature, but not very much more than the colder polar regions.

The net result, therefore, of this exchange of energy with outer space ought to be that the tropics should gain more energy than they lose and become boiling hot; the opposite should obtain in high latitudes, with the poles suffering a net loss of heat, and becoming colder and colder.

Now it may well be that this scenario does indeed apply in that zany world that exists Through The Looking Glass, since the Walrus and the Carpenter, you may recall, were moved to wonder

Why the sea is boiling hot,

And whether pigs have wings.

But in the real world nature avoids this unpleasant situation by arranging for a constant transfer of energy from the equator towards the poles.

The transfer is brought about by what might be called the moving parts of the Earth-atmosphere system, the ocean currents and the winds. In our hemisphere a northerly wind carries cold air equator-wards, and a southerly wind brings warm air towards the pole, neatly engineered in mid-latitudes by the circular flow of winds around depressions and anti-cyclones. The ocean also plays an important part: some 25 per cent of the total poleward transfer of heat is brought about by warm currents travelling pole-wards and by cold ocean currents drifting towards the equator.