Science was much easier two thousand years ago. Today, we can neither explain nor predict the ferocity of an El Nino. Aristotle, however, would have had no difficulty in identifying the cause of the disastrous droughts in the southern hemisphere.

Aristotle would almost certainly have blamed Hale-Bopp and Hyukatake for the problems now being linked to El Nino. "The fact that comets when frequent foreshadow wind and drought must be taken as an indication of their fiery constitution," he wrote. His book on meteorology frequently returns to a discussion of the nature of comets and shooting stars, which do not fit in with his general picture of an orderly universe. His view was that comets were formed from a coagulation of fire. "Hence the air is necessarily drier and the moist evaporation is so dissolved and dissipated by the quantity of the hot exhalation as not to condense readily into water." So comets lead to drought - which makes the current disastrous El Nino easy to explain through our recent visitation by two large comets.

Our modern mathematical model of the world is rather more complicated. You have to see the whole weather business as an exercise in fluid dynamics. It starts with a sphere whose surface is covered about two-thirds with water and one third with land. Then you set it rotating once a year about a source of heat, just hot enough to lead to a constant evaporation of water. Meanwhile the sphere we started with is rotating once a day about its own axis, which is inclined at an angle of about 30 to the plane of its rotation about the heat source. Oh, and the sphere is also cloaked in a robe of gases, which can absorb some types of radiation from the heat source, but let through others. Meanwhile, the land and water retain heat at different rates, which leads to comparably different effects on the air above them.

So far so good: the temperature differential will create north-south winds as warm air rises (it's warmer near the sphere's surface because the land and sea retain warmth better than the air), and those winds will be bent in an east-west direction by the sphere's rotation. But when the warm air rises to a cooler level, its capacity to hold moisture will decrease, leading to condensation and eventual rain. So what we have essentially is a mechanism of evaporation, wind and condensation that transfers water from one place to another.

There are just a few more things we have to take into account for an accurate mathematical model. One of these, which seems to have little measurable effect, but must play some part, is the smaller sphere rotating once a month around our main body, whose gravitational effect causes the waters to move to and fro. Then there's the fact that the surface of our sphere is far from smooth, but has great irregularities that create obstacles for the wind.

To make things still more complex, the orbit of the sphere about its heat source varies gradually over a cycle of about 95,000 years, while its axis also wobbles, but in a 23,000 year cycle, and the tilt of its axis to its plane of rotation also changes over a 41,000 year cycle. While none of these can be blamed for the occasional shower of rain, they do seem, however, to be in some way responsible for little climatic disturbances such as the Ice Ages.

Sometimes it seems easier to go back to Aristotle and blame the comets. But it's interesting that he wrote of "the fact" that comets foreshadow drought. I wonder if he was right.

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