Earth's nearly fixed rotation rate and the nearly constant length of day don't just happen by accident. Consider Figure 9.1, a view of Earth from above the North Pole showing two latitude circles, one in the tropics and one in the mid-latitudes (think of the largest circle as the equator). Note the greater circumference of the tropical circle compared to that of the mid-latitude circle. In twenty-four hours (the time to make one rotation), point T on the tropical latitude must travel faster to the east than point M (which lies due north on the mid-latitude circle) because Point T must travel a greater distance than point M. We say that point T has greater eastward speed and, as a result, greater angular momentum than point M (angular momentum is a measure of a spinning object's tendency to continue spinning - a fast-spinning ice skater has more angular momentum than one spinning slowly). Moreover, at increasingly high latitudes, eastward speed and angular momentum decline rapidly until completely vanishing at the pole. A question that arises is: with the sizable tropics zipping east and with prevailing winds from the west over much of the earth helping to speed up the planet's rotation, how does the Earth maintain a nearly constant rotation rate?

An important part of the answer is that, in the tropics, prevailing winds in the troposphere blow generally from the east. Near the surface, these prevailing easterlies are called trade winds. Though the trades blow more from the northeast than due east, they, for all practical purposes, blow in the opposite direction to the Earth's rotation. In turn, friction between the westward-bound trades and the Earth tends to slow down the Earth (and the trades too!). The total coverage of the tropical easterlies amounts to about 40% of the total surface area of the Earth, so their impact on keeping the Earth's rotation rate in check is substantial. The trades, plus other weather features and mechanisms in the tropics and mid-latitudes, help to even out and redistribute angular momentum across the globe. Thus, the total angular momentum of the Earth-atmosphere system (considered as one and inseparable) is nearly constant, a balanced budget that keeps the rotation rate and the length of day almost perfectly fixed.

We're not done yet with the Earth's rotation rate. The news-grabbing El Nino of 1997-98, which produced a record sea-surface warming in the eastern tropical Pacific that had rippling effects on weather patterns across the globe, was directly linked to an aberrant altering of the length of the day. According to an article published in Science News near the height of El Nino in January 1998, the Earth's rotation rate was consistently slow from mid-March to late November 1997. This unusually persistent slow-down caused an increase in the length of the day by as much as four-tenths of a millisecond during this period (given that El Nino has been blamed for the high cost of lettuce, among many other things, why not a longer day?).

How can El Nino alter the length of the day? How can El Nino influence global weather patterns? What causes the trades? What's the general weather pattern in the tropics?

Poignant, fundamental questions, indeed. The answers to the first three are involved but within our grasp. The last question has a short and a long response. The short answer is rather general - "monotonous and slow to change." The longer, more specific version needs some elaboration.