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Water Cycle

There are several components to what is called the water or hydrological cycle. The one we are most familiar with is the cycle of water evaporating into clouds, which subsequently gets returned to the Earth as precipitation. There is also the cycle of the ocean currents, which can greatly affect the weather of especially coastal areas (for example, the Gulf Stream brings warm water from the Caribbean to England, and consequently London, which is about the same latitude as Hudson's Bay, has a much warmer climate than what it would otherwise).

    Another aspect to the water cycle is that of the ice ages, which occur over a long time scale of several thousand years. The origin of ice ages is currently understood in terms of what are called Milankovitch cycles. As the following diagram indicates, the Earth spins around on its axis, and like a toy top, this axis precesses (wobbles) around in a wide circle.

Figure 14.8: Earth's precession
\epsfysize=6 cm

This precession occurs once every 26,000 years. As well, there is a small change in the angle of the earth's tilt from the vertical (ranging from about 24.5o to 21.5o), with a period of about 41,000 years, as well as an effect that, over the course of 100,000 years, the earth's orbit around the Sun goes from fairly elliptical to fairly circular. The net effect of these motions is that the axis of the Earth gradually changes its orientation. At present the Northern Hemisphere is tilted away from the Sun during the winter, which is actually the period at which the Earth is closest to the Sun. In about 11,500 years, though, the Earth will be tilted towards the Sun during the winter months. Milankovitch proposed that the appearance and disappearance of ice ages are tied into these orbital effects and the subsequent increase or decrease in solar heat reaching a given area of the Earth.
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