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Fig 23.20.

Circulation patterns on the Earth.The closed loops represent the major cells within the atmosphere, and the open arrows represent the general wind directions at the surface.

Fig 23.20.

Circulation patterns on the Earth.The closed loops represent the major cells within the atmosphere, and the open arrows represent the general wind directions at the surface.

north pole. We are standing at point B, which is far from the equator. We want to aim at a target on the equator, at point A. We point directly at A and shoot. The bullet travels towards the equator with the speed that it left the gun. Its velocity has a slight horizontal component to the left due to the Earth's rotation. Object A has an even larger motion to the left, since it is at the equator, and is moving faster than we are. The result is that by the time the bullet reaches the equator, A has moved to the left of its path, and the shot misses, going behind the target. A similar thing happens if someone at A shoots at B. The bullet leaves A with a large horizontal speed, and misses B by getting ahead of it. To the observer on Earth, it is almost as if there is an additional force present.

In general, air flows from high to low pressure areas in the atmosphere (Fig. 23.20). Air traveling from the poles to the equator will reach the equator behind the point at which they were initially launched. Air traveling from the equator to the poles will arrive ahead of the point at which it was aimed. This produces a circulating pattern. Air flowing from a high to a low, going toward the equator, will lag behind the low. Air flowing in the other direction will get ahead. This results in a counterclockwise circulation around the lows and clockwise circulation around the highs in the northern hemisphere. The opposite situation prevails in the southern hemisphere. This pattern is very evident in the circulation around hurricanes, which have very low pressure centers. A typical hurricane is shown in Fig. 23.21.

Recently, major improvements have been made in our understanding of the large-scale atmospheric circulation. Computers are used to model the Earth's atmosphere in considerable detail. The equations governing the fluid flow and energy balance are solved, taking such details as terrain into account. For such a model to be successful in predicting the movement of weather systems over periods of a few days, or even possibly weeks, it is important to have detailed information on conditions all over the Earth. Data are continuously collected from a variety of ground stations, from ships and weather buoys at sea, and from balloons sent up from various places on a regular schedule. Satellite observations are also important. The observations over the ocean are particularly important, since much of the energy that passes into the atmosphere is stored in the water. As the models are refined and the data gathering is improved, the predictions become better and better. We have now even reached the point where we can use the same approach to try to understand the global atmospheric properties of other planets, as we will see in the next few chapters.

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