El enfoque sistémico y los modelos de relaciones públicas

Unit 2: Patterns of Circulation and Surface Pressure

3.0 Main Content

3.1 Observed Pattern of Circulation

Expectedly, equatorial regions should be characterized, by relatively low pressures with weak surface winds, this condition are referred to as Doldrums by the mariners. A second region of calm on Earth’s surface, according to the three-cell model, would be around latitude 30°. In this region, air moving downward from both the Hadley and Ferrell cells collides as it reaches Earth’s surface, producing regions of high pressure. As in the doldrums, the regions around latitude 30° are characterized by weak and unpredictable winds, Sailors named these regions the horse latitudes. The regions between the horse latitudes and the doldrums (between 0° and 30° latitude) are those in which surface winds flow toward the equator. That flow is not directly from north to south or south to north because of the Coriolis effects. Instead, winds in these regions tend to blow from the northeast to the southwest in the northern hemisphere and from the southeast to the northwest in the southern hemisphere. Because the winds tend to be strong and dependable- the sorts of wind upon which sailing ships depend- these winds have long been known as the trade winds. The intersection of the Ferrell and polar cells around latitude 60° is another region at which surface flows of air meet. One, from the Ferrell cell, consists of relatively warm air flowing toward the poles. The other, from the polar cell, consists of much colder air flowing toward the equator. The point at which these two systems meet is called the polar front and is characterized by some of the world’s most dramatic storms. The prevailing direction of surface winds with the Ferrell and polar cells is determined by the Coriolis Effect. In the Ferrell cell, winds tend to blow from the southwest to the northeast in the northern hemisphere and from the northwest to the southeast in the southern hemisphere. In the polar cell, the predominant air movements are just the opposite of the prevailing westerlies: from northeast to southwest in the northern hemisphere and from southeast to northwest in the southern hemisphere.

Self-Assessment Question

 Differentiate between following patterns of circulation: Doldrums, polar Cell and Horse Latitudes?

3.2 Patterns of Surface Pressure

Conceptual models of meteorological phenomena have only limited applicability in the real world because a number of factors depart from the ideal conditions used to develop the models. These factors ensure that actual weather conditions will be far more complicated than the general conditions described above. For example, both the Hadley and Ferrell models assume that Earth has a homogeneous composition and that the sun always shines directly over the equator. Neither condition is strictly true. Most parts of the planet are covered with water and land masses are distributed unevenly. The flow of air in any one cell, therefore, may be undisturbed for long stretches in one region (as across an ocean), but highly disrupted in another region (as across a mountain range).Charts showing air pressure at various locations on Earth’s surface are useful tools for meteorologists, because air flows from regions of higher pressure to those of lower pressure. Such charts indicate that certain parts of the planet tend to be characterized by unusually high or low pressure centers at various times of the year. Eight semi-permanent high- and low-pressure cells that reappear every year on a regular basis have been identified.

A permanent high pressure zone persists in Bermuda throughout the year. A semi-permanent low pressure zone- the Icelandic low- is usually found north of the Bermuda high and tends to shift from east to west and back again during the year. During the winter in the northern hemisphere, a semi-permanent high that exists over Siberia disappears and is replaced by a semi-permanent low over India each summer. The existence of these semi-permanent highs and lows accounts for fairly predictable air movements over relatively large areas of Earth’s surface.

Self-Assessment Question

 Itemize the factors which limit the real world application of the model of atmospheric circulation?

3.3 The Jet Streams

During World War II, an especially dramatic type of atmospheric air movement (the jet streams) was discovered. A bombing raid over Japan, a sortie of B-29 bombers found themselves being carried along with a tail wind of about 186 mph (300 km/h). After the war, meteorologists found that these winds were part of permanent air movements now known as the jet streams. Jet streams are currents of air located at altitudes of 30,000–

45,000 ft (9,100–13,700 m) that generally move with speeds ranging from about 30–75 mph (50–120 km/h). It is not uncommon, however, for the speed of jet streams to be much greater than these average figures, and velocities as high as 300 mph (500 km/h) have been measured.

The jet streams discovered in 1944 are formed along the polar front between the Ferrell and polar cells. For this reason, they are usually known as polar jet streams. Polar jet streams usually travel on a west to east direction between 30°N and 50°N latitude.

Commercial aircraft often take advantage of the extra push provided by the polar jet streams when they travel from west to east, although the same winds slow down planes going in the opposite direction. The path followed by jet streams is variable. They may break apart into two separate streams and then rejoin or remain as a single stream. They also tend to meander north and south from a central west-east axis. The movement of the jet streams has a major effect on weather in mid-latitude regions. Since the end of World War II, jet streams other than those along the polar front have been discovered. For example, tropical easterly jet stream has been found to develop during the summer months over Africa, India, and Southeast Asia. Some low-level jet streams have also been identified. One of these is located over the Central Plains in the United States, where

topographic and climatic conditions favor the development of unusually severe wind systems.

Self-Assessment Questions

 Discuss the implication of Jet Stream on commercial Aviation?

 Name four types of Jet streams discovered after the Second World War?

3.4 Other Violent Wind Systems

A number of air movements are not large enough to be described as forms of global circulation, although they do cover extensive regions of the planet. Monsoons, for example, are heavy rain systems that sweep across the Indian subcontinent for about six months of each year. They are caused by movement of air from Siberia to Africa by way of India and back again. During the winter, cold, dry air from central Asia sweeps over India, across the Indian Ocean, and into Africa. Relatively little moisture is transported out of Siberia during this time of the year. As summer approaches, however, the Asian land mass warms up, low pressures develop, and the winter air movement pattern is reversed. Winds blow out of Africa, across the Indian Ocean and the Indian peninsula, and back into Siberia. These winds pick up moisture from the ocean and bring nearly constant rains- the monsoons- to India for about six months.

Self-Assessment Question

 Discuss the causes and effect of Monsoons?

4.0 Conclusion

In conclusion, the pattern of atmospheric circulation and surface pressure exhibits a number of features, this makes The Earth's atmospheric circulation varies from year to year, but the large-scale structure of its circulation remains fairly constant.

5.0 Summary

The Ferrell hypothesis presupposes that there should be relatively little surface wind near the equator. In this region, surface winds should flow toward the equator from the Hadley

cells and, when they meet, rise into the upper atmosphere. This makes equatorial regions to be characterized by a number of features like: Doldrums, horse latitudes and polar front. Jet streams and monsoon.

6.0 Tutor-Marked Assignment

a) Discuss the implication of Jet Stream on commercial Aviation?

b) Name four types of Jet streams discovered after the Second World War?

c) Enumerate the causes and effect of Monsoons?

d) Differentiate between: Doldrums, polar Cell and Horse Latitudes as patterns of circulation?

7.0 References/Further Readings

Ahrens, D. C. (2006), Meteorology Today. Pacific Grove, Calif.: Brooks Cole.

Palmer, T. and Renate H. eds. (2006), Predictability of Weather and Climate. New York:

Cambridge University Press, 2006.