Atmospheric dynamics

Scientists speculate that, while global warming may not increase the number of storms occurring on Earth, it may increase their average severity.

Background

In the eigthteenth century, Edmond Halley, for whom Halley’s comet is named, charted the monsoons and the trade winds, making the first known meteorological map. In an effort to understand the trade winds, Halley correctly surmised that the Sun-warmed air over the equator would rise high into the atmosphere and then flow toward the poles. He further supposed that the air would cool off and sink at the poles, and then return to the equator as a surface wind, but he could not explain why the trade winds came from the northeast, or even the east, instead of from the north.

The Role of the Coriolis Effect

Earth is about 40,000 kilometers around at the equator, and it rotates once in twenty-four hours, so at the equator the land, sea, and air are rushing eastward at nearly 1,700 kilometers per hour. At 45° north latitude, by contrast, Earth is only about 28,000 kilometers in circumference, so a point located at that latitude travels eastward at about 1,200 kilometers per hour.

Consider a parcel of air at rest with respect to the land at the equator. Suppose that it is filled with red smoke so that its location is easily seen. Now let the parcel move northward; because of its eastward momentum, it will also be moving eastward with respect to the land north of the equator. This eastward deflection of northward-moving air parcels is called the Coriolis effect and is named for Gaspard-Gustave de Coriolis, who studied it in 1835. Also as a result of the Coriolis effect, if an air parcel at rest with respect to the land at some point north of the equator begins moving southward toward the equator, it will be deflected westward, because its eastward momentum will be less than that of the ground below it.

Fifty years after Halley’s surmises, George Hadley explained the direction of the trade winds by referring to the Coriolis effect. Hadley believed that air parcels heated at the equator rose high and then migrated north to the pole. Cooled during the journey, the parcel would sink to the surface and head back south. Because of the Coriolis effect, the southward moving air would be deflected westward. This would explain the trade winds north of the equator, and this proposed air circulation route was called a Hadley cell.

The American meteorologistWilliam Ferrel pointed out that atmospheric dynamics could not actually be that simple, since the prevailing winds at midlatitudes are westerlies, not easterlies such as the trade winds. Ferrel suggested that the Hadley cell extended only to about 30° latitude north of the equator, where the cooled air sank and returned to the equator. The Ferrel cell lies between about 30° north latitude and 60° north latitude. Air rises at 60°, flows southward, cools and descends at 30°, and flows northward and from the west near the ground—hence the westerlies. The Polar cell extends from 60° to the pole, with air rising at 60° and sinking at the pole. The cells of the Southern Hemisphere mirror those of the Northern Hemisphere.

Jet Streams

Where the Ferrel cell meets the Polar cell, the temperatures and pressures of the air masses are generally different. These differences give rise to winds blowing north from the Ferrel cell toward the Polar cell, but this wind is soon deflected eastward by the Coriolis effect. Hemmed in between the Polar and Ferrel cells, the wind becomes the polar jet stream—a river of air 160 to 500 kilometers wide, 1 kilometer deep, and generally 1,500 to 5,000 kilometers long. Several discontinuous segments of the jet stream together might come close to circumnavigating the Earth. These segments wax and wane over time and sometimes disappear completely.

The polar jet stream forms at the tropopause, 7 to 12 kilometers above sea level. (The tropopause is the transition region between the troposphere below and the stratosphere above.) The speed of the jet stream averages 80 kilometers per hour in the summer and 160 kilometers per hour in the winter, but it can reach speeds of up to 500 kilometers per hour. While it generally flows eastward, sometimes it also meanders hundreds of kilometers south and then back north.

A second jet stream, the subtropical jet stream, occurs between the Hadley and the Ferrel cells, but since the tropopause is higher there, this jet stream is between 10 and 16 kilometers above ground level. This jet stream tends to form during the winter, when temperature contrasts between air masses are the greatest. Other low-level jet stream segments may form near the equator. The jet streams of the Southern Hemisphere mirror those of the Northern Hemisphere. Studies show that jet streams help carry carbon dioxide (CO₂) from where it is produced to other parts of the world.

There are practical reasons for studying jet streams. Jet streams influence the paths of storms lower in the atmosphere, so meteorologists must take them into account in their forecasts. Pilots flying from Tokyo to Los Angeles can cut their flight times by one-third if they can use the jet stream for a tailwind. One percent of the energy of the world’s jet streams could satisfy all of humanity’s current energy needs. Someday, it may be possible to use balloons to lift windmills into the jet streams, but they would need to be tethered to the ground to keep them from being blown along with the jet stream.

Oscillations

India was stricken by a severe famine and drought in 1877 because the monsoons failed. In response, Gilbert Walker headed a team at the Indian Meteorological Department using statistical analysis on weather data from the land and sea looking for a link to the monsoons. They eventually found a link between the timing and severity of the monsoons and the air pressure over the Indian Ocean and over the southern Pacific Ocean. The team found that high pressure over the Pacific meant low pressure over the Indian Ocean, and vice versa. Walker named this alternating of pressures the Southern Oscillation and linked it with the monsoon. It has since been linked to other weather phenomena.

Normally, there is a large region of high pressure in the Pacific just off the coast of South America. The trade winds near the surface blow westward from this high-pressure region to a low-pressure region over Indonesia. The winds pick up moisture as they cross the Pacific and deliver it in the monsoons over Indonesia, India, and so forth. Energy from the condensation of moisture heats the air and causes it to rise higher; then, the air flows back eastward to the South American coast, cools, and sinks to complete the Walker cycle.

Just after Christmas, a warm current flows south by the coasts of Ecuador and Peru. In some years, that current is stronger and warmer, and then it brings beneficial rains to the South American coast—a Christmas gift. The event is called El Niño (little boy) with reference to the Christ Child. It is also called the El Niño-Southern Oscillation, or ENSO. The trade winds weaken, and warm water surges eastward to the South American coast. Air rises as it is heated by the warm coastal waters, and air comes from the west to replace the air that rose. As a result, the trade winds are reversed and blow eastward. The winds that carried moisture from the Indian Ocean toward the equator are weakened, so the monsoons are weakened. Low pressure develops over the Indian Ocean and pulls the subtropical jet stream south. The displaced jet stream brings more rain to East Africa and drought to Brazil. Central Asia, the northwestern United States, and Canada experience heat waves, while Central Europe experiences flooding.

After a few years, the El Niño event weakens and things return to normal—except that two-thirds of the time nature overshoots “normal,” and La Niña (little girl) appears. The west-blowing trade winds return but are much stronger than normal. Cool water rises from the deep and forms a cool region off the west coast of South America. Colder-than-normal air blows over the Pacific Northwest and over the northern Great Plains, but the rest of the United States enjoys a milder winter. The Indian monsoons strengthen, and the subtropical jet stream returns to its normal position. Eventually, things quiet down and return to normal.

Since El Niño begins when warm water collects off the western coast of South America, global warming will probably increase the frequency and intensity of El Niño. The accompanying heat waves, flooding rains, and droughts will likely be more severe.

Fronts and High-Pressure Air Masses

A weather front is the boundary between two air masses of different densities. They normally differ in temperature and humidity. Cold air is denser than warm air, so the air of an advancing cold front wedges beneath and lifts warm air. This upward motion produces low pressure along the front, and as the air lifts and cools, moisture condenses and forms a line of clouds or showers along the front. Light rain may begin 100 kilometers from the front, with heavy rain beginning 50 kilometers from the front.

Cold fronts generally come from the north and head south, while warm fronts generally come from the south and head north. The leading edge of an advancing warm front takes an inverted wedge shape, such that the high cirrus clouds that mark the approach of the front may be hundreds of kilometers ahead of the front’s ground-level location. The cloud base continually lowers as the front approaches, and with enough moisture present rain may extend 300 kilometers in front of the ground-level front. Fronts are the principal cause of nonrotating storms.

Sinking air forms a high-pressure area at the surface. This air is usually dry, having delivered its moisture elsewhere, rendering the sky clear. If there were any warm, moist air, it would be prevented from rising by the descending air of the high-pressure area, so it could not form clouds and rain. Many of the world’s deserts form where the circulating air of the Hadley cell descends. Any high-pressure area that remains in place for a long time can cause drought. Winds blowing outward from the high-pressure area will be turned by the Coriolis effect in a direction established by a simple rule: If—in the Northern Hemisphere—one stands so that the low-pressure area is on one’s left, the wind will be at one’s back. In the Southern Hemisphere, the wind would be at one’s front.

Hurricanes and Low-Pressure Air Masses

Rising air forms a low-pressure area near Earth’s surface. Higher-pressure air from outside the area will flow toward the low-pressure area (like water running downhill), but it will be turned by the Coriolis effect and slowly spiral inward. This air will eventually be caught up in the rising air currents, will cool off as it rises, will be heated as its moisture condenses, and will then rise higher and contribute to the updraft. At this point, the low-pressure area has become a storm. If its rotation speed is over 120 kilometers per hour, the storm has become a hurricane. Global warming calculations predict that if CO₂ levels increase, the number of hurricanes occurring annually will not increase, but the average intensity of these storms will increase.

Context

More than a century ago, Sir Gilbert Walker began the process of describing the world’s weather in one comprehensive model. The weather in each part of the world is tied in some fashion to the weather elsewhere—through jet streams, circulation cells, or movement of air masses. As Earth’s global climate undergoes changes, the interrelationship of the planet’s weather systems becomes more important than ever, since the climate change will affect not only individual weather patterns and events but also the way in which those individual events affect one another and Earth’s weather generally.

Key Concepts

• air parcel: a theoretical house-sized volume of air that remains intact as it moves from place to place
• Coriolis effect: in the Northern Hemisphere, the westward deflection of southward-moving air and the eastward deflection of northward-moving air—caused by Earth’s rotation
• stratosphere: the atmospheric region just above the tropopause and extending up about 50 kilometers
• tropopause: the transition region between the troposphere and the stratosphere
• troposphere: the lowest layer of the atmosphere—in which storms and almost all clouds occur—extending from the ground up to between 8 and 15 kilometers high

Bibliography

Lutgens, Frederick K., Edward J. Tarbuck, and Dennis Tasa. The Atmosphere: An Introduction to Meteorology. 10th ed. Boston: Prentice Hall, 2006. Textbook covering atmospheric structure, air masses, circulation, storms, oscillations, the changing climate, and more.

Lynch, John. The Weather. Buffalo, N.Y.: Firefly Books, 2002. An outstanding book for beginners, lavishly illustrated and well written. Goes one layer deeper than typical introductory books, but it is not difficult to understand. Includes a short chapter on global warming.

Walker, Gabrielle. An Ocean of Air: Why the Wind Blows and Other Mysteries of the Atmosphere. New York: Harvest Books, 2007. Well written and easily read. Elucidates how the atmosphere behaves and how people learn about it.