Meteorology

XI

Human Induced Global Warming

In 1988, the United Nations Environment Program and the World Meteorological Organization established the Intergovernmental Panel on Climate Change (IPCC) to assess the environmental, social, economic, and scientific information available on climate change. The IPCC consists of more than 200 leading earth scientists. Their Second Assessment Report, published in 1995, concluded that the earth’s average surface air temperature has increased by between 0.3 and 0.6 Celsius degrees (between 0.5 and 1.1 Fahrenheit degrees) in the past 100 years. Their report states that this warming should continue and that global average surface temperature will increase by between 1.0 and 3.5 Celsius degrees (between 1.8 and 6.3 Fahrenheit degrees) by the year 2100 (see Global Warming). If such a warming should occur, sea level should rise by between 15 cm and 95 cm (6 in and 37 in) by the year 2100, with the most likely rise being 50 cm (20 in). Such a rise in sea level might have a damaging effect on coastal ecosystems. Other changes brought on by this warming might include a shift in the world’s wind and rainfall patterns, which might put added stress on important agricultural areas, especially those in the western United States that depend on irrigation water from reservoirs and streams.

Many climate scientists believe that human activity is responsible for global warming. They attribute the main cause of global warming to the burning of fossil fuels, which increases the concentration of carbon dioxide (CO₂) gas in the atmosphere. Carbon dioxide levels, presently about 360 parts per million (ppm), have increased 28 percent in the past century. The IPCC estimates that the concentration of CO₂ in the atmosphere will surpass 500 ppm, an increase of another 40 percent, before the end of the 21st century.

Carbon dioxide warms the atmosphere through a process known as the atmospheric greenhouse effect. The atmospheric greenhouse effect is caused by certain gases in our atmosphere, called greenhouse gases, selectively absorbing and emitting infrared radiation, or heat energy. The two most plentiful greenhouse gases are water vapor (H₂O) and carbon dioxide (CO₂). Other less plentiful (and hence less important) greenhouse gases include nitrous oxide (N₂O), methane (CH₄), and chlorofluorocarbons (CFCs).

A greenhouse gas is like a filter; it allows the shorter wavelengths of radiant energy (such as visible light) to pass through it, but it absorbs some of the longer wavelengths of radiant energy (such as infrared radiation). Visible sunlight readily passes through the greenhouse gases to reach the earth’s surface, where it warms the surface. The earth’s surface, which is much cooler than the sun, emits radiant energy in the form of longer infrared waves. The greenhouse gases absorb some of these infrared waves emitted by the earth’s surface. When greenhouse gases absorb infrared energy, they share this energy with other gases and the atmosphere warms. The greenhouse gases also emit infrared radiation. Some of the emitted radiation travels back to the earth’s surface, where it warms the earth again. By preventing the rapid escape of infrared energy to space, greenhouse gases act as an insulating layer around the earth, keeping its surface much warmer than it would be if these gases were not present.

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The atmospheric greenhouse effect is a natural effect that has been occurring for billions of years. Indeed, without it, the earth would be a frozen planet with an average temperature of about -18° C (about 0° F). Due to the greenhouse effect, the earth’s average surface temperature is a comfortable 15° C (about 59° F).

It is not the greenhouse effect that concerns scientists, but the enhancement of the greenhouse effect by human induced increases in the levels of greenhouse gases. Climate models predict that the world’s average surface temperature should rise by between 1 and 3.5 Celsius degrees (1.8 and 6.3 Fahrenheit degrees) by the year 2100. However, these models show that increasing the concentration of carbon dioxide to 500 ppm and keeping everything else constant only accounts for a global warming of less than 1 Celsius degree (1.8 Fahrenheit degrees). This slight warming, however, would increase the air’s capacity for holding water vapor. The added water vapor, the most plentiful greenhouse gas, would enhance the atmospheric greenhouse effect by producing a positive feedback on the climate system. A positive feedback occurs when an initial change is reinforced by another process. In this situation, the increase in temperature causes an increase in water vapor, which absorbs more of the earth’s infrared energy, thus accounting for the rest of the warming.

The interactions between the earth and its atmosphere are complex. There are many uncertainties in the climate system, especially with regard to clouds (which tend to cool the earth by reflecting sunlight) and the oceans (which act as a huge storehouse of heat energy). It is difficult to prove that increasing concentrations of greenhouse gases are responsible for the recent global warming. Most climate scientists contend, however, that at least part of the warming is due to human induced greenhouse gases.

XII

Atmospheric Optics

Atmospheric optics is the study of how light interacts with the atmosphere and objects in it. It explains, for example, why a mirage occurs, how a rainbow forms, why sunsets are red, and why the sky is blue.

A

Mirages

A mirage occurs when an object appears displaced from its true position. Atmospheric mirages are created when light is bent, or refracted, as it travels through layers of air with differing densities (see Optics). Changes in air density are usually caused by changes in air temperature. If the air near the ground is much warmer than the air above, light from the sky will bend up into an observer’s eyes so that an observer looking down at the distant ground sees light from the sky. The image of sky where the distant ground should be produces the mirage of a watery pavement, or water resting on hot desert sand. When the light from an object is bent, making the object appear higher than it actually is, a superior mirage occurs. When an object appears lower than it actually is, the mirage is called an inferior mirage.

B

Rainbows

A rainbow is an arc of concentric colored bands that spans a section of the sky. For a rainbow to form, rain must be falling in one part of the sky and the sun must be shining from behind the observer. Rainbows form when sunlight enters a raindrop and the various wavelengths of visible light, representing the different colors, begin to slow and bend. Violet light bends the most and red light bends the least. Most of the light passes through the raindrop. But the refracted light that hits the back of the drop at a certain angle (called the critical angle) is reflected off the back of the drop. The light is then refracted, or bent, a second time as it emerges from the drop. Because each color bends differently, each color emerges from the drop at a slightly different angle, producing a spectrum of colors. Because only a single color from each drop reaches an observer, it takes many raindrops, each one reflecting light back to an observer at slightly different angles, to produce the colors of a primary rainbow.

Fainter, secondary rainbows often form above the primary rainbow. Secondary rainbows form when sunlight enters a raindrop at such an angle that two reflections occur inside the raindrop. The second reflection weakens the light intensity and causes a reversal of colors. The weakened light that emerges produces a dimmer rainbow.

XIII

History of Meteorology

The scholars of ancient Greece were interested in the atmosphere and its related phenomena. About 340 bc Greek philosopher Aristotle wrote Meteorologica, a treatise on natural philosophy. His works, although speculative, represented the sum of knowledge about the natural science, including weather and climate. At that time, anything that fell from the sky (including rain and snow) and anything that was in the sky (including clouds) were called meteors, from the Greek word meteoros, meaning “high in the sky.” From meteoros comes the term meteorology. Several years later, Theophrastus, a pupil of Aristotle, compiled a book on weather forecasting, called the Book of Signs. His work consisted of ways to foretell the weather by noticing various weather-related indicators, such as a ring around the moon, which is often followed by rain. The work of Aristotle and Theophrastus remained a dominant influence in the study of weather and in weather forecasting for nearly 2000 years.

Although weather records were kept for different locations as early as the 14th century, meteorology did not become a genuine natural science until the invention of weather instruments. These instruments gave scientists data, so that the physical laws could be tested. Italian physicist and astronomer Galileo invented a crude thermometer in the late 1500s. Italian mathematician and physicist Evangelista Torricelli, a student of Galileo, invented the barometer in 1643. A few years later, French mathematician-philosophers Blaise Pascal and René Descartes, using a barometer, demonstrated that atmospheric pressure decreases with increasing altitude. In 1667 Robert Hooke, an English scientist, invented an anemometer for measuring wind speed. In 1714 German physicist Gabriel Daniel Fahrenheit worked on the boiling and freezing of water, and from that work he developed a temperature scale. In 1780 Horace de Saussure, a Swiss geologist and meteorologist, invented the hair hygrometer for measuring humidity.

The science of meteorology benefited from advances in other sciences, technology, and mathematics. In 1660 Irish-born English scientist Robert Boyle discovered the relationship between pressure and volume of a gas. English meteorologist George Hadley, in 1735, used physics and mathematics to explain how the earth’s rotation influences the trade winds in the tropics. By flying a kite in a thunderstorm in 1752, American statesman and scientist Benjamin Franklin demonstrated the electrical nature of lightning. French chemist Jacques Charles, in 1787, discovered the relationship between temperature and volume in a gas. In 1835 French physicist Gaspard de Coriolis mathematically demonstrated the effect that the earth’s rotation has on atmospheric motions.

The first system of classifying clouds was formulated by French botanist and zoologist Jean-Baptiste Lamarck in 1802. In 1803 Luke Howard, an English naturalist, devised a better system of classifying clouds. In 1806 British Admiral and hydrographer Francis Beaufort invented a wind scale for mariners. Enough weather information was available in 1821 that William Redfield, an American saddle maker and amateur meteorologist, was able to draw a crude weather map. By the 1840s ideas about winds and storms were partially understood. Meteorology got a giant boost in 1843 with the invention of the telegraph. Weather observations and information could now be rapidly disseminated.

A significant milestone in meteorology took place about 1920 when a group of Norwegian scientists, led by Vilhelm Bjerknes, and including Tor Bergeron, developed a model explaining the life cycle of a middle latitude storm system. These ideas were expanded as upper air observations became available from aircraft and radiosondes. By 1940 upper-level measurements of temperature, pressure, humidity, and wind gave atmospheric scientists a three-dimensional view of the atmosphere.

Weather radar became available to scientists during the early 1940s. At the same time, high-flying military aircraft discovered the existence of jet streams—swiftly flowing air currents that girdle the earth. In 1946 American chemist and Nobel laureate Irving Langmuir and American atmospheric physicist Vincent Schaefer found that tiny pellets of dry ice could induce supercooled liquid water droplets to crystallize. During the same year, Bernard Vonnegut, an American chemist, discovered that silver iodide crystals could cause these same water droplets to freeze. These events ushered in an active period of cloud seeding.

The atmospheric sciences advanced again in the 1950s when high-speed computers were able to solve the mathematical equations that describe the behavior of the atmosphere. Today, computers not only plot the observations and draw the lines on surface and upper-level maps, but they also predict the state of the atmosphere up to five days into the future.

In 1960 the National Aeronautics and Space Administration (NASA) launched Tiros 1, the first weather satellite. Subsequent satellites have been more sophisticated and have been capable of monitoring more aspects of the atmosphere. In the mid-1990s, the National Weather Service upgraded its conventional radar with a network of 135 Doppler radar units that are capable of peering into severe thunderstorms, unveiling hail, tornadoes, and strong winds.

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