Atmosphere Surface Conditions and Weather

Earth's atmosphere is divided into five layers: the troposphere, from 0 to about six miles (0 to 10 km); the stratosphere, from six to 31 miles (10 to 50 km); the mesosphere, from 31 to 50 miles (50 to 80 km); the ionosphere, from 50 to 310 miles (80 to 500 km); and the exosphere, from 310 miles and up (500 km and up). Most of the atmosphere is close to the surface of the Earth, and there is no clear top to the atmosphere, as it simply becomes thinner and thinner with height as it merges into space. In the United States people who travel above an altitude of 50 miles (80 km) are designated as astronauts.The altitude of 62 miles (100 km) is often used as the boundary between atmosphere and space. For spacecraft traveling back to Earth, at 75 miles (120 km) atmospheric effects become noticeable during reentry.

The troposphere contains approximately 80 percent of the total mass of the atmosphere. Jet planes generally fly near the top of this layer. In the Earth's troposphere there is strong convection, that is, mixing due to temperature or density differences in a liquid or gas. One example of convection is boiling oatmeal in a pot: Heat put in at the bottom of the pot causes the water and oatmeal at the bottom to expand (some water is converted into gas, which rises as bubbles). Almost every material expands when it is heated, and since it retains its original mass, it becomes less dense. Lowing the density of the material at the bottom makes it buoyant, and it rises to the top. Cooler material from the top sinks to the bottom to take its place, and the cycle continues. Gentle convection can create "cells," regular patterns of circulation with one side moving up and the other down. More violent convection can become turbulent, with currents moving on many length scales. In general, though, convection is caused by a density difference between adjacent pieces of material; the light piece will rise, and the denser piece will sink, initiating a cycle.The differences in density can be caused by temperature, as in the boiling oatmeal case, or the differences in density can be caused by differing compositions. In the troposphere temperature falls with increasing altitude, from a global average of approximately 60°F (17°C) at sea level to approximately -61°F (-52°C). This means that colder, denser gases from the upper troposphere will be continuously sinking down to displace the more buoyant, warmer, lower atmosphere. The troposphere is thus always mixing, and its currents are wind, creating weather. In addition to the density differences in a given column of the troposphere, the differences in temperature between the polar regions and the equatorial regions also cause density differences that drive winds, and therefore weather. The top of the troposphere is known as the tropopause, marking the height at which the temperature in the atmosphere begins to rise again. The height of the tropopause varies with pressure and weather, from four to 10 miles (6 to 16 km). Temperature increases in and above the stratosphere because of solar heating, and since each higher layer is hotter than the one below it the layers are unlikely to mix: Higher temperatures create less dense material, so the higher material is buoyant with respect to all that lies beneath it. All planets with atmospheres have a troposphere in which temperature falls with height, and a stratosphere where the temperature rises with heat (see figure on page 98).

Above the stratosphere lies the mesosphere, which reaches from 31 to 50 miles (50 to 80 km) altitude. Temperatures in the upper mesosphere fall as low as -99°F (-73°C), though as do temperatures at other altitudes they vary with latitude, weather, and season. Many meteors burn up in the mesosphere; there are enough gas particles in the mesophere to create sufficient friction to burn the tiny grains of interplanetary dust that are seen from the surface as shooting stars.

The two highest layer of the atmosphere, the ionosphere and exo-sphere, together are called the thermosphere. In the ionosphere strong solar radiation energizes the atmospheric gases to the point that they ionize and become electrically charged and heated. Temperatures in the ionosphere are highly dependent on solar activity, and can rise to 3,600°F (2,000°C). The electrical charges in the ionosphere also bounce radio waves, allowing radio transmissions to travel above the Earth, bounce off the ionosphere, and travel back to a point beyond the horizon from their origination.

The exosphere extends to 6,250 miles (10,000 km) above the Earth's surface, where it merges into space. Atoms and molecules are constantly escaping to space from the exosphere if they have enough energy to overcome the pull of gravity. Hydrogen and helium are the major components of the sparse atmosphere at this great height.This is also the height at which many satellites orbit the Earth, and so the exosphere is often thought of as space.

The POLAR spacecraft is orbiting the Earth at a distance of 3,700 to 17,000 miles (6,000 to 28,000 km or 2 to 9 Earth radii), in the ionosphere, the exosphere, and slightly beyond. After averaging measurements over 93 orbits, the researchers found that there is an average electron density of one electron every 10 cubic centimeters near the poles, and about one electron every three cubic centimeters near the equator.These plasma densities in the remote atmosphere of

ATMOSPHERIC CONSTITUENTS OF THE EARTH

Constituent

Chemical symbol

Fraction

Fraction on Mars

nitrogen

N2

0.7808

0.027

oxygen

O2

0.2095

0.013

argon

Ar

0.0093

0.016

water

H2O

0.3 to 0.04 ppm

300 ppm

carbon dioxide

CO2

345 pm

0.95

neon

Ne

18 ppm

2.5 ppm

ozone

O3

0 to 12 ppm

0.1 ppm

helium

He

5 ppm

methane

CH4

1.7 ppm

krypton

Kr

1 ppm

0.3 ppm

hydrogen

H2

500 ppb

nitrous oxide

n2o

300 ppb

carbon monoxide

CO

100 ppb

700 ppm

chlorofluorocarbon

CFC-12

380 ppt

chlorofluorocarbon

CFC-II

220 ppt

(Note: ppm means parts per million; 1 ppm is equal to 0.000001 or 1 x 10 6; ppb means parts per billion; 1 ppb is equal to 0.000000001 or 1 x 10-9; ppt means parts per trillion; 1 ppt is equal to 0.000000000001 or 1 x I0-12.)

(Note: ppm means parts per million; 1 ppm is equal to 0.000001 or 1 x 10 6; ppb means parts per billion; 1 ppb is equal to 0.000000001 or 1 x 10-9; ppt means parts per trillion; 1 ppt is equal to 0.000000000001 or 1 x I0-12.)

the Earth are controlled by the interactions between the magnetic field and the solar wind, and these are the source regions of the plasma that creates the aurora borealis and aurora australis.

Profile of Earth's Atmosphere

The Hubble Space Telescope orbits at 375 miles (600 km)

-----------------------------------jio miles (500 km)-------------

Exosphere

-----------------------------------260 miles (500 km)-------------

The Russian space station Mir orbits at about 250 miles (400 km).

The high temperatures here apply to the individual and highly scattered gas molecules; a person at this height would feel exceptionally cold.

The space shuttles commonly orbit at 175 miles (280 km)

Above about 125 miles (200 km) satellites feel

Thermosphere virtually no air resistance.

The space shuttles commonly orbit at 175 miles (280 km)

Above about 125 miles (200 km) satellites feel

Thermosphere virtually no air resistance.

Mesosphere

Stratosphere Troposphere

Ozone layer (15 miles, 25 km) .Highest çlqu_ds (75.mite, I2_km_)___6 m¡¡es (JQ Veather occurs in the troposphere_

Mesosphere

Stratosphere Troposphere

Temperatures in the thermosphere are highly influenced by solar activity and range from 900 to 2,700°F (500 to 1,500°C).

Temperature

A cross section of Earth's atmosphere shows how it merges into space.

The atmosphere on Earth is dominated by nitrogen, and Earth is one of the very few planetary bodies for which this is true (the others are Neptune's moon Triton, Saturn's moon Titan, and Pluto). While the atmosphere of Earth's neighbor Venus is rich in carbon dioxide, on Earth most of the carbon dioxide is tied up in carbonate rocks. The water content of Earth's atmosphere is highly variable; a given molecule of water has a residence time of about one week in the atmosphere before it is precipitated out. The main constituents of Earth's atmosphere are listed in the table on page 97, "Atmospheric Constituents of the Earth," where they are compared to the atmosphere of Mars.The life on Earth has made its atmosphere rich in nitrogen and oxygen, setting it apart from the atmospheres of its nearest neighbors.

The familiar forms of weather on Earth are, for the most part, duplicated on other planets. Hurricanes and typhoons are seen clearly on the gas giant planets, in particular Jupiter. Dust storms are common on Mars. Lightning is seen on Jupiter as well as other planets. Water rain appears to be unique in the solar system at the present time, though it probably occurred on Mars in the past. On Earth, water rain can freeze incrementally in the upper atmosphere into hailstones, like the huge example of a compound stone shown in the upper color insert on page C-7.

Storms and rain, along with snow and hail, create excitement and destruction on a daily basis somewhere in the world. Shown in the upper color insert on page C-8, some of the largest storms are tropical cyclones, low-pressure systems with circular (or "cyclonic") surface winds and rain and thunderstorms. If these storms have surface winds of less than 39 miles per hour (17 m/sec), they are known as tropical depressions. With higher sustained surface wind speeds, the storms are known as tropical storms, and if winds reach 74 miles per hour (33 m/sec), the storms are known by a bewildering number of terms. They are hurricanes if they are in the North Atlantic Ocean, the Northeast Pacific Ocean east of the dateline, or the South Pacific Ocean east of 160E, but they are known as typhoons in the Northwest Pacific Ocean west of the dateline, as severe tropical cyclones in the Southwest Pacific Ocean west of 160E or Southeast Indian Ocean east of 90E, as severe cyclonic storms in the North Indian Ocean, and as tropical cyclones in the Southwest Indian Ocean. The satellite image of the 1977 hurricane Heather shows the typical cyclonic pattern of

This satellite image of Hurricane Heather, taken in 1977, clearly shows the storm's typical spinning structure. (National Oceanic and Atmospheric Administration/Department of Commerce)

these storms, as does the above image of the 1979 typhoon Kerry, off the coast of Australia (also on page 101).

One of the most destructive hurricanes was Andrew, which hit the Bahamas, Florida, and Louisiana in 1992. Examples of damage by the hurricane are shown in two images. In the first image (on page 102), one of the cars belonged to a CNN reporter, and the other to a Hurricane Center employee, who learned of the damage to the car by watching CNN.The second image (shown in the lower color insert on page C-6) depicts a one-by-four-inch board that was driven through the trunk of a Royal Palm in the high winds of the hurricane.

The daily weather of the Earth is violent enough, but climate scientists have demonstrated that weather was more violent and extreme at times in the past, and is likely to become more violent and extreme in the future, as well.

For the last several hundred years people have known that the energetic output of the Sun is variable.This can be seen in sunspot and aurora variations, as well as in the amount of solar wind that makes it past the Earth's magnetic field and reach the Earth's surface. Over the

past 1,800 years, there have been nine cycles of solar energy output. Some of the periods of lowest solar output have names, such as the Oort, Wolf, Sporer, Maunder, and Dalton Minima. The Maunder Minimum is perhaps the most famous, because of its strong effects on civilization in Europe during recorded time. It occurred between 1645 and 1715, and even dedicated scientists with observatories were hard-pressed to see more than one or two sunspots per decade.At the same time, the Earth experienced what is called the Little Ice Age, from about 1600 to 1800 (scientists disagree about the proper beginning of the Little Ice Age; some put the start as early as 1400). Throughout the world glaciers in mountainous areas advanced. In many parts of the world average temperatures fell and harsh weather was more common. It was a time of repeated famine and cultural dislocation as many people fled regions that had become hostile even to subsistence agriculture.

In 1645 the glacier Mer de Glace on Mont Blanc in France was advancing on Chamonix due to the unusually cold temperatures. In warmer times there was less winter snow and more of it melted in the

Typhoon Kerry was one of the Pacific's major storms in 1979.

(National Oceanic and Atmospheric Administration/ Department of Commerce)

These cars were lifted and spun by the high winds of Hurricane Andrew in 1992. (National Oceanic and Atmospheric Administration/ Department of Commerce)

summer, leaving the glacier stationary. During the Little Ice Age, heavier winter snows and less summer melting weighed the glacier with additional layers of snow, pressing the deeper layers into ice, and the glacier moved more rapidly, deforming and flowing downhill under the force of its own weight like a spreading pile of bread dough. The advancing glacier had already engulfed and destroyed farms and even small villages. The Chamonix villagers wrote to the Catholic bishop of Geneva for help. The bishop actually came to Chamonix and performed a rite of exorcism, after which the glacier receded for some time, before once again inexorably advancing. Similar scenes of fear and destruction happened in other mountain areas. The North Atlantic was filled with icebergs both farther south and for more of the year than in current times, changing the cycles of fisheries and the availability of food. Eskimos came in kayaks as far south as Scotland. In China severe winters killed orange groves that had survived in Jiangxi province, around the lowerYangzi River, for centuries.

Temperatures during the Maunder Minimum period were only about one-half of a degree Celsius lower than the mean for the 1970s,

pointing out humankind's extreme sensitivity to small changes in average temperature, and the scientists Pang and Yau found that temperature decline consistent with the decrease in average solar radiance. Then, from 1795 to 1825, came the Dalton Minimum, along with another dip in Northern Hemispheric temperatures. Since that time, however, the Sun has gradually brightened to the current Modern Warm Period.

The small temperature variations in the Maunder Minimum caused violent changes in weather that strongly influenced human civilization. At the present time on Earth, average temperatures are rising. The overall average surface temperature on Earth is between 59 and 68°F (15 to 20°C), though at different places on the surface the average temperature varies between about —40 and 104°F (—40 to 40°C). About 31 degrees of the heat on Earth is due to the insulation of the atmosphere, called the greenhouse effect. Some amount of greenhouse effect is normal, caused primarily by the presence of carbon dioxide in the atmosphere.Venus's surface is hot enough to melt lead because of the greenhouse effects of carbon dioxide in its atmosphere. On Earth, the greenhouse effect is controlled by a cycle in which carbon dioxide is released continually from volcanoes, combines with calcium from weathered continental rocks, and is deposited as limestone (primarily as calcium carbonate, or CaCO3) into the oceans. When oceanic plates subduct into the mantle, their carbon, in the form of calcium carbonate, is erupted back onto the surface through the volcanoes at subduction zones.Water is among the controlling factors of this cycle: Both Venus and Mars lost their water, and so their atmospheres are dominated by carbon dioxide.

Though carbon dioxide is the dominant natural greenhouse gas, there are other chemicals that control climate when they are in the atmosphere. Many of these, including chlorofluorocarbons, are produced by humans. Over the past 100 years the average temperature on Earth has risen by between one-half and one degree due to an increased greenhouse effect caused by chemical input from man. Though this addition may seem tiny, it already appears to be having an effect. Increased average temperature does not in the short run necessarily cause the weather to be hotter; instead it causes greater variations in temperature across the globe and therefore stronger driving forces for winds and weather. As the greenhouse effect increases weather is likely to get more violent and more erratic; some scientists believe these effects are already apparent in the increased numbers of hurricanes.

Human contributions to the environment are inexorably driving the climate on Earth to extremes. Though the rock record and chemical data from ice cores show that the Earth's climate has often been at extremes in the past, this is little consolation to the people who have to find a way to live through it. Adversaries of change in the habits of society correctly state that the Earth is well within the range of climates that have existed in the past, but these are not climates that humankind has shown any ability to cope with. Many times in Earth's past the climate has moved into periods of heat (with attendant violent storms) or complete freezing, and has recovered, but never has humankind had to live through these periods. The surface of the Earth is already a challenging place, full of volcanoes, mountains, hurricanes, and other marvels full of scientific interest; it is necessary for the future of our civilization that we do what is possible to prevent further climate change.

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