24.1 The Atmosphere

Edmund Hillary and Tenzing Norgay were the first to climb Mount Everest, the world’s highest mountain.

24.1 The Atmosphere

How does the atmosphere affect conditions on Earth?

The atmosphere is the layer of gases that surrounds Earth.

Earth’s Protective Layer

The atmosphere forms a protective boundary between Earth and space and provides conditions that are suitable for life.

24.1 The Atmosphere

The atmosphere protects Earth’s surface from continual pounding by meteoroids, chunks of rock and metal from space.

The atmosphere also protects Earth from much of the high-energy radiation from space.

Earth’s Protective Layer

24.1 The Atmosphere

The atmosphere holds in heat and helps to moderate Earth’s temperatures. The atmosphere also provides the gases that are essential for life.

• Carbon dioxide in the atmosphere is essential for plants and some other organisms to carry out photosynthesis.

• Oxygen is produced during photosynthesis and released into the atmosphere.

Earth’s Protective Layer

24.1 The Atmosphere

What is Earth’s atmosphere composed of?

Composition of the Atmosphere

Earth’s atmosphere is a mixture of nitrogen, oxygen, water vapor, and many other gases, in which tiny solid and liquid particles are suspended.

24.1 The Atmosphere

The atmosphere is a mixture of different gases with a fairly uniform composition up to an altitude of about 80 kilometers.

• Two gases—nitrogen and oxygen—make up more than 99 percent of clean, dry air.

• The amount of water vapor in air varies from 0.02 percent in cold, dry air to more than 4.0 percent in warm, moist air.

Composition of the Atmosphere

24.1 The Atmosphere

Dry air is a mixture of many gases. Air also contains water vapor, which makes up between 0.02 percent and 4.0 percent of air’s volume.

Composition of the Atmosphere

24.1 The Atmosphere

Various amounts of water droplets and solid particles are suspended in the atmosphere.

Some solid particles can be seen as floating dust, but most particles are microscopic.

Composition of the Atmosphere

24.1 The Atmosphere

How do pressure and density vary with altitude?

Air pressure is the force exerted by the weight of a column of air on a surface.

Air Pressure

As altitude increases, air pressure and density decrease.

24.1 The Atmosphere

The atmosphere has weight because of Earth’s gravity.

• As a result of this weight, the atmosphere exerts pressure.

• Pressure is the force exerted on a surface divided by the area over which the force is exerted.

• At sea level, air pressure is about 101,325 newtons per square meter, or 1013.25 millibars.

Air Pressure

24.1 The Atmosphere

Effect of Altitude

Air pressure changes with altitude.• Near Earth’s surface, the column of air includes the

entire depth of the atmosphere, so the pressure and density are high.

• As altitude increases, the depth of the column of air above decreases, so the pressure decreases.

• At high altitudes there is very little air in the column above, so air pressure is lower.

Air Pressure

24.1 The Atmosphere

The atmosphere is densest close to Earth’s surface.

Air PressureStandard cruising altitude for jet liner 10,000-12,000 m

Mt. Everest, Asia, 8848 m

Mt. McKinley, Alaska, 6194 m

Mt. Kilimanjaro Africa, 5963 m

Mexico City, 2240 m

Denver, Colorado, 1600 m

Washington, D.C. 9 m

24.1 The Atmosphere

About half of the total mass of the atmosphere is found below an altitude of 5.6 kilometers.

Climbers on Mt. Everest have trouble breathing without an oxygen mask because there are many fewer oxygen molecules per cubic meter of air at the summit than at sea level.

Air Pressure

24.1 The Atmosphere

Measuring Air Pressure

Scientists measure air pressure with an instrument called a barometer.

• As air pressure increases, the column of mercury rises.

• As air pressure decreases, the column of mercury falls.

• At sea level, the average air pressure is about 760 millimeters of mercury, or 29.92 inches of mercury.

Air Pressure

24.1 The Atmosphere

An aneroid barometer is a smaller, more portable type of barometer.

Aneroid barometers use a metal chamber that expands and contracts with changes in air pressure.

Air Pressure

24.1 The Atmosphere

A. Changes in air pressure cause the airtight chamber of this aneroid barometer to flex in and out.

Air Pressure

24.1 The Atmosphere

B. Changes in air pressure cause the level of mercury in the open glass tube of this antique mercury barometer to move up or down.

Air Pressure

24.1 The Atmosphere

What are the characteristics of the major layers of the atmosphere?

Layers of the Atmosphere

The four layers of the atmosphere are the troposphere, the stratosphere, the mesosphere, and the thermosphere.

24.1 The Atmosphere

The atmosphere has four principal layers—the troposphere, the stratosphere, the mesosphere, and the thermosphere. The ionosphere overlaps the lower thermosphere.

Layers of the Atmosphere

24.1 The Atmosphere

The Troposhpere

The troposphere is the lowest layer of Earth’s atmosphere.

• This layer contains almost all of the atmosphere’s water vapor and suspended particles.

• Most weather takes place in the troposphere.• Weather is the condition of the atmosphere in a

particular place at a particular time.

Layers of the Atmosphere

24.1 The Atmosphere

• The height of the troposphere ranges from about 9 kilometers above the poles to 16 kilometers above the tropics. The average height of the troposphere is about 12 kilometers.

• In the troposphere, temperature generally decreases as altitude increases.

• This characteristic of the troposphere accounts for the extremely low temperatures on Mount Everest.

Layers of the Atmosphere

24.1 The Atmosphere

The Stratosphere

The stratosphere extends from an altitude of about 12 kilometers to about 50 kilometers.

• The temperature of the stratosphere remains nearly constant at about –57°C, to an altitude of about 20 kilometers.

• Above 20 kilometers, temperature increases as altitude increases, to about 0°C.

Layers of the Atmosphere

24.1 The Atmosphere

The upper stratosphere is warmer because of the presence of the ozone layer, a region of high ozone concentration.

• Ozone is a highly reactive gas whose molecules are composed of three oxygen atoms (O3).

• Ozone absorbs ultraviolet (UV) radiation in sunlight. The energy absorbed from UV radiation is converted into thermal energy.

Layers of the Atmosphere

24.1 The Atmosphere

• Because UV radiation can be harmful to living things, the presence of the ozone layer is extremely important to life on Earth.

• In humans, UV radiation can cause a deadly form of skin cancer.

• Certain chemical pollutants have been depleting ozone in the stratosphere, but international agreements and national policies appear to have stabilized the ozone layer.

Layers of the Atmosphere

24.1 The Atmosphere

The Mesosphere

The mesosphere begins at an altitude of about 50 kilometers and extends to about 80 kilometers.

• In the mesosphere, temperature decreases as altitude increases. At the top of the mesosphere, the temperature approaches –90°C.

• The air is very thin in the mesosphere, but most meteoroids that enter the atmosphere burn up there.

Layers of the Atmosphere

24.1 The Atmosphere

The Thermosphere

The outermost layer of the atmosphere, the thermosphere, begins at an altitude of about 80 kilometers and extends outward into space.

• No boundary marks the end of the atmosphere. Instead, the thin air of the thermosphere gradually merges with space.

Layers of the Atmosphere

24.1 The Atmosphere

• In the thermosphere, temperature increases rapidly with altitude, from about –90°C to more than 1000°C.

• Recall that temperature is a measure of the average kinetic energy of molecules.

• Because gas molecules in the thermosphere absorb solar radiation and move very fast, the temperature is quite high.

Layers of the Atmosphere

24.1 The Atmosphere

The Ionosphere

The ionosphere is not a distinct layer of the atmosphere but rather a region of charged particles, or ions, that overlaps the lower thermosphere.

• Molecules of nitrogen and oxygen lose electrons as they absorb high-energy wavelengths of solar radiation.

Layers of the Atmosphere

24.1 The Atmosphere

• The molecules become positively charged ions.

• These ions are most dense between the altitudes of 80 and 400 kilometers.

• An aurora is a colorful display of light in the sky, produced when charged particles from the sun are attracted to Earth’s magnetic poles and cause ions in the ionosphere to glow.

Layers of the Atmosphere

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Auroras provide spectacular displays of light in the polar regions.

Layers of the Atmosphere

24.1 The Atmosphere

Assessment Questions

1. Which is not one of the ways the atmosphere protects life on Earth? a. by protecting Earth from asteroids and other debris in

space

b. by absorbing harmful radiation from the sun

c. by moderating Earth’s temperature

d. by converting UV rays into molecules of oxygen

24.1 The Atmosphere

Assessment Questions

1. Which is not one of the ways the atmosphere protects life on Earth? a. by protecting Earth from asteroids and other debris in

space

b. by absorbing harmful radiation from the sun

c. by moderating Earth’s temperature

d. by converting UV rays into molecules of oxygen

ANS: D

24.1 The Atmosphere

Assessment Questions

2. Which layer of the atmosphere contains the ozone layer? a. the troposphereb. the stratospherec. the mesosphered. the ionosphere

24.1 The Atmosphere

Assessment Questions

2. Which layer of the atmosphere contains the ozone layer? a. the troposphereb. the stratospherec. the mesosphered. the ionosphere

ANS: B