Ch. 23 The Atmosphere

Ch. 23.1 Characteristics of the Atmosphere

Composition of the Atmosphere

 Most abundant elements—nitrogen, oxygen, and argon.

 Most abundant compounds—carbon dioxide and water vapor.

 Water vapor is added through evaporation and transpiration, and removed through condensation and precipitation. Its concentration varies from 4% to less than 1%.

 Ozone—a form of oxygen (O 3 instead of O 2 ) that exists in the upper atmosphere—important because it absorbs harmful ultraviolet rays.

  Ozone can be destroyed by human pollutants such as chlorofluorocarbons (CFC’s).

Also present is

atmospheric dust

—suspended mineral particles from the land and salt crystals from the ocean.

 78% Nitrogen  21% Oxygen  0.9% Argon  0.1% other (including

carbon dioxide

)

Oxygen in the Atmosphere

 Oxygen is both removed and replaced in the atmosphere at a constant rate, so the percentage of oxygen remains in a state of balance.

 Removed by: animals, bacteria, plants, burning of fuels and forests, and the weathering of rocks.

 Replaced by: land and ocean plants through photosynthesis.

Nitrogen in the Atmosphere

 The

nitrogen cycle

maintains a constant amount of nitrogen in the atmosphere.

 Nitrogen moves from the air, to the soil, to plants and animals, and again back to the air.

 Nitrogen-fixing bacteria convert atmospheric nitrogen into useful nitrogen compounds taken up by plants, which are then eaten by animals.

 Denitrifying bacteria release nitrogen back to the air when plants and animals decay or excrete waste.

Atmospheric Pressure

  Atmosphere is held by Earth’s gravity.

99% of atmosphere’s mass is within 32 km of the surface.

 Remainder extends upwards towards space for hundreds of kilometers, getting thinner with increasing altitude.

 Weight of the atmosphere presses on the surface.

 A column of air one square inch at its base, at sea level, and extending upward to the upper edge of the atmosphere, weighs 14.7 pounds.

 Force of the air on the surface can be expressed in newtons (101,325 N).

 Atmospheric pressure is the ratio of the force of the air to the area of the surface on which it presses.

 Atm. press.

decreases

with increasing altitude.

Mercurial Barometer



Barometers

are instruments that measure atmospheric pressure.

 In a mercurial barometer, the atmospheric pressure presses on a bowl of mercury and forces it up a tube.

 At sea level, mercury would be forced up the tube to an average height of 760 mm.

Aneroid Barometer



Aneriod

—without liquid.

 Contains sealed metal container at a vacuum.

 Sides of container flex inwards or outwards depending on air pressure.

 Pointer attached to container. It moves along a scale, and indicates the atmospheric pressure.

Layers of the Atmosphere

 Pressure gradually decreases with increasing altitude, but

temperature

shows

distinct differences

with increasing altitude.

 Temp. differences are due to the way solar energy is absorbed as it moves downward through the atmosphere.

 Four distinct layers based on temperature differences.

The Troposphere

 Layer closest to the earth, in which nearly all weather occurs.

 Almost all water vapor and carbon dioxide found in the troposphere.

 Temp. decreases with increasing altitude, due to increasing distance from earth’s surface (and distance from the warming effect of sunlight absorbed by earth’s surface).

 Rate of temperature decrease is about 6.5° C per km.

The Stratosphere

 Extends upward from the troposphere to a height of about 50 km.

 Contains most of the ozone in the atmosphere

(the ozone layer

)

.

 About -60° C at its base, but temp. begins rising in upper stratosphere as altitude increases…due to direct absorption of solar energy by ozone.

The Mesosphere

 Layer above the stratosphere, extending to an altitude of about 80 km.

 Temperature decrease as altitude increases…the coldest layer, dropping to about -90° C.

 Very thin air, but thick enough to burn up most meteors.

The Thermosphere

 Layer above the mesosphere. Temperature increases steadily with altitude, due to absorption of short wavelength solar energy by oxygen and nitrogen.

 Although very high temperatures (over 2000°C recorded), air is so thin and its molecules so far apart that little heat is transferred.

 Upper boundary of thermosphere not accurately determined.

The Ionosphere

 Within lower region of thermosphere (80 – 550 km).

 Solar rays strip electrons from gas molecules, forming ions and free electrons.

 There are four layers of the ionosphere.

 Free electrons can reflect radio waves back to earth.

The Exosphere

 Region of atmosphere where it blends into the vacuum of interplanetary space.

 Extends thousands of km above the surface.

Air Pollution

 Substances in the atmosphere harmful to people, animals, plants, or property.

 Results mainly from the burning of fossil fuels.

 Gases from fossil fuels combine with water to form

acid rain

, which kills plants and fish.

 Sometimes, cold air becomes trapped under warm air…this condition is a

temperature inversion

. The air is unable to disperse, and pollutants build up (common in Los Angeles).

 The result is

smog

(smoke and fog).