5.1STRUCTURE AND

COMPOSITION OF THE ATMOSPHERE

Composition of the Atmosphere

The atmosphere is a mixture of gases surrounding Earth.

Nitrogen (78%), the most common atmospheric gas, is released when dead plants and dead animals break down and also when volcanoes erupt.

Oxygen (21%), the second most common atmospheric gas, is made by phytoplankton and plants.

The remaining 1% of the atmosphere is made up of argon, carbon dioxide, water vapor and other gases.

Atmospheric Pressure Gravity pulls gas molecules

toward Earth, causing atmospheric pressure, which is the force per unit area that is exerted on a surface by the weight of the atmosphere. Strongest at Earth’s surface

more air is above you. Avg. pressure at sea level can be

expressed as any of the following:○ 1013.2 millibars (mb)○ 1 atmosphere (atm)○ 760 mm. Hg (mercury)○ 30 inches Hg (mercury)

Layers of the Atmosphere

The Earth’s atmosphere is divided into four layers based on temperature.

The name of each layer gives you clues about its features:Troposphere (tropo = turning or change)Stratosphere (strato = layer)Mesosphere (meso = middle)Thermosphere (thermo = heat)

Troposphere Lowest layer of the atmosphere, lying next to Earth’s surface.

Temperature decreases with altitude.

Contains almost 90% of the atmosphere’s mass.

Nearly all weather occurs in this layer.

The zone known as the tropopause represents the upper boundary of the troposphere.

Stratosphere The atmospheric layer above

the troposphere.

Lower stratosphere is extremely cold, but temperature increases as altitude increases in the stratosphere. Rises because ozone, (O3), in

the stratosphere absorbs UV radiation from the sun, warming the air protects life on Earth.

The zone known as the stratopause represents the upper boundary of the stratosphere.

Mesosphere

The middle layer of the atmosphere.

Coldest layer as well.

Temperature decreases as altitude increases, just like the troposphere.

The zone known as the mesopause represents the upper boundary of the mesosphere.

Thermosphere Uppermost atmospheric layer.

Temperature again increases steadily with altitude because nitrogen and oxygen atoms absorb solar radiation. This releases thermal energy.

No data to determine its upper boundary. Blends with the vacuum of space (exosphere).

Ionosphere

The lower part of the thermosphere is considered the Ionosphere.

As a result of N2 atoms and O2 atoms absorbing the radiation, temperature in the thermosphere rises, and gas particles become electrically charged ions.

Ions within the ionosphere can radiate energy as shimmering lights called auroras, a.k.a. the Northern and Southern Lights.

Radiation

The Earth receives energy from the sun by radiation, which is the transfer of energy as electromagnetic waves move through space.The sun radiates enormous amounts

of energy, but the Earth only receives about two-billionths of this energy.

This small amount still drives weather cycles and makes the Earth a habitable environment.

Conduction If you ever got burned when you

touched something hot, you have experienced another form of energy transfer.

Conduction is the transfer of energy through contact of one object to another object.Thermal energy is always transferred

from warm to cold areas.

When air molecules come in direct contact with the warm surface of the Earth, thermal energy is transferred to the atmosphere.

Convection The transfer of energy by the

circulation, or movement, of a liquid or gas is known as convection.Most thermal energy in the

atmosphere is transferred by convection.

○ Warm air rises, cool air sinks.

Convection should also sound familiar to you from talking about Earth’s interior.

○ Warm material rises, cool material sinks.

The Radiation Balance For Earth to remain habitable for humans, the amount of energy

received from the sun needs to balance the amount of energy returned to space.Solar energy absorbed by Earth is eventually reradiated into space as

thermal energy.

Every day, the Earth receives more energy from the sun. The balance between incoming energy and outgoing energy is known as the radiation balance.Out of the incoming solar radiation from the sun:○ About 25% is reflected and scattered by clouds and air.○ About 5% is reflected by Earth’s surface.○ About 20% is absorbed by clouds and the atmospheric gases.○ About 50% is absorbed by Earth’s surface.