The Atmosphere
Chapter 11
Earth Science
2013-2014
Atmospheric Composition
• Ancient Greeks believed that air was an
element that could not be broken down.
• We now know that air is made of a
combination of many gases.
• These gases form Earth’s atmosphere.
Atmospheric Composition
• 99% of the atmosphere is made of nitrogen
and oxygen.
• 1% is everything else: argon, hydrogen, carbon
dioxide, water vapor, and other gases
Key Atmospheric Gases
• The amount of water vapor at any given time
or place changes constantly.
• The percentage changes with the seasons, the
altitude of a particular mass of air, and with
the surface features beneath the air.
Atmospheric Composition
• The percentages of nitrogen and oxygen are
important for life.
– 78% nitrogen
– 21% oxygen
– 1% everything else
Key Atmospheric Gases
• The level of carbon dioxide and water vapor
regulate the amount of energy the
atmosphere absorbs.
• Water is the only substance in the atmosphere
that exists in three states: solid, liquid, and
gas.
– When water changes state, heat is absorbed or
released, which leads to weather and climate.
Key Atmospheric Gases
• Dust and salt
– Dust is carried into the atmosphere by wind.
– Salt is picked up from ocean spray.
– Both play a role in cloud formation.
• Ice
– Third solid found in the atmosphere in the form of
hail and snow.
Ozone
• A gas formed by the addition of a third oxygen
atom to an oxygen molecule.
• Exists in small quantities mainly in a layer well
above Earth’s surface.
• Absorbs UV radiation from the sun.
• It is thinning.
Structure of the Atmosphere
exosphere
stratosphere
thermosphere
mesosphere
troposphere
Lower Atmospheric Layers
• Troposphere
– Layer closest to Earth’s surface.
– Contains most of the mass of the atmosphere.
– General decrease in temperature from bottom to
top.
– Upper limit – tropopause – varies in height from
around 16 km at the tropics to 9 km or less at the
poles.
Lower Atmospheric Layers
• Stratosphere
– Made up primarily of concentrated ozone.
– Ozone absorbs more UV radiation than air, so the
stratosphere is heated.
– Temperature gradually increases to the top of the
layer.
– Upper limit – stratopause – located about 50 km
above Earth’s surface.
Upper Atmospheric Layers
• Mesosphere
– No concentrated ozone.
– Temperature decreases.
– Upper limit – mesopause.
Upper Atmospheric Layers
• Thermosphere
– Contains a tiny portion of the atmosphere’s mass.
– Temperature increases with height to more than
1000°C.
– Molecules are so far apart that it would not feel
warm to a human.
Upper Atmospheric Layers
• Exosphere
– Outermost layer of Earth’s atmosphere.
– Light gases (low mass) are found here – helium
and hydrogen.
– No clear boundary between the exosphere and
space.
– Fewer and fewer molecules until you are in space.
Solar Fundamentals
• Radiation
– The transfer of energy through space by visible
light, UV radiation, and other forms of
electromagnetic waves.
– All substances with temperatures above absolute
zero emit radiation.
– About 35% is reflected into space.
– About 15% is absorbed by the atmosphere.
– Only about 50% is absorbed by Earth’s surface.
Solar Fundamentals
• Radiation
– Different areas absorb energy and heat up at
different rates.
– Does not heat air directly.
– Air is heated by conduction and convection.
Solar Fundamentals
• Conduction
– The transfer of energy that occurs when
molecules collide.
– Example: Put a pot of water on the stove. The
element heats the lower molecules which then
heat the molecules above them.
– Substances MUST be in contact with one another.
– Affects only a very thin layer near Earth’s surface.
Solar Fundamentals
• Convection
– The transfer of energy by the flow of a heated
substance.
– Hot air rises, cools, falls, heats, rises.
– Over and over again.
Temperature vs. Heat
• Temperature
– A measure of how rapidly or slowly molecules
move around.
– More molecules or faster-moving molecules in a
given space generate a higher temperature.
– Fewer molecules or slower-moving molecules in a
given space generate a lower temperature.
Temperature vs. Heat
• Heat
– The transfer of energy that occurs because of a
difference in temperature between substances.
– The direction of heat flow depends on
temperature.
– Heat flows from hot to cold.
Temperature vs. Heat
• Measuring Temperature
– Degrees Fahrenheit (°F)
– Degrees Celsius (°C)
– Kelvin (K)

9
TF  TC  32.0
5
5
TC  (TF  32.0)
9
T  TC  273.15
• SI unit

• No negatives. 0 K is absolute zero.

Temperature vs. Heat
• Dew Point
– The temperature to which air must be cooled at
constant pressure to reach saturation.
• Saturation – the point at which the air holds as much
water vapor as it possibly can.
• Condensation
– When matter changes state from a gas to a liquid.
– Water falls as rain.
Vertical Temperature Changes
• Temperature is cooler at higher elevations (i.e.
mountaintop).
• Lifted condensation level – the height at
which condensation occurs.
• Adiabatic lapse rate – rate at which
unsaturated air to which no heat is added or
removed will cool.
Air Pressure and Density
• Air Pressure
– You are used to the pressure that the air exerts on
you.
– Fish are adapted to live under the pressure of the
water.
– Pressure increases with depth in the ocean.
– Pressure decreases with height on land.
Air Pressure and Density
• Density
– Proportional to the number of particles of air
occupying a particular space.
– Varies with temperature:
Temperature
Pressure
Density
Increases
Increases
Decreases
Decreases
Decreases
increases
Temperature Inversions
• An increase in temperature with height in an
atmospheric layer.
• Keeps the warm air lower and allows the cooler
air to rise.
• Can lead to pollution problems.
• Has a great effect on weather conditions.
Wind
• Air moves in response to unbalanced heating and
cooling of Earth’s surface.
• Imbalances create areas of high and low
pressure.
• Wind moves from areas of high pressure to areas
of low pressure.
• Changes with height in the atmosphere.
Relative Humidity
• Humidity
– The amount of water vapor in the air.
• Relative humidity
– The ratio of water vapor in a volume of air relative to
how much water vapor that volume of air is capable
of holding.
– Varies with temperature
– Warm air is capable of holding more moisture than
cool air.
– Expressed as a percentage.
Cloud Formation
• Clouds form when warm, moist air rises,
expands, and cools in a convection current.
• As the air reaches its dew point, the water
vapor in the air condenses around
condensation nuclei.
– Condensation nuclei – small particles in the
atmosphere around which cloud droplets can
form.
Cloud Formation
• Clouds can form when wind encounters a
mountain and the air has no place to go but
up.
– Orographic lifting
– Warm air cools, water vapor condenses, and a
cloud forms.
Cloud Formation
• Stability
– The ability of an air mass to resist rising.
– Determined by how rapidly any given mass of air
cools.
– Temperature of surrounding air masses and the air
mass itself also determine cooling rate.
Cloud Formation
• Latent Heat
– The energy that is stored in water vapor and is not
released into the air until condensation occurs.
– When condensation occurs, latent heat is released
and warms the air.
– Water vapor holds lots of energy.
Types of Clouds
• Classified by their altitude and shape.
• Developed by English naturalist Luke Howard
in 1803
Types of Clouds
• Height
– Cirro
• Describes high clouds with bases starting above 6000 m
– Alto
• Describes middle clouds with bases between 2000 m to
6000 m.
– Strato
• Refers to low clouds below 2000 m.
Types of Clouds
• Shape
– Cirrus
• Latin meaning: “hair.”
• Describes wispy, stringy clouds.
– Cumulus
• Latin meaning: “pile or heap.”
• Describes puffy, lumpy-looking clouds.
– Stratus
• Latin meaning: “layer.”
• Describes featureless sheets of clouds.
– Nimbus
• Latin meaning: “cloud.”
• Describes low, gray rain clouds.
Cloud Formation
• Low Clouds
– Stratocumulus or layered cumulus – covers much
or all of the sky in a given area
– Stratus – i.e. lifted fog
• Middle Clouds
– Altocumulus – resemble white fish scales
– Altostratus – dark, but thin veils of clouds
Cloud Formation
• High Clouds
– Cirrus – wispy, indistinct appearance
– Cirrostratus – continuous layer – transparent to very
dense
• Clouds of Vertical Development
– Clouds can grow vertically if the air that makes up a
cumulus cloud is unstable enough.
– Cloud is warmer than the air and thus rises.
– Can reach 18,000 m with the top part frozen.
Precipitation
• Coalescence
– The process in which cloud droplets collide and
join together to form a larger droplet.
• Precipitation
– Includes all forms or water, both liquid and solid.
– Rain, snow, sleet, and hail.
The Water Cycle
• The constant movement of water between the
atmosphere and Earth’s surface.
• Receives its energy from the sun.
– Causes liquid water to change into a gas 
evaporation.
– Evaporated water rises into the atmosphere.
– Water vapor cools and condenses to form clouds.
– Water droplets combine and falls to Earth as
precipitation.
The Water Cycle
Condensation
Evaporation
Precipitation
Runoff
Groundwater