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Atmosphere
Composition
• The atmosphere is composed or made of
certain substances that are gases at “normal”
temperatures.
– Nitrogen: 78%
– Oxygen: 21%
– Carbon Dioxide: <1%
– Argon: <1%
– Water vapor: <1%
Atmosphere layers
• Thickness: since the
Earth is spinning, it
bulges slightly around
the equator.
• The atmosphere is thus
thicker at the equator
where it bulges and
thinner at the poles.
Troposphere
• The troposphere stretches from the surface to
up to 20 kms in the air. (About the distance
from here to Orland Park)
• Almost all air movement occurs in the
troposphere.
• All weather occurs in the troposphere.
• The troposphere is the warmest and densest
layer, but cools near the top.
Stratosphere
• The stratosphere extends from 20 to 50 km in
the air.
• The stratosphere contains a layer of ozone (O3)
that absorbs ultraviolet light before it can
reach Earth’s surface.
• The stratosphere warms near its top because
of the energy the ozone layer absorbs.
• As altitude increases, air pressure continues to
decrease.
Mesosphere
• Temperatures continue to drop as you rise
through the mesosphere.
• Falling meteors experience enough friction in
the mesosphere that they heat up to become
glowing hot: falling stars.
• Most meteorites burn up completely as they
fall.
• Ice clouds are sometimes found in this layer.
Thermosphere
• In this layer, temperatures begin to rise again.
As the atmosphere gets thinner (density and
pressure decrease) the remaining particles
absorb so much energy from the sun that they
are very hot, if rarely found.
• This is where the International Space Station
and most satellites are found.
Exosphere
• The exosphere is what we would consider
“outer space.”
• Here, gases are so thin and spread out, that
the temperature, pressure and density are
basically zero.
• In the exosphere, there is so little matter to
absorb electromagnetic energy that
electromagnetic waves can go on forever.
Atmosphere Conditions
• Temperature increases with direct sunlight
and longer days.
• Temperature: average temperature increases
closer to the equator, and decreases closer to
the poles.
Atmosphere Conditions
• Humidity: humidity measures how
much water vapor (water in gas
phase) is in the air. High humidity
means there is lots of water vapor.
• Humidity can increase as
temperature increases.
Atmospheric Conditions
• Pressure: Pressure is the force on an area. In the
atmosphere, that force is caused by gravity pulling
a column of air towards the planet.
• The weight of that air causes pressure.
• Pressure decreases with altitude, because there is
less air pressing down from above.
The water cycle
• Evaporation – As liquid or
solid water is heated, the
fastest moving molecules
escape as gas: water
vapor.
• Evaporation rates
increase as temperature
increases.
• Evaporated water moves
throughout the
atmosphere.
• Evaporated water is
relatively fresh or pure
water.
The water cycle
• Condensation: the process where water vapor
cools and forms liquid or solid drops.
• As evaporated water (water vapor) moves into
the upper troposphere, the temperature
decreases as altitude increases.
• Water vapor condenses around ice crystals
and dust to form droplets.
The water cycle
• Precipitation: this is when water droplets or
crystals become large enough that the air can
no longer hold them aloft, and they fall to the
ground as rain, snow, hail or sleet.
• Precipitation is fresh water.
The water cycle
• Ground water: Liquid precipitation that meets
the ground and soaks in gets filtered as it
passes through layers of soil and rock to
become groundwater.
• Runoff: water that cannot soak into the
ground moves along the surface as runoff.
This water joins together into creeks, stream,
rivers, lakes, seas and finally oceans.
Fresh Water
• Fresh water: only 1% of the water on earth is
fresh. The rest is all salt water.
• Fresh water is found trapped in ice, snow,
glaciers, streams, rivers and lakes.
• As groundwater filters through soil and rock,
salts and minerals dissolve into it, forming salt
water.
Brackish water
• As fresh water in rivers flows into the oceans
and seas, it mixes to form partially salty water
called brackish water.
• Sedimentation, evaporation and organic
processes all work to keep ocean water at the
same levels of saltiness.
The air cycle
• As visible and ultraviolet radiation is absorbed
by land or water, it heats up, and reflects
infrared radiation.
• The land or water then heats nearby air
through conduction.
• As the air near the ground warms, its density
decreases, and the air rises as a convection
current.
Greenhouse Effect
• As infrared radiation reflects
from the ground, it can go
back into the exosphere.
• Certain substances can absorb
infrared radiation, converting
the energy into molecular
motion, also known as
temperature increase.
• If present in the atmosphere,
these are called greenhouse
gases (Methane, carbon
dioxide, water vapor),
although glass can work
similarly.
• The movement of air as a
part of convection
currents causes wind.
There are three major
wind belts:
– The trade winds blow
out from the equator
from the east between
the equator and the
tropics. (Hadley Cells)
– The Prevailing Westerlies
blow in towards the
equator from the west
between the tropics and
the arctic circles. (Midlatitude/Ferrel Cells)
– The Polar Easterlies blow
away from the equator
from the east between
the arctic circles and the
poles. (Polar Cells)
Wind
The Coriolis Effect
• Winds blow directly away from
high temperature areas at the
equator.
• Because the Earth is rotating
towards the east
(counterclockwise when viewed
from above the North Pole,)
winds appear to move in relation
to the ground.
• They move West as they move
away from the equator, and East
as they move away from the
equator.
• This causes winds in the
northern hemisphere to move
counterclockwise, and those in
the southern hemisphere to
move clockwise.
Winds
• Winds always blow from high pressure (cold,
dense areas of air) to low pressure (hot, low
density areas of air.)
• Differences in temperature cause differences
in pressure, which cause winds.
Land and Ocean Winds
• Land heats and cools quickly,
and water heats and cools
slowly.
• During the day, land heats up,
and heats the air above it. That
air floats upwards, and colder air
from the ocean blows in as an
ocean breeze to replace it.
• During the night, land cools off,
but the ocean stays warmer,
heating the air above it. That air
floats upwards, and colder air
from the land blows in as a land
breeze to replace it.
Jet streams
• Air near the top of the troposphere moves
from West to East at high speed in currents
called jet streams.
• Jet streams can carry cold polar air towards
the equator, creating storm fronts.
Air masses
• Temperature differences can form differences
in pressure and density.
• Cold air has high pressure, and is denser.
• Warm air has low pressure, and is less dense.
• Bodies of moving air with differences in
density do not easily mix.
Air Masses
• Tropical masses: warm, with low pressure.
• Polar masses: cold, with high pressure.
• Maritime masses: forms over oceans, with
high humidity.
• Continental masses: form over continents, and
are dry.
• Air masses usually move in the same direction
as the wind belt they are in.
Air Fronts
• Weather changes as conditions in the
atmosphere change with moving air masses.
Cold fronts
• A moving mass of colder air forces its way under
the less dense warm air as it moves.
– This forces the warm air higher in altitude, cooling it.
The water vapor in the warm air can then condense to
form storm clouds, possibly causing precipitation.
– This warm air moves in the opposite direction as the
cold front.
– Cold fronts are sometimes called storm fronts, and
can move quickly to cause dramatic changes in
weather.
Warm Fronts
• A moving mass of warm air can collide with a
cold mass, sliding up on top of it.
• The warm air usually is moving in the same
direction as the cold air.
• As the warm air increases in altitude, it cools.
Condensation forms clouds and sometimes
precipitation.
Stationary Fronts
• If neither of two air masses
is moving with enough
force to slide over or under
the other, a stationary
front forms.
• At the boundary, warm air
cools and condensation
occurs, forming clouds and
precipitation. Ongoing
storms can result.
Occluded fronts
• An occluded front forms when two cold air masses move
together with a warm air mass between them.
• The warm mass is forced higher in altitude, and cools to
form clouds and storms.
Tornadoes
• When cold air masses slide up atop warm
masses, a vortex can form as they switch
positions, resulting in a small rotating tube of
air that.
• High wind speeds, short durations
Hurricanes
• As air passes over warm ocean water, it picks
up water vapor and heat.
• This humid, warm air rises and moves away
from the equator in a curve because of the
Coriolis Effect.
• As it rises, it cools. Condensation and then
precipitation forms. This rainfall warms the
air, causing more evaporation.
Hurricanes
• The constant evaporation of water and
heating of air creates an ongoing current of air
in a giant circle, curving to the east as it moves
away from the equator and to the west as it
cools and returns to the equator.
• Precipitation releases more energy into the
system as rain warms air as it falls,
accelerating the system further until hurricane
strength winds result.
Weather maps
• Weather maps represent cold fronts with blue
lines with semicircles.
• Warm fronts are represented with red lines
with triangles.