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Ch 18 The Atmosphere
Vocab: 18.1 – troposphere, temperature inversion,
stratosphere, ozone, mesosphere, thermosphere, greenhouse
effect
18.2 – water cycle, transpiration, precipitation, humidity,
relative humidity, dew point, barometric pressure, Coriolis
effect
18.3 – air mass, front, isobar, climate, topography
18.1 Characteristics of the Atmosphere
Layers of the Atmosphere
Earth’s atmosphere consists of a variety
of gases.
• 78% nitrogen, 21% oxygen, trace
gases
Layers – differ in temperature, density, and
amount of certain gases
• Troposphere: height 8 km - 18 km
– Closest layer to the crust
– Almost all weather occurs in troposphere
– Densest layer because of the weight of gases
above
– Temperature decreases (6°C/km) as altitude
increases
– Tropopause= top of troposphere
where temperature stops decreasing
– Temperature inversion= sometimes
cold air in troposphere gets trapped
beneath warm air; this is one reason
for smog pollution
• Stratosphere: height 8 km – 50 km
– Contains ozone layer – O3 – absorbs
sun’s energy (UV)
– Temperature increases as altitude
increases
– Very little water vapor – little
weather, no storms
• Mesosphere: height 50 km – 80 km
– Temperature decreases as altitude
increases
• Thermosphere: height 80 km – 480 km
– Very hot layer 980°C; gas molecules are
far apart
• Ionosphere
– In between thermosphere and
mesosphere, where electrically charged
ions are formed
– Electrons in ionosphere reflect radio
waves, better at night
– Auroras, colorful light displays occur:
aurora borealis
• Note: Temperature is the measurement of
heat in the atmosphere
• Fahrenheit to Celsius: °C = (5/9)(°F – 32)
• Celsius to Fahrenheit: °F = ((9/5)(°C)) + 32
Changes in the Atmosphere
When Earth solidified –
volcanic eruptions released
gases and created the
atmosphere. Oxygen was
not in this early
atmosphere. Bacteria and
single-celled organisms
lived in this early
environment.
Photosynthesis: Plants
use carbon dioxide to
produce oxygen while
making energy from
sunlight.
Respiration: Animals
use oxygen to
produce carbon
dioxide, making a
cycle.
Man-made chemicals can deplete the
ozone layer
• 1985 scientists reported a lower
concentration of ozone
• Thought to be caused by increase of
Chlorofluorocarbons (CFCs); gases
used as refrigerants and propellants
in spray cans which can break down
ozone molecules.
Greenhouse effect= natural condition
where atmospheric gases trap sun’s
Energy
• If too much heat is trapped, the global
temperature will rise. Some scientists say
that this is happening and damaging the
Earth.
18.2 Water and Wind
Water Cycle
• Movement of water in troposphere:
transpiration, evaporation, precipitation,
condensation, ground water
Humidity
• the amount of water
vapor in the air
–water molecules fit
between the air
molecules
• the amount of water in
the air depends on the
temperature
• at cooler temperatures air molecules
move more slowly allowing the water
molecules to condense and fall out of
the air
• at warmer temperatures air molecules
are moving too fast for the water
molecules to join up and fall out of
the air
Relative Humidity
• measure of the amount of water
vapor that the air is holding
compared to the amount that it can
hold at a specific temperature
• saturated= when the air contains all
the water it can possible hold
–saturated air = 100% relative
humidity
• dew point
– the temperature at which air is
saturated and condensation takes
place
– dew point changes with the amount
of moisture in the air
Condensation
• the change from vapor to liquid
• for water vapor to condense, air must be
cooled below its dew point
• cooling can happen by
– contacting a colder surface
– radiating heat
– mixing with colder air
– expanding when it rises
• condensing water vapor needs
something to hold onto to condense
– condensation nuclei – the tiny
particles on which water vapor
condense
•salt – ocean sea spray evaporates
•surfaces and nitrates – natural
sources and burning fuel
• condensed liquid is a fine mist as fog
when the warm ground meets the cold
air
Cloud formation
• clouds form when water vapor begins
to condense around small particles of
dust, salt and smoke in the
atmosphere
• Cloud classification: classified mainly
by shape and altitude
3 main types of clouds
• Stratus clouds
– appear when layers
of air cool below their
dew point
temperatures
– form at low altitudes:
up to 6 km
– smooth even sheets,
layered appearance
– associated with both fair weather
and precipitation
– drizzle comes when these clouds
are low and gray
– fog= when the air is cooled to its
dew point and condenses near the
ground it forms a stratus cloud at
ground level
• Cumulus Clouds
– fluffy, white clouds
with flat bases
– they form when air
currents rise, and
water condenses
– various altitudes
from 500 m to 12 km
– associated with fair
weather and
thunderstorms
• Cirrus Clouds
– appear wispy,
fibrous or curly
– high altitude: 6,000
– 11,000 km
– contain ice crystals
– associated with fair
weather and
approaching
storms
Other clouds are combinations of these
3 main types
• Nimbus Clouds
– added to one of the 3 main clouds
– dark clouds associated with
precipitation
– they are so full of water that no
sunlight penetrates them
Rain capacity
• when water droplets combine and
reach the size of 0.2 mm they become
too heavy and fall out of suspension
in the cloud
Air Pressure - (barometric pressure or
atmospheric pressure)
• pressure from air in atmosphere above
measuring instrument
• measured in inches mercury, millimeters
mercury or hectapasals
• Barometers
– Mercury barometer = more accurate
– Aneroid barometer = more portable; no
liquid
• overall pressure depends on
temperature, density, and amount of
water vapor in air
–
temperature =
pressure
–
temperature =
pressure
–
density =
pressure
–
density =
pressure
–
water vapor =
pressure
–
water vapor =
pressure
• high pressure means descending air
– when the air can’t rise, clouds can’t
form = nice weather
• low pressure means clouds = bad
weather
Winds
• Pressure gradient: the differences in
pressure that create winds
• Air is pushed from high-pressure to
low-pressure
• So, winds result from the uneven
heating of the atmosphere
• warm air rises faster creating a pressure
gradient force
Differences in pressure causes winds
• heated island surrounded by cooler
water becomes a region of low
pressure, causing winds to blow
toward the land
Land breeze occurs at night
• Winds are named for the direction
that they come from: North winds
come from the north.
• The Coriolis Effect – change in movement
due to rotation
• Winds normally blow from high-pressure
areas to low-pressure areas. However, the
Earth is rotating as well. Due to this effect,
the path of the wind does not move in a
straight line relative to land. Winds are
turned to the right (counter clockwise) in
the North Hemisphere and to the left
(clockwise) in the Southern Hemisphere.
• Northern Hemisphere: winds flow
clockwise around highs and
counterclockwise around lows
Circulation Cells, Pressure Belts, and Wind
• Due to Coriolis effect, the earth’s rotation
affects direction of wind
• Earth’s rotation=
wind starts= wind blows=
• The winds between the pressure zones are
named from the directions that they flow from
– North + coriolus effect= northeasterlies
– South + coriolus effect= southwesterlies
animation
• Both the Northern and Southern
Hemisphere have three wind belts: Polar
Easterlies, Westerlies, and tradewinds.
The winds move in vertical loops called
cells.
– Equator has a low-pressure belt,
because hot air rises.
– 30° N latitude has a high-pressure belt
– 60° N latitude has a low-pressure belt
Trade winds: Found between the
equator and 30° N and 30° S
• Warm and steady in both direction
and speed
Prevailing winds: Winds that blow
from the same direction on the
average
18.3 Weather and Climate
Weather
• present state of the atmosphere and
describes current conditions
• caused by the interaction of air,
water, and sun
• National Weather Service depends
upon meteorologists and satellites
Meteorologist
• Study the weather; specifically
temperature, air pressure, winds,
humidity, and precipitation using
machines such as satellites, Doppler radar,
computers, and instruments attached to
weather balloons
• Information weather maps predictions
Air Masses
• a large body of air with uniform
temperature and moisture content
• has the same properties as the surface
over which it develops
– cold usually forms in high altitude
– hot usually forms near tropics
– wet usually forms over an ocean
– dry usually forms over land
N. American air masses
Fronts
• A front is the boundary between air
masses of different densities
• Weather occurs at fronts due to the
interaction of the air masses
• Air moves from high pressure
systems to low pressure systems
• As the cold air converges into the low
pressure system, it pushes the
warmer, less dense air upward
• As the warm air rises it cools and
reaches its dew point and the water
vapor condenses forming clouds
• When a cold front meets a warm
front the air does not mix
– High pressure - cold air moves
under the warm air
– Warm air is pushed higher
– Wind begins
– Coriolis effect turns the wind and
makes it a circle counter clockwise
around the low pressure area
• Fronts usually bring a change in
temperature and always bring a
change in wind direction
• Most changes in weather occur at the
4 major front types
• Front symbols
Cold front
• Cold air mass advances and forces
warmer air mass to rise
• Rapid speed of advance causes steeper
slopes than warm fronts
• Can causes a narrow area of high winds,
violent thunderstorms, and sometimes
tornadoes
Cold Front
Warm front
• Less dense, warm air mass slides over a
departing cold air mass
• Gentle sloping front because of slower
movement
• As warm mass rises, it cools and
precipitation can occur over a large area
for one to two days
Warm Front
Stationary front
• When neither air mass is being displaced,
the front does not move
• Pressure differences cause a warm front or
cold front to stop moving for several days
• Light wind and precipitation across the
front for one to two days
Occluded front: 3 air masses coming
together: 2 types
• Occurs when cold, cool, and warm air
come together
• Cold occlusion: a cold front pushes
under a cool front and an overlying
warm front
• Warm occlusion: a cool front slides in
between a cold front and an
overlying warm front
• Both cause warmer air to rise; strong
winds and heavy precipitation
Occluded Front
Precipitation
• Rain, sleet, snow, hail
• When air rises high enough and in
large enough quantities, precipitation
occurs
• Warmer air holds more precipitation
• Higher the air rises, the more
moisture it can drop
• Rainy areas of the Earth (NW coast of
USA)
– windward side of mountain range
– storm areas
•hurricanes, typhoons, low pressure
zones, and fronts
•warm air rises, cools, and drops water
• areas favored by the global wind belts at
the equator where air has no choice but to
rise causing daily thunderstorms
Thunderstorms
• Heavy rain, lighting flashes, thunder
and sometimes hail
• Occur inside warm, moist air masses
and at fronts
• Warm, moist air moves upward
rapidly, cools, condenses, and forms
cumulonimbus clouds up to 10 km
high
– Water droplets fall, collecting other
drops
– The falling rain causes downdrafts
which form strong winds
• Lightning= a large spark caused by a
rapid uplift of air resulting in a build up
of electrical charges (+ & -); can travel
easily through water
• Thunder
– Results from the rapid heating of the air
around the bolt of lightning then its
cooling
– Moving molecules cause sound waves:
25° C to 30,000° C
Tornadoes
• Violent whirling wind that moves in
a narrow path over land and through
the clouds
• Most form along fronts in severe
thunderstorms as warm air rises, and
begins to rotate
• Wind shears: differences in wind
direction and speed: causes funneling
• Most common in the United States
during spring and early summer
• Begins as a funnel cloud of water
droplets
Hurricanes
• A large, swirling, low pressure
system that forms over tropical
oceans; also called cyclones (Indian
Ocean) and typhoons in (Pacific)
• Warm ocean water evaporates; warm
water vapor rises causing tropical
depressions (low-pressure area) that
can gain strength
• As water vapor rises, it condenses
into clouds… releases energy
Hurricane
• Occur when SE tradewinds meet up
with NE tradewinds in North
Atlantic
• Start in the north hemisphere
between 5° and 20° latitude; move
slowly and are powerful
Weather Maps
• Map showing weather conditions
including precipitation, wind speed,
and cloud coverage.
Isotherm
• Line that indicate temperature
• Connects points of the same temperature
– “iso” = same
– “therm” = temperature
Isobar
• Line drawn to connect points of equal
atmospheric pressure
• You can tell how fast the wind is
blowing in an area by how close the
isobars are; closer isobars = faster
winds
• When isobars create a closed loop,
this is the pressure center
– Remember lows usually mean
clouds and precipitation
– Highs mean fair weather
Climate = the average weather of a
region measured over years
• The Sun
• The sun is the driving force behind
weather… it supplies the energy for
air and water to heat and move which
causes winds
• Winds move in attempt to equalize
pressure and temperature… but
cause changing weather.
• Temperatures are
closer to the
equator because of
the direction of
sunlight
• Earth’s tilt and rotation cause the
seasons
– Northern hemisphere has summer
when it is tilted towards the sun…
Southern hemisphere is on opposite
seasons
– June 21 summer solstice – longest
day
– December 21 winter solstice –
shortest day
– March 21 vernal (spring) equinox –
day and night are equal
– September 22 autumnal (fall)
equinox – day and night are equal
Topography affects climate
• Hills, mountains, valleys etc affect
pressure systems and climate
• Air masses must rise to pass over
mountains… it cools, precipitation
falls on windward side.
• Deserts often form on the leeward
side of a high mountain range
• Broad flat surfaces (Great Plains)
have winds and air masses converge
and create thunderstorms and
tornadoes