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The Atmosphere
Structure and Temperature
Atmosphere Characteristics
 Weather: the state of the atmosphere at a
given time and place; “snapshot” in time
 Climate: based on weather observations
over many years (typically 30 years)
 Weather properties: air temperature;
humidity; type and amount of precipitation;
air pressure; wind speed and direction
Atmospheric Composition
 Mixture
 “Others”: Argon, CO2,
water vapor, traces of
other gases
 Water vapor: source of
clouds and precipitation;
absorbs heat from Earth
and solar radiation
 Condensation nuclei
(particulates) : dust, pollen
salt, soot, smoke
(necessary for cloud
formation as provides
water with something to
condense onto)
Height and Structure
 Atmosphere thins with
increased altitude (gravity
concentrates air at
surface)
 Temperature decreases
with altitude in
Troposphere (where
weather occurs) and
Mesosphere
 Temperature increases
with altitude in
Stratosphere (ozone) and
Thermosphere (O and N
absorb short-wave, high
energy solar radiation)
The Atmosphere
QuickTime™ and a
Sorenson Video 3 decompressor
are needed to see this picture.
Altitude and Pressure
 Air pressure is simply the
weight of the air above you
 The higher the altitude, the
fewer the air particles
 Fewer particles exert less
pressure, thus pressure
decreases with an
increase in altitude
 Air that rises from the
surface moves into layers
with less pressure. This
makes the air parcel
expand and cool, forming
clouds (adiabatic cooling).
Adiabatic
 Describes a change in temperature resulting
from the expansion or compression of air
 Air that rises will expand (due to less
pressure from surrounding particles) and will
cool adiabatically
 Air that descends will compress (due to
more pressure from surrounding particles)
and will warm adiabatically
Earth-Sun Relationships
 Solar energy is not distributed evenly over
Earth’s surface
 Varies with latitude, time of day, and season
of the year
 Unequal heating creates winds and drives
ocean currents
 Winds and ocean currents transport heat
from warmer to colder regions in an attempt
to balance energy differences
Earth’s Orientation
 Seasonal changes occur because Earth’s position
relative to the sun continually changes as it travels
its orbit.
 Due to the Earth’s tilt (23.5º) and revolution around
the sun, energy received at given latitudes
changes with time. This creates our seasons as
well as seasonal variations in severe weather.
 Seasons ARE NOT due to changes in distance
from the sun. (We have summer when farthest
from the sun, and winter when closest.)
Heating the Atmosphere
 Heat is the energy transferred from one object to
another because of a difference in their
temperatures.
 When energy is transferred to the gas atoms and
molecules in the air, particles move faster and air
temperature increases.
 When air transfers energy to a cooler object, its
particles move slower and air temperature
decreases.
 Three mechanisms of heat transfer: conduction,
convection and radiation
Conduction
 Transfer of heat through matter by molecular
activity
 Energy of molecules transferred by
collisions from one to another
 Heat flows from higher to lower
temperatures
 Air is a poor heat conductor, so conduction
only occurs between land and air in direct
contact with it
Convection
 Transfer of heat by mass movement or
circulation within a substance
 Takes place in fluids (ocean and air) and
solids (mantle)
 Air warmed at surface rises (less dense),
cools adiabatically and sinks (more dense)
 Most heat transfer within the atmosphere is
due to convection.
Radiation
 All objects (hot or cold) emit radiant energy,
although hotter objects emit more total
energy
 Hottest radiating bodies produce the
shortest wavelengths of maximum radiation
 Solar energy reaches earth by radiation
 Some energy (depending on wavelength) is
scattered, some reflected and some
absorbed at the Earth’s surface.
Temperature and Wind
 Temperature differences (due to unequal
heating of the Earth’s surface) create
pressure differences. (Colder air is
“heavier” so it exerts greater pressure.)
 Wind is the result of pressure differences.
Wind flows from areas of HIGH pressure to
areas of LOW pressure in an attempt to
make the atmosphere more uniform.
Factors Affecting Wind
 If Earth did not rotate and
friction between air and
Earth didn’t exist, air would
flow in a straight line from
high to low pressure.
However…
 Three factors combine to
control wind: pressure
differences, the Coriolis
effect, and friction.
Pressure Differences
 The greater the difference
in pressure, the greater the
wind speed.
 Isobars connect places of
equal air pressure. The
closer the isobars, the
faster the wind speed.
 Pressure Gradient: amount
of pressure change
occurring over a given
distance
Coriolis Effect
 Describes how Earth’s
rotation affects moving
objects.
 All free-moving objects or
fluids (including the wind)
are deflected to the RIGHT
of their path in the N.H.
and to the LEFT of their
path in the S. H.
 Wind actually moves in a
straight line, but the Earth
rotates beneath it, giving
the “illusion” of bending.
Earth’s Rotation and Weather
Systems
QuickTime™ and a
Sorenson Video decompressor
are needed to see this picture.
Friction
 Important only within a few kilometers of
Earth’s surface.
 Acts to slow air movement, which changes
wind direction.
 When air is above friction layer, the pressure
gradient causes air to move across the
isobars. At that time, the Coriolis effect acts
at right angles to this motion. The faster the
wind, the greater the deflection.
Pressure Centers and Winds
 Cyclones (low
pressure centers)
 Pressure decreases
from outer isobars to
center
 Winds flow
counterclockwise into
system
 Associated with
overcast skies and rain
 Anticyclones (high
pressure centers)
 Pressure increases
from outer isobars to
center
 Winds flow clockwise
out of system
 Associated with fair
weather and blue skies
Weather and Air Pressure
 Low pressure at surface
causes air to convergence
and rise. Rising air cools
adiabatically and forms
clouds.
 High pressure at surface
causes air to diverge (air
subsides from aloft and
warms adiabatically)
creating clear, blue skies.
Global Winds
 Recall that a non-rotating
Earth would create two
main convection cells with
air flowing from the poles
to the equator (high to low)
 The Earth’s rotation breaks
this convection into smaller
cells
 The behavior of winds
within these cells produce
global wind belts.
El Nino
 Under normal conditions,
trade winds and a strong
equatorial ocean current
flows toward the west.
 Flow encourages
upwelling of cold nutrientfilled water from below as
water above “pushed
away” from the wind.
 Food source for fish
More on El Nino…
 At irregular intervals of
three to seven years,
these warm
countercurrents become
unusually strong and
replace normally cold
offshore water with warm
equatorial waters (El Nino)
 Produces abnormal
weather patterns for
Ecuador and Peru
(excessive rainfall).
 Fishing industry impacted.
La Nina
 Opposite of El Nino
 Occurs when surface temperatures in the eastern
Pacific are colder than average
 Distinctive set of weather patterns
 Colder than normal air over Pacific Northwest and
northern Great Plains, but warmer over much of
the rest of the United States
 Greater precipitation over Northwest than usual
 Can increase hurricane activity; hurricane
damages 20X greater in U.S. during La Nina years