Chapter 14 – The Atmosphere
The modern atmosphere
• Two most abundant gases:
– 78% N2
– 21% O2
• Less abundant gases (< 1%)
– Argon
– Water vapor
– CO2 (only about .035%)
• Non-gaseous components
– water droplets
– dust, pollen, soot and other particulates
Fig. 17.6, p.437
Thermal Structure of Atmosphere: Troposphere
• Extends to about 12 km
(40,000 ft) elevation
• All clouds & water vapor
and most weather
• Temperature decreases as
elevation increases,
because chief source of
heat is radiated heat from
the earth’s surface
• Environmental Lapse rate:
6.4°C/1000m (3.5°F/1000ft)
Lapse rate: the rate at which
air temperature decreases
with altitude.
Tropopause: boundary
between troposphere and
stratosphere
Thermal Structure of Atmosphere: Upper Layers
• Stratosphere – heated
primarily by solar radiation
– Ozone (O3) layer
absorbs UV energy,
causing temperatures
to rise
– Above 55km
(stratopause) temps fall
again
• Mesosphere – thin air
(can’t absorb energy), very
cold up to 80km
• Thermosphere – above
80km, temps rise rapidly
(to just below freezing!)
Latent Heat
• Phase change:
change from a more
ordered state to a less
ordered one, or from a
less ordered state to a
more ordered one
• Latent heat – energy
stored or released
during a phase
change without
temperature change.
Latent Heat
• Sometimes heat transfer causes a
change of state – change from
solid to liquid, or liquid to gas.
This occurs without a change in
temperature. Examples:
– Water stores energy when it
changes from a liquid to a gas,
since it absorbs the energy, but
does not change temperature.
Evaporated of sweat stores
heat energy from your body and
carries it away.
– Water releases energy when it
changes from a gas to a liquid.
This is the source of wind
energy found in hurricanes.
Humidity
• H2O in air exists in 2
forms: vapor (invisible)
and liquid e.g. rain, fog,
clouds.
• Saturation: Maximum
quantity H2O vapor that
air can hold. Additional
H2O condenses into liquid
water.
• Saturation increases with
temperature (hot air can
hold more water vapor
than cold air.
• Conditions found on left
side of curve cause water
vapor to condense.
Relative Humidity
• Relative humidity (RH)- ratio
of actual amount H2O vapor in
air to the amount at saturation.
• Often, as the days warms up,
the air, more water will
evaporate into the air (actual
amount), but since the
temperature has increased, so
has the amount of water
needed for saturation
• Therefore, the RH stays about
the same.
RH =
Grams water vapor in air x 100%
Grams vapor at saturation
Relative Humidity
•
What happens if the
air now cools down?
• Actual amount of
water vapor in the air
stays the same but
amount needed for
saturation decreases.
• Therefore the RH
increases.
Relative Humidity
• When the relative
humidity reaches
100%, the air is
saturated, regardless
of actual amount of
water vapor present.
• At low temperatures,
100% RH means less
water vapor than at
higher temperatures.
Dew Point Temperature
RH =
Grams water vapor in air x 100%
Grams vapor at saturation
• Dew point temperature –
temperature at which RH
= 100 %.
• If saturated air cools
below the dew point
temperature, water vapor
will condense into liquid.
• At the dew point
temperature, clouds, fog
rain or dew appear.
Solar Energy (Insolation)
– Also called solar radiation, although NOT
radioactive!
– Composed of electromagnetic waves with
different properties depending on wavelength,
frequency
• Longwave (low frequency): includes heat
(infrared), radio waves
• Shortwave (high frequency): includes visible
light as well as ultraviolet, x rays, gamma rays
• Electromagnetic spectrum – shows EM
wavelengths by frequency and wavelength.
Fig. 18-2, p.430
Shortwave solar energy
• Much of the highest
frequencies (x-rays,
gamma rays) are
absorbed by oxygen
atoms in the
thermosphere and O2 gas
in the mesosphere.
• Much ultraviolet (uv)
absorbed by the ozone in
the stratosphere.
• Visible light waves (still
considered shortwave)
pass through atmosphere
and are absorbed by
earth.
Solar Radiation in the atmosphere
• Scattering and Reflection
• Absorption, which is often accompanied by re-emission
Reflection and Albedo
• Reflection–electromagnetic radiation bouncing of
from a surface without absorption or emission, no
change in material or energy wavelength
• Albedo – proportional reflectance of a surface
–
–
–
–
–
a perfect mirror has an albedo of 100%
Glaciers & snowfields approach 80-90%
Clouds – 50-55%
Pavement and some buildings – only 10-15%
Ocean only 5%! Water absorbs energy.
Typical Albedos of Materials on the Earth
Absorption and Emission
• Absorption of radiation – electrons of absorbing
material are “excited” by increase in energy
– Increase in temperature; physical/chemical change
– Examples: sunburn, cancer
• Emission of radiation – excited electrons return
to original state; radiation emitted as light or heat
– Example: earth absorbs short wave radiation from
sun (i.e. visible light) and emits longwave (infrared or
heat) into the atmosphere
Fig. 18-6, p.432
the Radiation Balance
• Sun emits EM radiation of all wavelengths, but
primarily shortwave (i.e. light).
– Earth’s surface absorbs this energy
– Most is re-emitted upward, as heat (longwave)
• Greenhouse Effect
– “greenhouse gases” (water vapor, carbon dioxide,
methane, etc.) let shortwave energy pass, but absorb
and longwave energy radiated upward by the Earth.
– this longwave energy is re-radiated in all directions,
some of it returning to the Earth’s surface. This is
what keeps our atmosphere at a livable temperature
of about 15 degrees C (59 degrees F).
Cloud Formation
• Clouds – made of water
droplets and/or ice
particles (not water vapor,
which is invisible)
• Cloud particles grow
around a tiny solid
surface; these are called
condensation nuclei
(singular nucleus).
• Examples include dust,
smoke, pollen, sea salt
pollutant particulates.
Cloud Formation
• clouds form when the air
is saturated (100%
relative humidity) and H20
vapor condenses.
• air must cooled to dew
point for saturation to
occur.
• Large scale cooling
occurs when an air mass
is lifted to a higher level in
the atmosphere, due to
the Adiabatic Principle.
The Adiabatic Principle
• When a gas
expands, it’s
temperature
decreases.
• When a gas is
compressed, it’s
temperature
increases.
• Air expands when
it goes up in
altitude, where
the air pressure is
less.
• Air compresses
when it goes
down, as air
pressure
increases.
The Adiabatic Process
The Adiabatic
Process
refers to a
temperature
change
caused by a
change in
pressure.
• Adiabatic
(expansional)
cooling – occurs
when rising air
expands due to
decrease in air
pressure.
• Adiabatic
(compressional)
warming - sinking
air compresses, due
to increase in air
pressure.
another example of adiabatic
warming – Santa Ana winds
Adiabatic Lapse Rates
Dry: 10°C/1000m
Wet: approx 5°C/ 1000m
<--- lifting condensation level
Cloud Formation
• Warm,moist air parcel gets
“pushed up” and cools
adiabatically.
• If not saturated, cools at the
dry lapse rate.
• Starts rising because it is
warmer and therefore less
dense than it’s surroundings
• gets “pushed” by some
mechanism (e.g.cold front,
mountains) to keep rising,
since it cools faster than its
environment.
• As it cools down, its RH
increases towards 100%.
Lapse Rates
Dry Adiabatic:
10°C/1000m
Wet Adiabatic: approx
5°C/ 1000m
Environmental: approx.
6.4°C/1000m
Cloud Formation
• Lifting Condensation Level:
Altitude where the
temperature of the parcel
drops to the dew point
temperature. At this
temperature, RH = 100% and
the parcel has become
saturated.
• Condensation begins around
any condensation nuclei that
are present, and tiny water
droplets form.
• “A Cloud is Born!”
Adiabatic Lapse Rates
Dry: 10°C/1000m
Wet: approx 5°C/ 1000m
Environmental: approx.
6.4°C/1000m
Cloud Formation
• When condensation occurs,
latent heat stored in the water
vapor is released, warming
the parcel, which causes it to
rise.
• Due to the latent heat, it cools
at a slower rate, the “wet”
lapse rate. There is a good
chance that it will cool less
quickly than the atmosphere
around it, and it will continue
to rise, condensing as it goes.
• This causes the cloud to have
“vertical development”.
Adiabatic Lapse Rates
Dry: 10°C/1000m
Wet: approx 5°C/ 1000m
Environmental: approx.
6.4°C/1000m
What makes air rise?
• Warm air is less dense (lighter) than cool air.
• Water vapor is less dense (lighter) than dry air.
• Air near earth’s surface warms, picks up water
vapor from near the surface of the Earth, then
rises.
• Adiabatic cooling lowers temperature to dew point,
which causes cloud formation.
• Air may be “helped” to rise by various means.
Clouds
Cloud families – based on height
1. Low: to 10,000 ft above the surface
2. Middle: 10,000-20,000 ft above the surface (prefix
alto)
3. High – above 20,000 ft high (prefix cirro)
4. Clouds with vertical development
Cloud classes - based on shape
– stratiform – layered clouds
– cumuliform - globular
Cirrus – high altitude clouds
• Formed from ice
crystals, not liquid
water drops.
• Strong winds at
altitude blow them
into wisps (“mare’s
tails”).
• No rain (or snow)
comes directly from
them, but they are
often an indication of
unsettled weather in
the near future.
Altocumulus – Middle-layer Clouds
“Mackerel Sky”
Low Stratus Clouds
• Horizontally layered,
sheet-like clouds.
• Form in relatively stable
conditions when air stops
rising as soon as
condensation occurs.
• Causes a gray overcast
sky that may persist for
days.
• Nimbostratus clouds
bring steady rain or
snowfall (nimbo = rain).
Cumulus clouds display vertical development
• Fluffy, white clouds with flat
bottoms and billowy tops
that may rise as much as 4
miles up in the air.
• Form from relatively
unstable air which
continues to rise after
condensation takes place.
Flat base of cloud is where
condensation started.
• Rain or snow comes in the
form of brief, intense
showers.
Other Cumulus Clouds
Fair weather
cumulus
Cumulonimbus
Lenticular clouds form as moist air flows up and over a
mountain peak. They are classified as altocumulus. Lenticular
clouds indicate high winds aloft.
Fog
Fog is a cloud that forms at or very close to the ground.
The picture shows an example of radiation fog, from air
near the earth’s surface which cooled by radiation during
the night. Fog usually disappears during the day, when
the sun warms the earth and it begins to emit heat.
San Diego Marine Layer
The marine layer is an example of an advection
fog: warm air over the ocean takes in water
layer, but as prevailing west winds blow it over
the land, the water vapor condenses
Precipitation
Precipitation
• Clouds sometimes release
moisture as precipitation
– Occurs as rain, hail, snow or
sleet
– 3 mechanisms that cause air
to rise, leading to cloud
formation and precipitation
• Orographic precipitation
• Cyclonic precipitation
• Convection-Convergence
precipitation
Orographic Precipitation
• Prevailing winds push air up against, and then over, a
mountain range
• This process is responsible for our deserts in eastern
San Diego County
Rainfall Map
• Orographic
precipitation occurs
on west side as air is
lifted over mountains.
• Great Central Valley
has less rain than
coastal regions.
• Death Valley and
Mojave Deserts are
rainshadow deserts.
Cyclonic Precipitation (Frontal Wedging)
• When moving warm and cold air masses meet
(a “front”), more dense cold air forces the less
dense warm air to rise.
• This happens when a storm (cyclone) passes
through
Convection-Convergence Precipitation
• Forms when warm, moist air
–
–
–
–
–
Heats up near a hot surface
Rises and cools adiabatically
Condenses and forms clouds
May form fair weather cumulus
If conditions are unstable, can lead to precipitation in the form
of thunderstorms.
Convection/convergence
Thunderstorms and Unstable Air
Unstable air conditions
caused by:
• surface air that is very warm and
moist
• the environmental temperature
lapse rate exceeds both dry and
wet adiabatic rate (relatively cold,
dry air aloft)
– Rising air remains warmer than
its surrounding air, causing
strong, persistent convection
(updrafts)
– Clouds with intense vertical
development – “towering
cumulus”
– Cumulonimbus clouds and
thunderstorms
Exploring Thunderstorms
• Vertically developing
clouds
– Reach very high altitude
– Are affected by high velocity
winds
– Become anvil-headed
• Strong convective flows
– Are created by precipitation
dragging cool air down
– And warm rising air taking its
place
• Hail and lightning are
destructive by-products of
thunderstorms
The force exerted by the
atmosphere
• Gas molecules in the atmosphere bounce
randomly, creating force in all directions.
• In addition, the force of gravity causes
atmospheric gas molecules to concentrate
at lower elevations. More force (in all
directions) at lower elevations.
Barometric Pressure - force exerted by the
gas molecules in the atmosphere over a
given area
• English unit is pound per square inch (psi). At
sea level, the atmosphere exerts about 14
pounds of force on a one by one inch area
(slightly bigger than a postage stamp).
• How many postage stamps fit on the top of your
head?
• How many pounds of force does the atmosphere
exert on the top of your head?
• Why don’t you have a big headache right now?
Barometric pressure
• At 5 km (about 3 miles) of elevation, there
are only half the gas molecules above you
that there are at sea level.
• Now your headache is due to altitude
sickness!
Measuring barometric pressure
• If you evacuate a tube
(i.e. remove all the air)
and put it in a dish of
liquid, the liquid will fill
the tube as the air
pressure pushes on the
liquid in the dish.
• If you tried this with a
dish of water, the water
would rise up to about 33
feet in the tube!
Measuring barometric pressure
• Using mercury, a very
heavy liquid, we find that at
normal sea-level barometric
pressure, the liquid in the
tube rises to a height of 760
mm (or 29.92 inches).
• This apparatus is the
original form of the
barometer, a device used
for measuring barometric
pressure.
Measuring barometric pressure
• What is a normal
barometric pressure
in the units reported
by the TV news
weatherman?
• Hint: the units are not
psi, and not
millimeters
Answer
• In the U.S., Mr. TV Weatherman reports
barometric pressure in inches, with normal
ranging from 29-31 inches (listen for it tonight).
• Overseas, Mr. TV weatherman probably
announces the barometric pressure in Millibars
(1013 millibars = normal sea level pressure)
• Meteorologists routinely use millibars for
scientific applications.
Measuring barometric pressure –
the modern way
• Mercury barometers are
dangerous and difficult to
use.
• Modern aneroid
barometers use changes
within a partially
evacuated chamber to
move the pointer to the
correct value.
This graph shows how
the barometric
pressure (x axis – in
millibars) decreases
as the elevation
increases and the
number of gas
molecules above gets
less and less.
Acid Rain
• Burning of coal and
petroleum release sulfur
and nitrogen oxide gases
• In moist air, these gases
form acids
• Atmospheric acids dissolve
in water droplets and fall as
acid rain
• The damage often occurs
over 100 miles away
• Acid rain damage in
Ontario, Canada can be
traced to coal-burning
power plants along the
Ohio River Valley
Photochemical Smog
• Incompletely burned
gasoline reacts with
nitrogen oxides and
oxygen in the
presence of sunlight
to become ozone
(O3).
• Ozone reacts again
with gasoline exhaust
to become smog
Ozone and smog
• Irritates the lungs and
aggravates asthma
• Increases the
liklihood of heart
disease
• A suspected
carcinogen
Ozone high and low
• The ozone layer in
the stratosphere is a
good thing because it
protects life on Earth
from harmful UV rays
• The Ozone in the
troposphere is a bad
thing because it
damages hearts and
lungs.
Ozone formation in stratosphere
Depletion of the ozone layer
• Ozone – O3 forms in the stratosphere and
absorbs UV energy, which can be harmful
• Halons and Chlorofluorocarbons (CFCs), – are
organic compounds that destroy ozone in the
ozone layer
Depletion of the ozone layer
• Ozone hole – reported in
1985 and linked to CFCs
in Antarctic ice clouds
• Many nations of the world
agreed to reduce or stop
use of these CFCs and
halons
• Most industrial countries
no longer produce CFCs.
• Since banning CFCs, the
hole may be decreasing