Transcript ATMO 201: Atmospheric Science

CHAPTER 2
ENERGY THAT DRIVES
THE STORMS
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Temperature measures the average speed
of air molecules
◦ This also means it is a measure of kinetic
energy
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But what is energy, really?
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Energy: the ability or capacity to do work on matter
Energy changes forms and transfers from one body to
another, but cannot be created nor destroyed
Potential energy: When something has the potential to
do work (it is up in the air, it can be burned, etc.)
Kinetic energy: the energy of motion (the wind, a car)
Everything has kinetic energy – the molecules that make
it up are moving, and the faster they move, the higher
the temperature
This reflects the internal energy of the substance
When this energy is being transferred, it is called heat
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Three ways for heat to be transferred:
◦ Conduction: Heat transfer within a substance:
touching a metal pan
Energy travels from hot to cold
Metal is a good conductor,
air is a poor conductor
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Three ways for heat to be transferred:
◦ Conduction: Heat transfer within a substance:
touching a metal pan
◦ Convection: Heat transfer by a fluid (such as water
or air): Warm, less-dense air rising
 In meteorology, we only call vertical motions
“convection”, and we use “advection” for horizontal
motions such as the wind
Energy has been transported upward
Remember: at the same pressure,
warm air is less dense than cold air
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Three ways for heat to be transferred:
◦ Conduction: Heat transfer within a substance:
touching a metal pan
◦ Convection: Heat transfer by a fluid (such as water
or air): Warm, less-dense air rising
 In meteorology, we only call vertical motions
“convection”, and we use “advection” for horizontal
motions such as the wind
◦ Radiation: Heat transfer that does not require the
substances touching or a fluid between them:
energy from the sun
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Our eyes can only
see radiation
between 0.4-0.7 μm
Radiation
travels in the
form of waves,
which move at
the speed of
light in a
vacuum
(186,000 miles
per second)
The shorter
the wave, the
more energy
it carries!
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Conduction: Only important very near the
ground (air is a poor conductor)
Convection: Many clouds form as a result of
convection, as warm, moist air rises
Radiation: Energy from the sun warms the
planet; causes daily changes in temperature,
and much more
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Everything with a temperature emits
radiation!
The amount of radiation emitted is
proportional to the 4th power of T: if we
doubled our temperature, we would emit 16
times more radiation (Stefan-Boltzmann Law)
Objects emit a radiation “spectrum” (a variety
of wavelengths)
Hotter objects emit most of their energy at
shorter wavelengths, as shown by Wien’s Law:
λmax = 2897/T
(The scale on the left is 100,000 times greater than the scale on the right)
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Solar radiation is often called “shortwave”
radiation
◦ Much of the solar radiation is in the visible part of
the spectrum – we can see the sun, and the
reflection and absorption of solar radiation allows
us to see other things
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Earth’s radiation is “infrared” or “longwave”
radiation
◦ Not visible to our eyes
◦ Transfers much less energy
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If the Earth is radiating energy all the time,
why is it not extremely cold and always
getting colder?
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Objects with a temperature don’t just emit, they
also absorb!
If something emits more than it absorbs, it will
cool, if it absorbs more than it emits, it will warm
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Objects that are
good absorbers
are also generally
good emitters
Consider an asphalt
road:
 During the day the
asphalt absorbs solar
radiation and warms
 At night the asphalt
emits infrared
radiation and cools
relative to its
surroundings
Day
Warm
Asphalt Road
(warms due to solar radiation)
Night
Cool
Asphalt Road
(cools by IR radiation)
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Objects that absorb all radiation hitting them and emit all
possible radiation at their temperature are known as
“blackbodies” (but they don’t need to be black)
The sun and the earth’s surface behave as blackbodies, but
the atmosphere does not
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Averaged over a long period
of time, the amount of
shortwave energy received
from the sun is equal to the
amount of longwave energy
emitted by the earth’s surface
– the planet is in radiative
equilibrium – on average, the
planet does not heat or cool
But this calculation gives an
average temperature of 255 K
(0° F) – a frozen earth!
What we actually observe,
however, is an average
surface temperature of 288 K
(59° F) – much more livable.
Why?
Radiative equilibrium:
incoming = outgoing
Infrared
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For instance, glass:
absorbs IR and UV, but
not visible light
Important selective
absorbers:
◦ Ozone (O3) in the
stratosphere: absorbs
ultraviolet (UV)
radiation: keeps us
from getting a
sunburn!
◦ Water vapor (H2O),
Carbon dioxide (CO2),
Methane (CH4):
absorb longwave
radiation, but not
shortwave
Percentage absorbed
UV
Ozone absorbs UV
Water vapor absorbs some IR
CO2 absorbs some IR
Can be:
 Absorbed by the atmosphere (19% of
incoming radiation: atmosphere is relatively
transparent to solar radiation)
 Reflected back to space by clouds, aerosols,
and the atmosphere (26%)
 Transmitted down to the surface
◦ This can be reflected (4%)
◦ Or absorbed by the surface (51%)
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The solar radiation
reflected by things in the
atmosphere (26%) and the
surface (4%) compose what
is called the planetary
albedo, which is observed
to be 30% (averaged over
the entire planet)
Each surface has a different
albedo – snow and clouds
are very reflective, water
and dark ground are not
Instead of being radiated straight out to space,
most of the longwave radiation is absorbed
by the atmosphere: the selective absorbers!
 6% is transmitted out to space
 94% is absorbed by the atmosphere
The atmosphere then emits radiation (it has a
temperature, after all!): the upward radiation
escapes to space, the downward radiation
comes down to the surface
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This downward longwave radiation warms the
surface
When this is accounted for, we can calculate
the average temperature of 288 K
Without the greenhouse effect, Earth’s
temperature would not be suitable for life!
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The greenhouse effect absolutely exists, and
it is very well understood scientifically
The greenhouse effect is simply this: The
surface of the Earth is warmer than it would
be in the absence of an atmosphere because
it receives energy from two sources: the Sun
and the atmosphere.
It is the enhancement of the greenhouse
effect that causes concern for global
warming…more on this later.
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What process could take heat from the
surface and transfer it to the atmosphere?
Fig. 2.13, p. 50
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Heat energy is required to change the phase of
water – this heat is “hidden” or “latent” – we
can’t measure it with a thermometer
Instead of being used to change the
temperature of the substance, the heat is used
to change the phase
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The evaporation of water from oceans and
lakes transfers heat from the surface to the
atmosphere
When warm, moist air rises and clouds form,
latent heat is released (condensation) – This
is “moist convection”, and is another way that
things are brought back into balance
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All of the previous information was for the
entire globe, averaged over long periods of
time
At a given location, there is very rarely
radiative equilibrium: during the day, we get
more solar energy than the earth emits, and
the opposite is true at night
This is why it warms during the day (energy
surplus) and cools at night (energy deficit)
Clouds are very important locally
◦ During the day, more clouds = higher albedo and
less shortwave radiation reaching the surface
◦ At night, clouds absorb longwave radiation and
emit back to the surface
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Radiation surplus in
the Tropics; deficit
near the poles
Do the poles get
colder and colder,
and the tropics
hotter and hotter
every year?
No! Circulations in
the atmosphere and
ocean transfer heat
from the Tropics to
the poles. More on
this later in the
semester.
“If you graduated from Harvard, do you think
you would know why it is warmer in summer
than in winter? Educators who surveyed
Harvard students on their graduation day in
1986 discovered that most of them could not
correctly answer this question.”
-- Harvard Gazette, 1997
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When the sun is directly overhead, the
radiation is concentrated over a smaller
area
When at an angle, that same energy is
spread out over a much larger area
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…at an angle of about 23.5°
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The tilt of Earth on its axis is the primary
reason there are seasons
In December, the Southern Hemisphere is
strongly tilted toward the sun; they get longer
days and the sun is high in the sky
◦ The Northern Hemisphere is tilted away from the
sun; we have shorter days and winter
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In June, the opposite is true
March and September are the “equinoxes”,
when the solar energy is maximized at the
equator
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If the sun is out for all 24 hours in Alaska,
why isn’t it hotter there than in College
Station where it’s only light for 14 hours?
Fig. 2.19, p. 56
Stepped Art
Fig. 3-8, p. 63
(incoming minus outgoing)
http://profhorn.meteor.wisc.edu/wxwise/Acke
rmanKnox/chap2/ERBE%20Net.html
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Equinox, “equal night”
◦ Day and night are the same length; sun is directly
over the equator (March 20 and September 22)
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Solstice, “sun stands still”
◦ Summer solstice: June 21 – longest day of year in
northern hemisphere
◦ Winter solstice: December 21 – shortest day of year
in northern hemisphere
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In meteorology, seasons are DJF (winter),
MAM (spring), JJA (summer), SON (autumn)
The “first official day of winter” on December
21 is the astronomical definition
Record high
Average high
Average low
Record low
http://www.srh.noaa.gov/hgx/?n=climate_cll
12 hrs daylight
15 hrs daylight
12 hrs daylight
9 hrs daylight
March
South
June
Sept.
Dec.
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The core: estimated to be
~15 million degrees
Celsius
The photosphere (what
we see) is about 6000°C
Sunspots: cooler, dark
regions
Corona: much hotter (2
million °C)
Chromosphere: cooler
region between the
photosphere and the
corona
Solar flares and
prominences: jets of gas
that shoot up into the
corona
Solar flares can disrupt
Earth’s magnetic field,
causing problems with
radio and satellite
communications
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Much like a bar magnet, Earth has a
magnetic field
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Charged particles from the sun, called the
“solar wind”, distorts its shape
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Charged particle from the solar wind “excites” atoms or
molecules in the upper atmosphere (thermosphere)
This causes the electron to jump to a higher energy level
When it returns to normal, it emits light
(Solar wind)
(Air molecule or
other atom)
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Different elements give off different color
light (oxygen is red or green, nitrogen is
red or violet)
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Northern hemisphere = “aurora borealis”
(northern lights)
Southern hemisphere = “aurora australis”
(southern lights)
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Auroras tend to happen where magnetic
field lines intersect the earth’s surface (at
high latitudes)
Number of nights per year with aurora
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UV-B radiation responsible for most sunburn, though
UV-A can also cause it
11 am to 3 pm: biggest threat of sunburn
NWS UV forecast:
http://www.nws.noaa.gov/view/national.php?prodtype=
ultraviolet