Transcript Energy: Warming the earth and Atmosphere

Chapters 2 and 19
Specific Heat
In the atmosphere, heat is transferred by conduction,
convection and radiation.
 Heat capacity is the heat energy absorbed to
raise a substance to a given temperature

Specific heat is the heat capacity of the
substance per unit mass; or the amount of
energy required to raise one gram of a
substance 1°C

High specific heat equates to slow warming and
vice versa
Latent Heat
Change of state or phase change represents change
between solid, gas, and liquid.
 Latent heat is the energy involved in the change of
state. latent heat is an important source of atmospheric
energy.
 Ice to vapor: absorb energy, resulting in cooling the
environment (melt, evaporation, sublimation)
 Vapor to ice: release energy, heating the environment
(freeze, condensation, deposition)


ice freezing water

melting
evaporatio
n 
water condensation water vapor

sublimatio
 n 
ice
H 2O vapor
deposition
 

Heat Transfer in the Atmosphere

Conduction: transfer heat from one molecule
to another in a substance
 Energy travels from hot  cold
 Air a poor conductor, metal a good conductor

Convection: transfer of heat by the mass
movement of a fluid (water or air)

Convection circulation happens naturally in
the atmosphere: warm air expands and rises
then cools and sinks; thermal cell
A thermal is a rising bubble of air
that carries heat energy upward
by convection  convective circulation
-Air that rises will expand and cool
-Air that sinks is compressed and warms
Radiation

Energy from the sun travels through the
space and the atmosphere in the form of
a wave (electromagnetic waves).

Radiation and Temperature
 All objects with a temperature greater than 00 K radiate
energy.
E  T Stefan - BoltzmannLaw
4
 As temperature of an object increases, the more total
radiation is emitted by the object.
pot
burner
Radiation

Radiation of the Sun and Earth
 Sun 6000 0K emits radiation, electromagnetic
spectrum (most energy emitted at λ~ 0.5 µm)
const .
max 
(Wien' s Law)
T
 Earth 288 0K radiates also energy (λ~ 10 µm)
 Shortwave radiation (high energy) from the Sun
 Longwave radiation (low energy) from the Earth
Radiation
Shorter λ

Longer λ
Environmental Issue: Sunburn
 UV index is a weather forecast product that indicates the
potential for sun burn due to high energy or short
wavelengths emitted by the sun.
Balancing Act

If the Earth is radiating energy all the time,
why is it not very cold?
 Radiative equilibrium
○ Absorb > emit = warming
○ Emit > absorb = cooling
Selective Absorbers

Good absorbers are good emitters at a
particular wavelength and vice versa.

Greenhouse effect: the atmosphere
selectively absorbs infrared radiation
from the Earth’s surface but acts as a
window and transmits shortwave
radiation
The earth’s surface would constantly emit IR
radiation upward, during day and night. Incoming
energy from the sun would equal outgoing energy
from the surface, but the surface would receive
virtually no IR radiation from its lower atmosphere.
(No atmospheric greenhouse effect.)
The earth’s surface air temperature would be
quite low, and small amounts of water found
on the planet would be in the form of ice.
The earth’s surface not only receives
energy from the sun but also IR energy
from the atmosphere. Incoming energy
still equals outgoing energy, but the added
IR energy from the greenhouse gases
raises the earth’s average surface
temperature to a more habitable level.
Greenhouse Enhancement





The atmospheric greenhouse effect occurs because the greenhouse
gases are selective absorbers  keeps the temperature of our planet
at a level where life can survive!!!
Global warming is attributed to an increase in greenhouse gases (see
Fig. 2.12, page 43):
 Carbon dioxide (CO2)
 Water vapor (H20)
 Molecular Oxigen (02) and Ozone (03)
 Methane (CH4)
 Nitrous Oxide (N20)
 Chlorofluorocarbons
Positive feedbacks continue the warming trend.
Negative feedbacks decrease warming.
Two potentially largest and least understood feedbacks in the
climate system are the clouds and the oceans.

Specular reflection is
reflection from a smooth
surface
 The reflected rays are
parallel to each other



Diffuse reflection is
reflection from a rough
surface
The reflected rays travel in
a variety of directions
Diffuse reflection makes
the dry road easy to see at
night
Diffuse Reflection
Refraction
Refraction
in a prism
The Rainbow
A ray of light strikes a
drop of water in the
atmosphere
 It undergoes both
reflection and refraction

First refraction at the
front of the drop
○Violet light will deviate the
most
○Red light will deviate the
least
Observing the Rainbow
If a raindrop high in the sky is observed, the red ray is
seen
 A drop lower in the sky would direct violet light to the
observer
 The other colors of the spectra lie in between the red and
the violet

Incoming Solar Radiation

Conduction, convection, and infrared radiation warm the
atmosphere from below, not sunlight or insolation from
above.

Air molecules are << than λ of visible light  more effective
scatterers of shorter (blue) λ than the longer (red) λ
 Scattering sun light (blue sky during daytime)
 Reflection of sun light, albedo (clouds ~ 60% albedo;
Water ~ 10% albedo; snow ~ 95% albedo)


White clouds scatter light
Black clouds have large cloud droplets which absorb light, rain likely
Further warming occurs during condensation as latent heat is given up to
the air inside the cloud.
Air in the lower atmosphere is heated from the ground upward. Sunlight warms
the ground, and the air above is warmed by conduction, convection, and
infrared radiation.
Cloud droplets scatter visible light in all directions; light
from many droplets turns a cloud white.
The sky appears blue because billions of air molecules selectively scatter the shorter
wavelengths of visible light more effectively than the longer ones.
The blue haze is caused by the scattering of blue light by extremely small particles
(hydrocarbons) smaller than the λ’s of visible light  the scattered blue light causes
the most distant mountains to become almost indistinguishable from the sky.
The Blue Ridge Mountains in Virginia.
Red Suns and Blue Moons

A thick atmosphere selectively scatters
all but red sunlight.

A low solar angle (sunrise or sunset)
causes light to travel through a greater
distance or thicker atmosphere.

Same process for a blue moon.
Bright red sky over California produced by the sulfur-rich particles from the volcano Mt.
Pinatubo during September,1992. The photo was taken about an hour after sunset.