A. Mechanisms of Heat
Transfer
Heat Transfer :
-
Always from warmer to cooler objects
Forms:
-
conduction: the transfer of heat energy by molecular activity
- molecule to molecule contact
-
convection: the transfer of heat energy by the movement of a mass
or substance from one place to another
-
radiation: can be transfered through a vacuum
Three Mechanisms of Heat Transfer
B. Forms of Atmospheric Radiant Energy:
Wavelength
-
Sun (shortwave): ultraviolet (.01-0.4), visible (0.4-0.7),
near infrared (IR) (0.7-4.0)
-
Earth-Atmosphere (longwave): thermal IR (4.0-100) .......
units are in micrometers (1 micrometer equals one-millionth of a meter)
Electromagnetic Energy Classified
According to Wavelength
C. Basic Radiation Laws:
-
all objects emit radiant energy
-
hotter objects radiate more total energy per unit area than cooler objects
-
the hotter the radiating body, the shorter the wavelength of maximum radiation
-
objects that are good absorbers of radiation are good emitters as well
-
the rate @ which solar energy falls on a surface located at the top of
the Earth's atmosphere is a constant (solar constant)
-
A "blackbody" is an object that is perfectly efficient at
absorbing and radiating radiation (blackbodies do not exist in nature,
but represent and ideal)
Intensity
of Solar Radiation as a Function of Wavelength
Intensity of Radiation Emitted by the Earth-Atmosphere System
as a Function of Wavelength
D. Paths Taken by INSOLATION
-
As insolation moves through the atmosphere three things can happen
to it:
-
reflection - occurs at the interface between two different
media (e.g. air & cloud) when some of the radiation striking the interface
is thrown back
-
albedo - the ratio of reflected to incident radiation (an
object w/albedo of 1 (or 100%) is a perfect reflector) - light colored
objects have high albedos; dark objects have low albedos
-
scattering - dispersal of radiation in all directions
-
absorption - radiation is converted to heat and emits
that heat (radiation) according to #2 (above)
[most absorption within the atmosphere is by oxygen, ozone, water vapor
and various aersols (solid and liquid particles)]
Direct Insolation - insolation that is transmitted directly through
the atmosphere to the earth's surface
Diffuse Insolation - insolation that is scattered &/or reflected
to the earth's surface
-
the direct and diffuse insolation ("total solar")are either
absorbed or reflected by the earth's surface
-
(that portion which gets absorbed is converted to heat)
E. Distribution of INSOLATION
-
the Earth-atmosphere energy system is in a state of balance; i.e., 100
units of energy come into the system and 100 units leave.
-
i.o.w.: the earth and atmosphere must return to space as much energy
as they take in - otherwise the earth's average surface temperature would
change
Budget: 100 Units of Insolation
30 units ...... reflected to space by clouds, the earth, and
the atmosphere (thus, the albedo for the Earth as a whole (planetary
albedo) is 30%
20 units ...... absorbed by clouds and the atmosphere
50 units ...... absorbed at the earth's surface
........................................................................
70 units .........radiated back to space by the Earth-Atmosphere system
-
the earth maintains a delicate balance between incoming and outgoing energy,
and essentially there is no yearly gain or loss of total energy, thus the
average temp of the earth remains fairly constant from year to year.
-
in contrast, the Earth's surface receives a surplus of energy, while the
atmos. exhibits a deficit
Average Distribution of Incoming Solar Radiation by Percentage
F. The (Atmospheric) Greenhouse Effect
Basic Principles:
-
while the earth behaves like a blackbody - the atmosphere does not!!
-
the gases that comprise the atmos. are "selective absorbers" - they
absorb some wavelengths and are transparent to others.
-
selective absorbers usually emit radiation at the same wavelength which
they selectively absorb at.
Important Selective Absorbers:
-
These gases are poor absorbers of visible (shortwave) radiation,
but good absorbers of infrared (longwave) radiation
Absorbtion of Radiation by Selected Components of the Atmosphere
The greenhouse effect thus works in the following manner:
-
direct and diffuse shortwave radiation from the sun is absorbed at the
earth's surface
-
the earth radiates longwave energy into the atmosphere where some of it
is absorbed by the various greenhouse gases
-
these gases gain kinetic energy and collide w/neighboring air molecules
which increases the average KE of the air, which results in an increase
of air temperature.
-
these same greenhouse gases also emit longwave radiation - some of which
is transmitted to the earth's surface where it is absorbed and thus heats
the ground.
-
the earth then reradiates longwave energy upward, where, once again, it
is absorbed by the greenhouse gases and warms the lower atmosphere
