1. Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 06: Radiation and Temperature
Jan-29-07
(1 of 14)
Further Reading: Chapter 04 of the text book
Outline
- solar radiation & earth’s temperature
- insolation and climatic regimes
- composition of the atmosphere

2. Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 06: Radiation and Temperature
Jan-29-07
(2 of 14)
Introduction
Last time we discussed radiation and distinguished between solar (or shortwave) and thermal
(or longwave) radiation
In addition, we discussed how temperature affects the intensity and wavelength of radiation
Today we want to look at how radiation can affect the temperature of a body
In particular, we want to:
Discuss how radiation from the sun determines the temperature of the earth’s surface
based upon a very simple model for the global earth system
Look at how solar radiation varies with latitude and time of year
We are also going to use this lecture to introduce some basic concepts used in atmospheric
sciences to describe the composition and state of the atmosphere

3. Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 06: Radiation and Temperature
Jan-29-07
(3 of 14)
Thermal Equilibrium
Amount of energy absorbed equals the amount of energy released (or radiated away)
If the input of energy exceeds the output, energy is added to the system and it will heat up
If the input of energy is less than the output, energy is removed from the system and it will
cool down
Hence, thermal equilibrium implies that there is no net heating of the system

4. Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 06: Radiation and Temperature
Jan-29-07
(4 of 14)
Global Radiative Energy Balance
The flux arriving at the top of the atmosphere is constant
This constant input of energy goes to heating the earth
Heating of the earth raises the temperature
As the temperature of the earth increases, this increases amount of longwave radiation it
gives off
Eventually, the temperature is just right so that the amount of longwave radiation given
off exactly balances the incoming solar radiation

5. Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 06: Radiation and Temperature
Jan-29-07
(5 of 14)
Solar Constant
Energy flux arriving at the top of the atmosphere measured
perpendicular to Sun’s rays
Equal to 1370 W/m^2 (Watts per square meter)

6. Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 06: Radiation and Temperature
Jan-29-07
(6 of 14)
Total Incident Solar Energy
Energy flux arriving at the top of the atmosphere
Esolar = 1400 W/m^2 (approx. to the solar constant)
Interception area of Earth = pi*r^2 = 129,000,000,000,000 m^2
Ein = 1.8 x 10^17 Watts

7. Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 06: Radiation and Temperature
Jan-29-07
(7 of 14)
Total Solar Energy Absorbed
Not all energy reaching the earth’s atmosphere is absorbed
About 33% is reflected back to space
Clouds
Ice and Snow
Plants
33% reflected away [(1.8 x 10^17) x 0.33 = 0.6 x 10^17]
Energy absorbed is (1.8 – 0.6) x 10^17
Eabsorbed = 1.2 x 10^17 Watts

8. Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 06: Radiation and Temperature
Jan-29-07
(8 of 14)
Radiation Energy Balance
Eabsorbed(incoming) = Ere-radiated(outgoing)
Ere-radiated(outgoing) = sT^4 times Area of Earth
T^4 = (1.2 x 10^17/Area)/s
T(earth) = 253 K (degrees Kelvin) or -20 Celsius
Water freezes at 273 K; The actual T(earth) is 290 K
That is because of the natural greenhouse effect

9. Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 06: Radiation and Temperature
Jan-29-07
(9 of 14)
Insolation (Daily)
So far we have been talking about global balances; this ignores regional and seasonal effects
For a particular point, we refer to incident solar radiation as “insolation”
For a given point, insolation depends on the
angle of the incoming sunlight and the duration
of sunlight
Angle of sun (solar zenith angle): if this is large,
the energy of the sun is spread over a larger area
and insolation goes down
Duration: the longer the point is exposed to the
sun, the more radiation it receives, hence its
insolation goes up
For daily insolation, maximum occurs at the
poles during the solstice because of duration
effects (i.e. the day is 24 hours long)
However, it also has the lowest insolation during
winter, hence it has a large annual temperature
range

10. Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 06: Radiation and Temperature
Jan-29-07
(10 of 14)
Annual Insolation
Over the year, however, equatorial regions have the highest insolation because of
the consistently low solar zenith angles
Because of this, the tropics have the highest average annual temperatures
The earth’s tilt results in
Substantial increase in mean insolation at high latitudes
Slight decrease in insolation at low latitudes

11. Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 06: Radiation and Temperature
Jan-29-07
(11 of 14)
Climatic Regimes
Areas of the globe defined based upon the annual insolation they receive (13 regimes)
Explain first order control on climate
Higher insolation and warmer climate at low latitudes (and vice versa)
Climates at higher latitudes characterized by strong seasonality

12. Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 06: Radiation and Temperature
Jan-29-07
(12 of 14)
Radiation and Temperature
Up until now we have looked at the general relationship between solar energy, the earth’s orbit
and global and latitudinal temperatures
For instance, we know that in the tropics its warm and has a constant temperature through the
seasons; for the polar regions, it is cool and the temperature changes throughout the seasons
NOTE: this doesn’t always hold. For instance London at 51N has cooler summers and
warmer winters than Boston at 42N
Now we want to discuss heterogeneity in temperatures. This deals with how energy interacts
with the cryosphere, oceans, biosphere, lithosphere, and atmosphere at different points on the
globe

13. Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 06: Radiation and Temperature
Jan-29-07
(13 of 14)
Composition of the Atmosphere
Starting with the atmosphere
First, we are interested in how energy is transferred through the atmosphere
This depends on its composition
The earth is 128,000km across
The atmosphere is gaseous envelope held
close to earth by gravity
97% of atmosphere is within 30km of the
surface
Originally the gases from the atmosphere seeped out from volcanoes
10% was CO2, 85% was H2O; No O2; it was also much warmer (100C)
As the earth cooled, the H2O condensed into clouds, rain, oceans
Photosynthetic organisms evolved, converting CO2 into O2
Evolution of land plants led to rapid conversion of CO2 into O2
Today we find most is N with O2 second
The most highly variable constituent is H2O: from %; depends on
temperature, location, dynamics

14. Natural Environments: The Atmosphere
GE 101 – Spring 2007
Boston University
Myneni
Lecture 06: Radiation and Temperature
Jan-29-07
(14 of 14)
State Variables
Temperature: Average kinetic energy of air molecules
Highly variable (-60 -> 50 C)
Study is called “thermodynamics”
Pressure: Average mass of air molecules above a given point
Related to potential energy
Study is called “dynamics”
Humidity: Average number of water molecules in a give volume of air
Water is one of the most important constituents of the atmosphere - influences
energy balance, water balance, and dynamics
Study is called “hydrodynamics”