AT 351
Lab 3 – The Energy Budget and Radiation
Due in lab: The Week of February 11th
100 Points Total
1. (10 Points) All things emit radiation. As the temperature of an object increases, more
total radiation is emitted each second. The Stefan-Boltzmann Law expresses this
mathematically:
E = σT4
…where σ = 5.67x10-8 Watts/(m2K4), E = Maximum rate of radiation emitted by each
square meter of surface area, and T is the object’s ‘blackbody’ temperature in degrees
Kelvin.
If some imaginary planet were to emit 240 Watts/ m2 of radiant energy, what would its
blackbody temperature be?
2. (30 Points) Heat is a measure of the transfer of energy from a body with a higher
temperature to a body with a lower temperature. There are three types of heat transfer:
1. Conduction
2. Convection
3. Radiation
The formation of a thunderstorm on a hot, sunny summer day is a great way to illustrate
the transfer of heat by all three mechanisms:
1. First, electromagnetic waves from the sun travel through the atmosphere and heat
the soil and vegetation molecules at the surface of the earth.
2. The excited molecules at the surface then transfer their energy to adjacent air
molecules in the atmosphere, thus heating the lower atmosphere.
3. Since warm air is LESS dense than cold air, the warm air parcel at the surface will
rise to replace colder air aloft.
(a) On the diagram below, please fill in the blanks with Conduction, Convection, or
Radiation, corresponding to the descriptions of how heat is transferred at points 1, 2 and
3.
(b) Warm air rises because it is less dense than cold air. As this warm air rises, the water
vapor it contains condenses, forming liquid cloud droplets and eventually rain. Warm air
that rises establishes updrafts, which sustain thunderstorms. Recalling your knowledge of
latent heat from the lab presentation, why does the air within the storm cloud remain
warm, sustaining updrafts?
(c) Often, if a storm cloud begins to rain, the storm will die. Again, recalling your
knowledge of latent heat, explain why the evaporation of rain below the cloud would
suppress the storm? (Hint: What drives updrafts that sustain thunderstorms?)
3. (25 Points) Pictured below is the electromagnetic spectrum.
The electromagnetic spectrum characterizes radiation by wavelength. As the wavelength
of light decreases the energy carried per wave increases.
The sun radiates light in the shortwave (SW) part of the spectrum, at wavelengths
between 0.4 and 0.7 μm, while the earth radiates light in the longwave (LW), between 5
and 20 μm.
(a) Recalling that 1 micron (μm) = 1.0 * 10-6 meters (m), and using both of the above
diagrams, what type of shortwave radiation does the sun emit?
(b) What type of longwave radiation does the earth emit?
(c) Does radiation emitted from the sun or the earth carry more energy per wave?
(d) Wien’s Law relates the peak wavelength of emitted light of an object to that object’s
temperature:
λmaximum = (2897 μm K) / T
…where T is in degrees Kelvin.
Using your answer from Question 1, what would you expect the peak wavelength of light
(in microns) of radiation emitted by this imaginary planet to be?
4. (10 Points) Rank the following in terms of their albedo from highest (1) to lowest (5).
_________ Asphalt
_________ Fresh Snow
_________ Moon
_________ Thin Clouds
_________ Calm Lake Surface
5. Short Answers (a) (10 Points) Why are carbon dioxide and water vapor called
selective absorbers?
(b) (15 Points) Explain how the earth’s atmospheric greenhouse effect works.