1. AOS 100: Weather and Climate
Instructor: Nick Bassill
Class TA: Courtney Obergfell

2. Miscellaneous
Homework Reminder
Note takers

3. Review of September 15th: Heat and Heat Transfer
Heat can be thought of as a form of energy transfer
The amount of heat gained by one object is equal to the amount lost by the other object
Heat transfer is why certain objects feel warm or cold
Specific heat is a measure of the heat energy needed to heat 1 g of an object 1º C
This is why water changes its temperature less during the day then the air

4. Review Continued
There are four primary mechanisms of heat transfer in the atmosphere:
- Conduction, convection, advection, and radiation
Conduction is the transfer of heat through an object molecule by molecule
Heat Conductivity is a measure of the effectiveness of different materials at conducting heat
This is one reason that 40º F water “feels” colder than 40º F air, even though they are the same temperature
Convection is heat transfer by the movement of fluids (gases and liquids) in the vertical (along gravity)
Less dense (i.e. warmer) fluids tend to rise and more dense fluids (i.e. colder) tend to sink

5. Review Continued
The lapse rate is simply a measure of the rate of change of temperature with height
If the temperature decreases by 15ºC in 2000 meters, you would say that the lapse rate is 7.5ºC/km
Advection is the transfer of heat through the horizontal movement of a fluid

6. Radiation
Unlike Conduction, Convection, and Advection … Radiation does not need a medium to travel through (i.e. it can travel through empty space)
Radiation carries energy in the form of electromagnetic waves
This is how the Earth receives energy from the sun
All objects emit and receive radiation

7. Radiation Continued
As objects emit radiation, they cool (i.e. lose energy), unless they are being warmed by some other process
Radiation can be –
Emitted (such as from the sun)
Absorbed (like sunlight on your face)
Reflected (visible light off a mirror)
Scattered (visible light as seen in a rainbow)

8. Wien’s Law
The emitted wavelength of radiation depends on the temperature of the object
Hotter objects emit radiation with a shorter wavelength
Shorter wavelengths are more energetic
This means that there is more energy in the radiation that hot objects emit
The wavelength of maximum emitted radiation (λ, in nm) is approximately: λ=2897/T (in Kelvin)

9. λ=2897/T

10. Stefan-Boltzmann Law
This law essentially states that warmer objects emit (transfer) more energy than colder objects
However, this isn’t a linear relationship
The amount of energy emitted is proportional to the temperature to the fourth power
Energy emitted = σ * T4 where σ is a constant (5.67 x 10-8 W m-2 K-4)
Really, we can think of it as E~ T4 (In Kelvin)

12. The Spectrum of the Sun’s Emittance
Notice that the largest component of the Sun’s emittance falls in the visible spectrum

13. A Comparison of Emittances
From:

14. Albedo
The albedo of an object is a measure of the amount of sunlight it reflects
For example, snow, ice, and clouds have a much higher albedo than grass, water, or dirt
The average albedo of the planet is about .3, which means on average the planet reflects 30% of the incoming solar radiation
This varies strongly from location to location (i.e. the arctic vs. forests or the ocean)

16. The atmosphere absorbs some radiation
However, this doesn’t happen uniformly for all wavelengths
Since the Sun’s radiation and the Earth’s radiation are almost completely separate wavelengths, these variations are important!
Much more of the Earth’s radiation is absorbed by the atmosphere than the Sun’s