Weather and Climate

aos.wisc.edu

Weather and Climate

Notes

• Enter forecasts this week for INL before

6PM

• You may be removed from the contest (by

the managers) if you consistently make

forecasts that are ‘extreme’

• Continue to fill out forecasting logs (for

extra credit)

• Reading: Martin: pages 1-24, 28-29

• Look over review sheet and “questions to

consider.” We’ll discuss answers next

Tuesday.


Weather and Climate

(Lecture 8)

Instructor: Prof. Michael C. Morgan

Teaching Assistant: Dianna N. Nelson


Questions from last lecture


Wien’s Law is a statement

A. Relating temperature of maximum

emission to wavelength

B. Relating intensity of emitted radiation to

temperature

C. Relating temperatures in degrees Celsius

to temperatures in degrees Fahrenheit


The Stefan-Boltzmann law states

that the hotter an object,

A. The greater the radiative flux from that

object

B. The smaller the radiative flux from that

object

C. Neither A or B - energy emitted from an

object is independent of the object’s

temperature


If an object emits more radiation than it

absorbs, its temperature will

A. increase

B. decrease

C. stay the same


Objects not only emit

radiation, but also absorb

radiation!

If an object:

Emits more than it absorbs => cools

Emits less than it absorbs => warms

Emits as much as it absorbs => no change


What is the radiative equilibrium

temperature of the earth?


What affects the rate at

which something emits and

absorbs radiation?

• Depends strongly on surface

characteristics:

– Color

–Texture

–Moisture

– Temperature


Blackbodies

• Definition: Any object that is a perfect

absorber and a perfect emitter.

‣Perfect absorber: Absorbs all

radiation that strikes it.

‣Perfect emitter: Emits the maximum

radiation possible at the given

temperature.


Radiative Equilibrium

• The sun constantly bathes the

earth in radiation.

• Earth is constantly emitting

infrared radiation.

• The average temperature at which

this occurs is called the radiative

equilibrium temperature.

• What is the radiative equilibrium

temperature of the Earth?


Why don’t we get sunburned

through a window?

• Glass is a

selective

absorber.

• Glass absorbs

some infrared and

ultraviolet

radiation.

• Glass does not

absorb visible

light wavelengths


Selective absorbers

• Objects that selectively absorb

and emit radiation, such as the

gases in our atmosphere.

• Our atmosphere absorbs some

wavelengths of radiation, and

is transparent to others.


Why can you get sunburned on

cloudy days?

Clouds are also selective

absorbers!


• Infrared radiation is

absorbed by

greenhouse gases.

• Clouds can absorb in

the “atmospheric

window”, where CO 2

and H 2 O don’t.


Kirchoff’s Law

• Objects that selectively absorb

radiation also selectively emit

radiation at the same wavelength.

• Definition: Good absorbers are

good emitters at a particular

wavelength, and poor absorbers

are poor emitters at the same

wavelength.


How does color fit into all this

talk about absorption and

emittance?

• Why is it hard to tell in the dark

what color your crayon is?

‣Color does NOT represent the

wavelengths at which an object is

emitting.

• Color represents the wavelengths

that an object is reflecting or

scattering.


Scattering

• When sunlight strikes very small objects, the

light itself is deflected forward, sideways,

and backwards – this is called scattering.

• Air molecules are more effective scatterers

of short (blue) wavelengths than long (red)

wavelengths.


Reflection

• Reflection differs from scattering in that

reflection sends more light backwards.

• The ratio of the amount of radiation

returning from a surface to the amount

initially striking that surface represents the

reflectivity of the surface, called “albedo”.


Albedo


Energy Balance

• The earth and its atmosphere combined

send off to space just as much energy

as they receive.

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