Midterm #1 - Wednesday!!!

Bring a scantron sheet!!!!!

No OH’s this week again (last time – I promise)

Midterm Review Tonight Broida Hall1610 starting 6:30!!

Energy in the troposphere Energy in the troposphere and at the Earth’s surface:and at the Earth’s surface:A topic not to make light ofA topic not to make light of

Chapter 4Chapter 4

Questions we will answer Questions we will answer today:today:

•What is electromagnetic energy?

•What happens to solar energy once it enters our atmosphere?

•Why is the sky blue? Why are sunsets red?

•What is a “green flash?”

•How does the Greenhouse Effect work?

So, which produces energy with shorter So, which produces energy with shorter wavelengths? The earth or the sun?wavelengths? The earth or the sun?

So, what happens to solar So, what happens to solar energy once it enters Earth’s energy once it enters Earth’s

atmosphere?atmosphere?

CONSEQUENCESCONSEQUENCES

• TransmissionTransmission

• Reflection -> AlbedoReflection -> Albedo

PROCESSESPROCESSES

• ScatteringScattering

• AbsorptionAbsorption

• RefractionRefraction

ScatteringScatteringThe redirection of light by The redirection of light by gas molecules or aerosols gas molecules or aerosols without altering its without altering its wavelengthwavelength

•Atmosphere is denser nearer Atmosphere is denser nearer

the Earth’s surfacethe Earth’s surface•Interaction between light & Interaction between light &

gas molecules increasesgas molecules increases•Scattering increases closer to Scattering increases closer to

the Earth’s surfacethe Earth’s surface

Rayleigh ScatteringRayleigh Scattering

Rayleigh scattering –When particles are much smaller than the wavelength of the incoming radiation, shorter wavelength radiation is scattered much more often than longer wavelength radiation.

Blue light is scattered much more than Red!!

Mie ScatteringMie Scattering

Mie scattering - when scattering particles when scattering particles are the same size as the wavelength of lightare the same size as the wavelength of light

These particles are aerosolsThese particles are aerosols

Mie scattering affects all visible wavelengths Mie scattering affects all visible wavelengths

a little stronger for blue than reda little stronger for blue than red

So why is the sky blue?So why is the sky blue?

•The most abundant type of solar energy is in the blue wavelength, so blue light is scattered more than anything else.

•Shorter wavelengths are also scattered more easily than long wavelengths

Why are sunrises & sunsets red?Why are sunrises & sunsets red?

Low Altitude Sun –

Solar radiation passes through more atmosphere and undergoes more scattering.

Why does this cause us to see red?

Earth’s AlbedoEarth’s Albedo

RefractionRefraction

•As light passes from one medium to another As light passes from one medium to another (space to atmosphere), it is forced to change (space to atmosphere), it is forced to change speed. speed.

•This causes light at different wavelengths to This causes light at different wavelengths to bend at different angles.bend at different angles.

•What are some examples of refraction in our What are some examples of refraction in our atmosphere?atmosphere?

AbsorptionAbsorptionAny solar radiation that is not Any solar radiation that is not reflected back to space is reflected back to space is eventually absorbed.eventually absorbed.Albedo = 31% (31% of incoming energy goes back out Albedo = 31% (31% of incoming energy goes back out to space)to space)

Earth’s absorbance = 100% - 31% = 69%Earth’s absorbance = 100% - 31% = 69%

Absorbed solar radiation is converted to:Absorbed solar radiation is converted to:

1.1. Chemical energy via photosynthesisChemical energy via photosynthesis

2.2. Heat -> longwave radiation Heat -> longwave radiation (& other processes)(& other processes)

Longwave RadiationLongwave Radiation

• We know that anything on earth that absorbs shortwave radiation must re-emit it as longwave radiation because earthly objects are much cooler than the surface of the sun:

• Remember Intensity = Intensity = σσTT44

• When T decreases, Radiant intensity goes down When T decreases, Radiant intensity goes down & WAVELENGTH INCREASES.& WAVELENGTH INCREASES.

Shortwave & Longwave EnergyShortwave & Longwave Energy

Earth’s Energy BalanceEarth’s Energy Balance

• Everything on Earth constantly emits longwave Everything on Earth constantly emits longwave radiation.radiation.

• On the whole, Earth emits just as much longwave On the whole, Earth emits just as much longwave energy to space as the shortwave energy that it energy to space as the shortwave energy that it absorbs.absorbs.

• RadiationRadiationnetnet = (SW - SW ) + ( LW - LW ) = (SW - SW ) + ( LW - LW )

AbsorptionAbsorption EmissionEmission

Greenhouse EffectGreenhouse Effect

components of the greenhouse effectcomponents of the greenhouse effectwater vapor: 60%water vapor: 60%

carbon dioxide: 25%carbon dioxide: 25%ozone: 8%ozone: 8%

other gases like nitrous oxide, methane, and CFCs make up the other gases like nitrous oxide, methane, and CFCs make up the remainderremainder

sun earth

compare the ideal curve for earth above with the actual

curve at left

Earth’s temperature without the greenhouse effect

Earth’s average temperature without the natural, enhanced greenhouse effect

(i.e., its “black body radiative equilibrium temperature”) would be

~256 K, or -17°C

Earth’s actual average temperature is ~288 K, or 15°C

the difference is due to the greenhouse effect!

The greenhouse effect on other planets

atm. pressure

(Earth = 1)

Tblackbody

(K)

Tsurface

(K)

Mars 0.007

Venus 90

what does this imply about the atmospheric

composition of these planets?

216

227 750

240

yes, both atmospheres are mostly

CO2

Confusion between the greenhouse effect & global

warmingthe greenhouse effect is one of the most well

validated theories in earth science

global warming is driven by greenhouse effect properties of the gases we are adding

to the atmosphere: CO2, N2O, CH4,CFCs

think of the natural, background greennhouse effect and the

anthropogenic, enhanced greenhouse effect cause by human activities

~45% of incoming radiation reaches the surface

Energy Balance in the Troposphere

Cloud-albedo and cloud-greenhouse forcing

Not all clouds are alike in their radiative effects (greenhouse effect

properties versus albedo properties)

Cre

dit

:htt

p:/

/ww

w.u

car.

edu

DV, Show the Energy Budget AnimationDV, Show the Energy Budget Animation

high, thin cirrus clouds have a net warming effect

(greenhouse effect > albedo effect)

low, thick stratus clouds generally have a net cooling effect (greenhouse effect <

albedo effect)

aerosols can also have a large impact on albedo and the

atmospheric radiation budget

focus on the Sahara -why does it have both high reflected light but also high losses of

longwave radiation?

now look at the Amazon, Central

Africa, and Indonesia - what

causes high reflected shortwave and low longwave

losses?

Some fundamental concepts

the surface is net positive, while the atmosphere is net negative - the two balance one another

out

taken as a whole, the earth is in radiative equilibrium over an annual cycle

36° N

36° S

“If there were no energy transfer the poles would be 25° Celsius cooler, and the equator 14° Celsius warmer!”