Lecture 6Solar vs. terrestrial radiation and the

“bare rock” climate model.

• Controls energy balance of Earth

• Is all around us all the time.

• Can be labeled

• by its source (solar, terrestrial)

• or its name (ultra violet, visible, near infrared, infrared, microwave, etc….)

• or by its wavelength (e.g. < 3 micrometers)

Radiation

• Which electromagnetic radiation waves have the shortest wavelength and highest frequency?

• a) gamma rays b) radiowaves

• c) x rays d) UV rays

i-clicker quiz:

• All matter with a temperature glows radiation energy.

• The idea of “Blackbody” radiation yields a powerful law of nature:

Review from last time:

FBB =σT4

• How much radiation energy is glowing based on T.

• The amount is really sensitive to temperature!

• T4 = T x T x T x T

Blackbody energy flux (W/m2)

Hotter things glow radiation at shorter wavelengths!

I-clicker quiz: In the following movie, the process of allows us to see energy move from a person to a chair through the process of .

• A: radiation, convection

• B: radiation, conduction

• C: convection, radiation

• D:conduction, radiation

• The Sun emits energy at a lot of wavelengths, some we feel warms us, most we see as visible light

Solar emission

Solar radiation has peak intensities in the shorter wavelengths, dominant in the region we know as visible,

but extends at low intensity into longwave regions.

Solar emission

• Images taken in thermal infrared wavelengths produce accurate measurements of temperature

Thermal Imaging

Thermal EmissionSome we can’t see!

SUN

EARTH

Terrestrial emission: The Earth emits radiation too. But at much lower temperatures, so therefore at longer wavelengths.

• Both sun & earth are almost perfect blackbodies!

• The hot sun radiates at shorter (visible) wavelengths that carry more energy

• Energy absorbed by the cooler earth is then re-radiated at longer (thermal infrared) wavelengths

i-clickersurvey

TheStephan-BoltzmanlawisF=σT4IfFhasunitsofW/m2andTistemperatureinK,whataretheunitsofσ?

A)W/m2KB)W/m2K4C)m2K/WD)m2K4/W

Long Waves = small photons

Short Waves = BIG PHOTONS

Waves and photons• Is light a wave?

• YES!

• Is light a particle?

• YES!

• All light travels at the same speed

• Think of short waves as BIG HEAVY particles

• Think of longer waves as small, lightweight particles

Most everything that happens on our planet…

(Recall from Lecture 3)

… Is a link on the chain of energy flowing out from the hot sun and dissipating into outer space.

Fine, but what actually happens to solar radiation energy once it enters the Earth’s atmosphere?

Energy from solar rays

Remember:Conservation of Energy

I = R + A + T

What happens when radiation meets matter.

I-clicker question: Does the Earth reflect solar radiation?

A: Yes

B: No

• Albedo: the fraction of incoming radiation that gets reflected

• Surface albedo varies according to the material • Spatially • Temporally

Reflection of radiation - jargon alert: “albedo”

- By Prof. Dargan Frierson - University of Washington

“Albedo”

https://www.youtube.com/watch?v=aj25vm8eN2M

Music to help you remember the unfamiliar word:

Diagram of the solar radiation “budget”

30% reflected by clouds, air, dust, and surface 19% absorbed by the atmosphere (mostly clouds)

51% absorbed at the surface

Get ready to nerd out!

We now have enough building blocks to do our first legit climate calculation…

We now have enough building blocks to do our first legit climate calculation…

Energy in = Energy outThe first law of thermodynamics requires that:

Watts in from solar radiation = Watts from terrestrial radiation

How many Watts come into the Earth from solar radiation?

• At the distance of the Earth’s orbit from the sun, a constant solar energy flux shines towards the Earth.

• We give this a special name:

• S = 1360 W/m2 = “the solar constant”

• S can be calculated for other planets too.

• Gets smaller the farther they are from the sun.

How many Watts come into the Earth from solar radiation?

So to find the Watts, we multiply S by the area of this disc in m2, over which the solar energy flux is absorbed.

S = Watts per square meter, constant

Wm2

x ? = W

• Formula for area of a circle?

• Area = πR2

= π x R x R

• R is radius of Earth.

• π ~ 3.14152

So far, Watts in = S x π x R x R

How many Watts come into the Earth from solar radiation?

… almost correct, but not quite…

Energy in = S x pi x R2

How many Watts come into the Earth from solar radiation?

Does the Earth absorb all the solar energy that strikes it?

No. 30% is reflected back to outer space. Only remaining 70% is

available for absorption. Need to multiply S by 0.7 = (1 - albedo)

What’s missing?

Energy in = (1 - α) x S x π x R2 (complete)

Wm2

solar flux

absorption areax m2

Wm2

non-reflected solar flux

So far,

Energy in = Energy outThe first law of thermodynamics requires that:

Watts in from solar radiation = Watts out from thermal radiation

= FBB x emission area

Blackbody energy

flux

Surface area of the Earth

(the whole Earth glows)

=This is the “Bare rock” climate model

=

The power of math:

..solar constant,

…and the albedo

We can solve for a planet’s temperature!This is the “bare rock” model: A climate prediction from laws of

energy balance, black body radiation and geometry!

If we know…

What temperature does the “bare rock” model predict?

• Solving for T predicts an equilibrium temperatue that is really cold: Minus eighteen degrees Celsius. -18C

• If Earth were this cold it would have:

• frozen oceans, miles of ice

• So something must be missing from the model…

• The atmosphere! The Earth is not a bare rock.

• If it were it would be real cold here on the surface.

• But the atmosphere blankets our rocky surface. This makes a big difference to the temperature.

Next time: The Greenhouse effect.

Why the atmosphere keeps us warmer than we should be.