Lecture 6 Solar vs. terrestrial radiation and the bare rock climate model.
Radiation 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)
i-clicker quiz: Which electromagnetic radiation waves have the shortest wavelength and highest frequency? a) gamma rays c) x rays b) radiowaves d) UV rays
Review from last time: All matter with a temperature glows radiation energy. The idea of Blackbody radiation yields a powerful law of nature: Blackbody energy flux (W/m 2 ) F BB = σt 4 How much radiation energy is glowing based on T. The amount is really sensitive to temperature! T 4 = T x T x T x T
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
Solar emission 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.
Thermal Emission Some we can t see! Thermal Imaging Images taken in thermal infrared wavelengths produce accurate measurements of temperature
Terrestrial emission: The Earth emits radiation too. But at much lower temperatures, so therefore at longer wavelengths. SUN Both sun & earth are almost perfect blackbodies! EARTH The hot sun radiates at shorter (visible) wavelengths that carry more energy Energy absorbed by the cooler earth is then reradiated at longer (thermal infrared) wavelengths
i-clicker survey The Stephan-Boltzman law is F = σt 4 If F has units of W / m 2 and T is temperature in K, what are the units of σ? A) W / m 2 K B) W / m 2 K 4 C) m 2 K / W D) m 2 K 4 / W
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 Long Waves = small photons Think of longer waves as small, lightweight particles Short Waves = BIG PHOTONS
(Recall from Lecture 3) Most everything that happens on our planet
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
What happens when radiation meets matter. Remember: Conservation of Energy I = R + A + T
I-clicker question: Does the Earth reflect solar radiation? A: Yes B: No
Reflection of radiation - jargon alert: albedo Albedo: the fraction of incoming radiation that gets reflected Surface albedo varies according to the material Spatially Temporally
Music to help you remember the unfamiliar word: Albedo https://www.youtube.com/watch?v=aj25vm8en2m - By Prof. Dargan Frierson - University of Washington
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
The first law of thermodynamics requires that: Energy in = Energy out 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/m 2 = 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? S = Watts per square meter, constant W m2 x?=w So to find the Watts, we multiply S by the area of this disc in m2, over which the solar energy flux is absorbed.
How many Watts come into the Earth from solar radiation? Formula for area of a circle? Area = πr 2 = π x R x R R is radius of Earth. π ~ 3.14152 So far, Watts in = S x π x R x R almost correct, but not quite
How many Watts come into the Earth from solar radiation? So far, Energy in = S x pi x R 2 What s missing? solar W flux m 2 absorption x m 2 area 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) Energy in = (1 - α) x S x π x R 2 (complete) non-reflected solar flux W m 2
The first law of thermodynamics requires that: Energy in = Energy out 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: If we know..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!
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.