Light, Heat and Temperatures on Earth Temperatures on Earth or any planet are primarily determined by 1: The irradiance of sunlight (Presented in Previous Powerpoint). 2: The rate at which planets emit radiation back to space. Irradiance is the rate energy is transmitted divided by the area it crosses, given in Watts per square meter, Wm-2. When Earth is at its average distance from the Sun, direct sunlight on top of the atmosphere is about 1366 Wm-2 (called the solar constant). This means that if sunlight shines directly into a window 1 m wide and 1 m high and no light was lost when it passed through the atmosphere then the room will be about as bright as if it were lit by thirteen 100 Watt bulbs and one 60 Watt bulb. Electric room heaters put out about 1500 Watts, so 1 m2 of sunlight shining in a window can warm a room. All objects emit radiation, and the hotter they are the faster they radiate. On each planet temperature reaches a level where heat is radiated back to space as fast as it is absorbed from the Sun. This balance leads to the Fundamental Equation of Climate. Secondary factors that affect temperature are caused by the nature and composition of the atmosphere, clouds, oceans and ground including snow and vegetation. These factors include 1: Reflection of sunlight 2: Retention of heat by the atmosphere 3: Heat transport by air and water 4: Penetration of heat below the surface 5: Evaporation and Condensation
Understanding Natural Processes and Phenomena Using Systems and Cycles SYSTEM Input Rate The Fundamental Equation of Systems Change = Input – Output (Rate) (Rate) (Rate) Output Rate Often, Input for one System or Reservoir is Output for another. Cycles may occur when there are both Inputs and Outputs
Components of Systems and Cycles S = Reservoir Storage: Total Amount of Material or Quantity F: = Flux: Flow Rate of Material or Quantity into or out of the Reservoirs t = Residence Time: Average Time Spent in Reservoir (time to fill or empty reservoir) Feedback: Change of flow rates as reservoirs shrink or expand Human Cardiovascular System Reservoirs: Body Blood Supply, SBODY = 5120 cm3. Heart Blood Supply, SHEART = 120 cm3. Flux (Normal Cardiac Output), F = 80 cm3 s-1. Residence Time RBODY = 64 s RHEART = 1.5 s Feedback: During exercise the Heart pumps faster, arteries open and F increase by a factor of 5 or 6. SBODY = 5120 FH→B = 80 SHEART = 120 FB→H = 80
Diet Begins Feedback: Impact of Dieting Feedbacks are common in economic, biological, geological, atmospheric, and oceanic systems. They occur when input and/or output rates are affected by changes in storage. The feedbacks greatly complicate the behavior of and changes in these systems. When you first go on a diet it is pretty easy to lose weight. The first weight loss is mostly water and your metabolism is fast. But as time goes on the body fights the weight loss by slowing the metabolic rate. Your body also tells you that you are being starved and makes you feel ravenous. Both of these are negative feedbacks that oppose and slow weight loss. A positive feedback would be that you eat even less and lose weight more rapidly as you see the results of dieting on your beautiful body.
Heat, Temperature, and Changes of Phase When heat is added to the Atmosphere, Earth and Ocean it can do 3 things 1. Raise Temperature, T 2. Change Phase: Evaporate Water or Melt Ice 3. Do Work: Expand Air Consider the daily (annual) temperature cycle. T rises when heat enters a system more rapidly than it leaves (excess). All night (winter) T decreases because heat escapes to space faster than it enters since there no (little) sunshine (deficit). T begins to rise soon after dawn (in Spring) because its heating exceeds the rate of heat lost to space. Even though the Sun reaches its highest point in the sky and is strongest at Noon (Summer Solstice), T continues rising for another two to four hours (one to two months) because the Sun’s heating rate is still larger than the outgoing radiation rate. This time difference is called the lag (of the seasons). But by late afternoon (late Summer) the Sun gets so low in the sky and its rays are so weak that T begins to fall. The temperature range and length of the lag depend on how much material must be heated and cooled. The larger the mass, the longer it takes to change T and the less that T changes. Thus, over the oceans, where sunlight penetrates to great depths and so, must heat a great mass, the range of T is small and the lag of the seasons is large. Inland, the Sun hardly penetrates the ground, which grows extremely hot very quickly but cools quickly at night or during winter.
Daily Cycle of Temperature and Heat Flux Min and Max T occur when Incoming and Outgoing Heat Fluxes are Equal DAILY T T Incoming Solar Radiation Outgoing Terrestrial Radiation Heat Flux Heat Deficit Heat Deficit Heat Excess Midnight Sunrise Noon Hour Sunset Midnight
Transporting Heat • (see next two slides for illustrations) • Radiation – Energy is transported through “empty” space by Electromagnetic waves that travel at the speed of light • Conduction – Sensible heat is transported from hotter regions to colder regions of a motionless volume of material by movements of individual electrons, atoms and molecules. • Convection – Sensible heat is transported by winds or currents of fluids from warmer to colder regions and Latent Heat is transported by winds from more humid to less humid regions of the atmosphere.
HOT COLD Convection Conduction Boiling Melting
Black Body Radiation – Planck’s Law, Wien’s Law and the Stefan-Boltzmann Law Every object with T > 0 K emits radiant energy. (Put your hands near your face to feel the heat!) Joseph Stefan and Ludwig Boltzmann proved that the irradiance, I, of a perfectly efficient radiator (called a Black Body because it absorbs all the light that shines on it) is proportional to the 4th power of the absolute temperature, T. Thus, if an object’s T doubles it will radiate 16 times as fast. No wonder we feel so hot in front of a fire. The Stefan-Boltzmann Law can be rearranged to solve for the equilibrium T of a planet if we know the radiation it receives from the Sun. We call this the Fundamental Equation of Climate. Black bodies emit radiation over a wide range of wavelengths. Wilhelm Wien found that the wavelength of the most intense radiation is inversely proportional to T. Thus the hotter the object, the shorter the waves it emits efficiently, so that as objects heat up they first glow red hot then white hot and finally blue. (This helps us estimate the temperature of star from their color. Max Planck found exactly how much radiation a black body would emit at every wavelength. Planck’s Law represents the Birth of Modern Physics. We will show that it gives rise to the Atmospheric Greenhouse Effect.
T=5800 K UV VISIBLE IR Planck Radiation Perfect Radiators (Black Bodies) Stefan -Boltzmann Law: Irradiance, I is proportional to T4 Wien’s Law: Most Intense Irradiance occurs at This determines the color of a glowing object
Earth’s Equilibrium Temperature and the Radiation Balance Earth attains Equilibrium Temperature when incoming Solar Radiation equals outgoing Terrestrial Radiation. Solar Radiation consists of much shorter waves than Terrestrial Radiation because the Sun is much hotter than Earth. This separation of wavelengths makes the Greenhouse Effect possible by blocking outgoing long waves more efficiently than incoming short waves Solar Radiation reaching Earth Terrestrial Radiation leaving Earth Incoming Radiation Blocked by Atmosphere Outgoing Radiation Blocked by Atmosphere Greenhouse 5800 K 278 K Ultraviolet Infrared
Terrestrial Radiation and Earth’s Temperature We can estimate the equilibrium temperature of any planet by rearranging the Stefan-Boltzmann Law, and setting outgoing terrestrial radiation equal to incoming solar radiation. This produces The Fundamental Equation of Climate: Simple Version Since Iavg = 342 W m-2 TEQUIL = 278 K = 5oC
Complicating Factor #1: Albedo Albedo is the % of light that an object reflects. Reflected light does not contribute to heating. Therefore white objects such as cloud tops and snow reinforce cold. Since A = 30% Tequil = 255K = -18oC 30 Nov 2006 Midwest Ice Storm
Albedo and Temperature The higher the Albedo the Colder the Planet Space Refected sunlight, AISunis wasted ISun ISun Atmosphere Cloud On a black planet with no atmosphere, all sunlight is absorbed at the surface On Earth, a fraction, A, of sunlight is reflected by air, aerosols, clouds, snow, water, soil, rocks, and vegetation. Snow Ice Snow Ice (1-A)ISun absorbed The ground then radiates Long Infrared waves up to space at the maximum rate, ISun = sTBB4 Earth Earth The ground then radiates Long Infrared waves back up to space at the reduced rate, (1-A)ISun = sTP4
Earth’s Albedo: The more sunlight is reflected, the cooler the climate. But ice and snow reflect much sunlight and spread when the climate cools.
The Nature of Feedbacks Some Light Reflected when Some Land Covered with White Paint Some Light Reflected when Some Land Covered with White Snow As T Falls, Snow Spreads – More Light is Reflected so T Falls More Positive Feedback Falling T does not Spread PaintNo Feedback Sunlight Ground Temperature Falls Somewhat when Some Light is Reflected Temperature Falls Somewhat when Some Light is Reflected T T time T Falls More when Snow Spreads and More Light is Reflected
Snow, Ice Albedo Positive Feedback Mechanism Extra Snow and Ice Perhaps by Chance More Light Reflected Less Absorbed Temperature Falls Even More Snow and Ice Even More Light Reflected Less Absorbed Temperature Falls More Ice Age Begins ??
Hot atmosphere levels are those that efficiently absorb solar radiation of some wavelength. Molecules atoms, and ions in the Thermosphere absorb short waves of ultraviolet radiation (UV-C). Oxygen (O2) and Ozone (O3) in the Stratosphere absorb medium wave UV (UV-B). The surface absorbs all remaining wavelengths (mainly visible and short wave infrared IR). Cold atmosphere levels are those that do not absorb any wavelength of radiation efficiently.
Layers of the Atmosphere Thermosphere Mesosphere Stratosphere Troposphere UVC heating of O UVB heating of O3 VIS heating of Rock
Complicating Factor #2: The Greenhouse Effect Sunlight (short wave) or solar radiation easily penetrates the atmosphere. It is absorbed, transformed, and then re-emitted as long wave*****, infrared terrestrial radiation, which has great difficulty escaping from the atmosphere. Even though most of the atmosphere is N2 (78%) or O2 (21%), infrared waves are only absorbed and reemited by the minor gases with three or more atoms (CO2, H2O, CH4, O3). Some of the terrestrial radiation heads back down to the ground. This extra radiation has warmed Earth by some 35oC (63oF) and makes our planet habitable. Now, we are rapidly increasing Greenhouse gases so the atmosphere retains heat more efficiently. This extra heat has accumulated in the oceans so climate has warmed. *****Note: Reflected Radiation retains its wavelength. Absorbed Radiation is later emitted with a different wavelength. The analogy is that if someone squirts whip cream in your face, the whip cream that falls off is still whip cream, but the whip cream that you eat changes form when you digest and emit it. Run ABSORB_EMIT
Terrestrial Radiation Spectrum Measured by Satellite Upwelling Terrestrial Radiation absorbed (blocked) by the atmosphere’s greenhouse gases Upwelling Terrestrial Radiation from the Ground . Computer climate models have radiation modules that can simulate these wiggles, which are caused by emissivity and absorptivity that vary with wavelength.
The Atmospheric Greenhouse Effect Space Only a little IR from the ground escapes directly to space. Most of the IR radiation is absorbed by the atmosphere’s Greenhouse gases (H2O, CO2, O3, CH4) Sunlight, which consists of Short waves penetrates atmosphere easily …which then radiates the IR both up to space … The absorbed IR heats the atmosphere And down to the ground Atmosphere (the Greenhouse) …The ground then radiates Long Infrared waves upward The ground absorbs the IR radiation from the atmosphere, raising ground temperature. This is Greenhouse warming. …The sunlight is absorbed at the ground, which heats up Earth
Within a year after starting the CO2 measurements, David Keating realized that it was increasing. This increase has continued ever since. The wiggles represent the seasons.
Secular trend due to burning fossil fuels and forests The annual cycle of CO2 in the atmosphere has a range of 7 ppm. Excess of decay over growth of vegetation in the North Hemisphere Fall and Winter adds 7 ppm and net growth in Spring and Summer removes it. If there were only net growth and it continued at its present rate (without feedback), it would take 378/7 = 54 years to remove the CO2.
Sunshine, Radiation and The Daily Temperature Cycle near and in the Ground T T During the Day, intense sunshine heats the ground more than the air. Infrared radiation travels up from the surface while IR radiation from the atmosphere travels down to the ground. At night, more IR radiation travels up from the ground than down from the atmosphere, so the surface gets colder than the air above. Enough cooling in humid air can produce fog. Underground Temperature variations are largest near the surface and decrease with depth. The greater the conductivity of the ground the deeper heat and cold penetrate and the smaller the daily and annual range of temperature at the surface. UNDERGROUND TEMPERATURE VARIATIONS