Announcements Final Grades will be determined by: Homework 10% Midterms (1 & 2) 50% Final 40% Alternative Web page where homework assignments and previous lectures can be found is: www.atmos.umd.edu/~meto200 (or on WebCT: www.courses.umd.edu)
Outline for Lecture 4 • Laws of Radiation (reviewed) • Radiation Emitted by the Earth • Heat Budget • Latitudinal Heat Balance • Temperature (Chapter 3) 2/6/2003
What the Heck was that? • Every day we’ll look at some aspect of the weather • Today, take a look at different forecast models and a little on what goes into a forecast—in particular, the system passing through today.
Temperature drop after the system passes is due to cold, dry air being pulled down from the North afterwards Eta Temp Temperature rise ahead of the system is due to warm, moist air being blown up from the south ahead of the system Time goes from right to left
Review of the Average Distribution of Incoming Solar Radiation +100% from Sun -5% backscattered to space -50% absorbed by Earth -20% absorbed by clouds -20% reflected from clouds -5 reflected from land-sea 0 leftover visible light infrared light visible light
Examples of gases which absorb radiation in the atmosphere • Ozone absorbs UV radiation • Water Vapor, CO2 absorb IR Radiation Absorption
Examples of gases that allow radiation to transmit through them • Nitrogen and Oxygen are mostly • transparent to both IR and UV radiation… • but they do absorb higher forms of • radiation… Transmittance
Scattering is responsible for Blue Skies and Red Sunsets Gas molecules more efficiently scatter the shorter wavelengths of visible light (blue and violet) than the longer wavelengths (red and orange). Back scattered light (same wavelength, same intensity) • Aerosols and clouds • scatter and reflect radiation
Blue skies are produced as shorter wavelengths of the incoming visible light (violet and blue) are selectively scattered by N2 and O2 – which are much smaller than the wavelength of the light By the evening, when the sun is low on the horizon, all the blue light is scattered out leaving mostly red and orange light.
Larger particles (haze, fog, or smog) scatter light more equally in all wavelengths. When there is a fog event the sky appears white because no wavelength of light is preferentially scattered.
Absorption by Earth’s Surface and Atmosphere The size and characteristics of a “particle” are important in determining its optical properties. This is why gases are selective in what types of radiation they will absorb, and as fate would have it, many of the gases in our atmosphere do not absorb incoming visible radiation. The Earth’s surface absorbs incoming radiation more readily than does the atmosphere. This helps account for the fact that the half of the solar radiation that reaches the earth’s surface is absorbed while only 20% of this energy is absorbed directly by the atmosphere.
Green Flash The Green Flash http://virtual.finland.fi/finfo/english/mirage.html
Diffused light results from scattering. If scattering did not occur, areas not in direct sunlight would be dark…. shadows would be darker than often observed. Reflected light: bounces back at the same angle at which it strikes the surface and with the same intensity. Scattered light:a larger number of weaker rays all traveling in different directions.
Laws of Radiation Objects that are good absorbers are also good emitters. Dull, black objects absorb ~90% of radiation striking them. These “blackbodies” radiate the maximum intensity of radiation possible for a given temperature. We are interested in the actions of blackbodies because: The Earth’s surface and the Sun approach being blackbodies (perfect radiators) as they absorb and radiate with nearly 100 percent efficiency for their respective temperatures.
Laws of Radiation (Box 2-3 in your textbook) = Rate of radiation emitted by a body (W/m2) = 4 = The Stefan-Boltzmann constant (W/m2K4) = Temperature (K) Stefan-Boltzmann Law JS LB Josef Stefan, in 1879, found an empirical relation between the power per unit area radiated by a “blackbody” and the temperature. About 5 years later Ludwig Boltzmann also derived the same result. It is for that reason that this relationship is called the : Stefan-Boltzmann Law
Wien’s Displacement Law = wavelength of radiation = / = The Wien constant = Temperature (K) Wilhelm Carl Werner Otto Fritz Franz Wien experimentally found that the wavelength of maximum radiation of a thermal body is proportional to the inverse of its temperature (known as "Wien's law") using an oven with a small hole as an approximation to a theoretical blackbody.Wien received the 1911 Nobel Prize for his work on heat radiation.
Sunsets reviewed Short wavelengths(blue, violet) of visible light are preferentially scattered over longer wavelengths. When the sun is overhead (high sun angle), we mostly observe the scattered blue light. When the sun has a low angle (late in the day) the blue light is removed leaving only red and orange colors.
Radiation Emitted by the Earth Recall all objects emit radiation. Recall the Earth is cooler than the Sun and, therefore, emits less radiant energy than the Sun. The difference in the radiant energy between the earth and the sun is critical to understanding how the atmosphere is heated. Gases are excellent absorbers. When radiation is absorbed, energy is converted to internal molecular motion (Rise in Temperature).
N2 absorbs slightly higher energy radiation than is emitted by Earth. O2 and O3 absorb high energy shortwave radiation. H2O is the only significant absorber of incoming (and outgoing) radiation. The gases which are the most efficient absorbers of radiation are important in heating the atmosphere. EARTH SUN Absorptivity of selected gases in the atmosphere
“Atmospheric window” is the zone in the electromagnetic spectrum where no gases efficiently absorb outgoing radiation…well, almost no gases. Ozone does absorb in that band. Anthropogenic, “ground level” ozone is present in the troposphere. Compared to water vapor, ozone does not create a large portion of the heating in the atmosphere
The “Greenhouse Effect” If Earth had no atmosphere it would experience an average surface temperature far below freezing. The role the atmosphere plays in heating the atmosphere is termed the “greenhouse effect”. By contrast, Mars which has a much thinner atmosphere (with no water!) experiences a diurnal variation in temperature of ~60°C. 24 hour temperature change for Mars
Venus and the Runaway Greenhouse Effect On the other end of the spectrum… Atmospheric temperatures on Venus range from 850-900°F because of a dense atmosphere of carbon dioxide. Venus is shrouded in a hot, cloud filled atmosphere composed mainly of CO2
Earth, Mars, Venus • Earth, Mars, and Venus each had similar masses and, similar initial atmospheric composition • Primordial (first) atmosphere (H and He) was blown away by sun • Ancient atmosphere was outgassed by the planet: CO2, Water Vapor, Nitrogen, Argon, but NOT Oxygen!
Earth, Mars, Venus Earth • Was cool enough so that water vapor condensed to form oceans • Was large enough to hold onto its hydrogen and therefore water vapor, H2O • This allowed CO2 to dissolve in the ocean and be turned into rock (limestone) • Oxygen comes from life (later).
Venus Hot enough to boil away any ocean—hydrogen escapes No ocean, so CO2 never dissolves, stays in atmosphere Serves as a blanket for a runaway greenhouse effect (900°F!) Mars Sun is far away—cold! Gravity small so that hydrogen escapes, hence very little H2O, water. CO2 can’t dissolve Cold enough to make solid CO2 icecaps So what happened?
The Heating of the Atmosphere The Greenhouse effect keeps Earth much warmer than it otherwise would be.
A Greenhouse? • Something of a misnomer—in reality, greenhouses work by eliminating cooling by convection. This “greenhouse” works by reducing cooling from radiation
The Role of Clouds in Heating the Earth “Cloud-radiation interactions and feedbacks are at the top of every recent list of high-priority research topics in climate modeling because they dominate the response of climate models to imposed forcings.” “Sensitivity studies demonstrate how crucial it is that observational evidence be brought to bear to determine which, if any, of the tested parameterizations realistically represents how clouds behave in the actual atmosphere.” -Richard C. J. Somerville Scripps Institution of Oceanography University of California, San Diego