Weather Studies Introduction to Atmospheric Science American Meteorological Society

1. Weather StudiesIntroduction to Atmospheric ScienceAmerican Meteorological Society Chapter 3 Solar and Terrestrial Radiation Credit: This presentation was prepared for AMS by Michael Leach, Professor of Geography at New Mexico State University - Grants

2. Case-in-Point • Recurring patterns of seasons and seasonal change have been important to humans since the beginning of their existence • Stonehenge • Earliest portions date to 2950 BC • Aligned to summer solstice and mid-winter sunset • Predicts solar and lunar eclipses • Other locations • Native Americans near present-day St. Louis • Wooden posts arranged in circles (Woodhenge calendars) • Nubian Desert of southern Egypt • Predates Stonehenge by 2000 years • These devices predict important events

3. Driving Question • How Does Energy Flow Into and Out of the Earth-Atmosphere System? • Energy = the ability to do work • 1st law of thermodynamics – energy cannot be created or destroyed, although it can be converted from one form to another • Example heat energy → kinetic energy of winds • This chapter examines: • Electromagnetic radiation and laws that govern it • How this reacts with the Earth-atmosphere system • Conversion of solar radiation to heat • Earth emission of infrared radiation • The greenhouse effect

4. The Electromagnetic Spectrum • Terms • Electromagnetic radiation – energy transmitted through space or materials as waves (e.g., solar radiation). It has both electric and magnetic properties • Electromagnetic spectrum – composed of radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and Gamma rays • Wavelength – distance between successive wave crests or troughs • Wave frequency - number of wave crests that pass a given point per second (hertz, Hz) • Inversely proportional to wavelength • Speed of electromagnetic radiation: • 300,000 km/sec (186,000 mi/sec) • Commonly called the “speed of light” • Ultraviolet – beyond violet. Short-wave radiation • Infrared – below red. Long-wave radiation

5. The Electromagnetic Spectrum

6. Wavelengthis InverselyProportional toWaveFrequency

7. The Electromagnetic Spectrum • Terms, continued • Visible radiation – that portion of the spectrum perceptible to the human eye • Violet end – 0.40 μm • Red end – 0.70 μm • μm = micrometer = one millionth of a meter • Microwave radiation – wavelength = 0.1 to 1000 mm • Microwave ovens • Some used for radio communication (weather radio) • Radio waves • Wavelengths range from a fraction of a centimeter to hundreds of kilometers • Frequency up to a billion Hz • FM – 88 million to 108 million Hz

8. Radiation Laws • Blackbody – at a constant temperature, it absorbs all radiation it receives and emits all the energy it absorbs • It is a perfect absorber and a perfect emitter • Surfaces of real objects may approximate blackbodies for certain wavelengths of radiation • To make all of the mathematical laws simple: • The wavelength of most intense radiation emitted by a blackbody is inversely proportional to its absolute temperature (Wien’s displacement law) • Both sun and Earth are nearly blackbodies. The sun is much hotter than the Earth, therefore, its most intense radiation is at a much shorter wavelength than the Earth’s.

9. Wien’s Displacement Law λmax = C/T, where λmax is the wavelength of most intense radiation, C is a constant of proportionality, and T is absolute temperature

10. Radiation Laws • The total energy flux (E) emitted by a blackbody across all wavelengths is proportional to the 4th power of its absolute temperature (T), E ~ T4 Sun Earth

11. Inverse Square Law • Doubling the distance from the sun reduces solar radiation by 1/4

12. Input of Solar Radiation • Sun – composed of hydrogen and helium • Source of solar energy is nuclear fusion reaction • 4 hydrogen protons fuse to form one helium nucleus • Excess mass in this fusion is converted to energy, E = mc2 • Some of this energy is used to bond the helium nucleus • The rest is radiated off to the sun’s surface and into space • The photosphere, or visible surface of the sun, is cooler than the interior and is convective • These convective cells are called granules • Sunspots = cool areas on the sun’s surface • Accompanying bright areas are called faculae • changes in numbers of sunspots/faculae may affect Earth’s climate • Chromosphere – sun’s atmosphere of superheated gases, mostly hydrogen and helium • Corona – the outermost portion of the sun’s atmosphere

13. Solar Altitude • Solar radiation more directly overhead concentrates solar energy in a small area • Solar radiation that comes in at an angle spreads the solar energy over a larger area • Concentrated energy provides for more heat per unit surface area = hotter ground temperatures

14. Solar Altitude and Latitude • The noon solar altitude always varies with latitude because the Earth presents a curved surface to the incoming solar beam • In equinox example, the solar altitude is 90 degrees at the equator and decreases with latitude (towards the poles) • Noon solar radiation striking horizontal surfaces per unit area is most intense at equator

15. Additionally, the incoming solar radiation has more atmosphere to pass through at low angles of incidence. • The atmosphere is not completely transparent to solar radiation • Low angles of incidence allow for more atmospheric scattering, reflection, and absorption of solar radiation

16. Solar Altitude • Intensity of solar radiation striking local Earth surfaces varies over the year • Inclination of Earth’s axis causes the Northern Hemisphere to be tilted toward the sun for part of the year, and away from the sun for part of the year • When the North Pole is tilted toward the sun, the Northern Hemisphere receives more solar radiation • This is spring or summer in the Northern Hemisphere • When the North Pole is tilted away from the sun, the Northern Hemisphere receives less solar radiation • This is fall or winter in the Northern Hemisphere

17. Solar Altitude & Procession of Seasons • At the June 21 Solstice the sun is directly overhead (90° altitude) at the Tropic of Cancer • 23.5° N latitude • Beginning of Northern Hemisphere Summer • At the September 23 Equinox, the sun is directly overhead (90° altitude) at the equator • 0° latitude • Beginning of Northern Hemisphere Fall • At the December 21 Solstice the sun is directly overhead (90° altitude)at the Tropic of Capricorn • 23.5° S latitude • Beginning of Northern Hemisphere Winter • At the March 21 Equinox, the sun is directly overhead (90° altitude) at the equator • 0° latitude • Beginning of Northern Hemisphere Spring

18. Procession of the Earth Around the Sunand the Seasons Northern Hemisphere tilted away from the sun Northern Hemisphere tilted toward the sun

19. Perihelion and Aphelion

20. Average Daily Solar Radiation at Differing Latitudes

21. Circle of Illumination Equinox N. Hemisphere summer solstice N. Hemisphere winter solstice

22. Path of the Sunat the equatorat N. Hemisphere midlatitudesat the North Pole

23. Variation in the length of daylight increases with increasing latitude

24. Solar Radiation and the Atmosphere • The solar constant is the rate at which solar radiation falls on a surface located at the outer edge of the atmosphere and oriented perpendicular to the incoming solar beam when Earth is at a mean distance from the sun – averages 1.97 cal/cm2/min (1368 W/m2) • Some solar radiation passing through the Earth’s atmosphere interacts with gases and aerosols via scattering, reflection, and absorption • Law of energy conservation → Within the atmosphere, % solar radiation absorbed (absorptivity) + % scattered or reflected (albedo) + % transmitted to Earth’s surface (transmissivity) = 100%

25. Solar Radiation and the Atmosphere • Scattering • Particles can disperse solar radiation • Scattering is wavelength dependent • Preferential scattering of blue-violet light by oxygen and nitrogen molecules • That is why the daytime sky is blue • Reflection • A special case of scattering when some radiation striking a surface is backscattered • Law of reflection → Angle of incidence (i) = Angle of reflection (r) • Albedo = (reflected radiation)/(incident radiation)

26. Solar Radiation and the Atmosphere • Absorption • Converts radiation to heat energy • UV absorbed in stratosphere – chemical reactions involved in formation and dissociation of ozone • Significantly reduces the intensity of UV that reaches Earth’s surface • Causes marked warming of upper stratosphere

27. The Stratospheric Ozone Shield

28. Why is the Southern Hemisphere Spring Ozone Hole Over Antarctica? • Circumpolar vortex cuts off Antarctic atmosphere • Loses ozone through absorption of UV radiation • Circumpolar vortex weakens in spring • Warmer, ozone rich air invades • Replenishes ozone • Cold Antarctic stratosphere with stratospheric ice accelerates the reaction with CFCs as a catalyst • No comparable ozone hole in Arctic due to warmer temperatures and weaker circumpolar vortex • The Montreal Protocol was an international agreement to limit CFC production • Violators receive economic sanctions from other signing countries

29. The Antarctic Ozone Hole

30. Chemicals in the Ozone Layer • Chemicals that threaten ozone layer have natural and industrial sources • They enter the stratosphere through deep tropical convective currents

31. Solar Radiation and the Earth’s Surface • The lighter the surface, the higher the albedo • Albedo can vary with solar altitude • Water has highest albedo at lowest solar altitude • Near 100% at sunrise and sunset • This decreases rapidly as solar altitude increases • Global average oceanic albedo = 8% • 92% of solar energy reaching oceans is absorbed

32. Solar Radiation and the Earth’s Surface Albedo of water surface as a function of solar altitude

33. Solar Radiation and the Earth’s Surface • Water absorbs red light more efficiently; more green and blue light is scattered to our eyes, explaining the color of the open ocean

34. Global Solar Radiation Budget • Earth’s surface is the principal recipient of solar heating and is the main source of heat for the atmosphere, which is evident in the vertical profile of the troposphere • Global radiative equilibrium → solar radiational heating of the Earth-atmosphere system is balanced by emission of heat to space in the form of infrared radiation

35. Greenhouse Warming • Greenhouse effect – heating of Earth’s surface and lower atmosphere by strong absorption and emission of infrared radiation by certain atmospheric gases • These gases are called greenhouse gases • Recall that Earth emits infrared, or long-wave radiation as its most intense radiation, and the sun emits ultraviolet and visible as its most intense radiation • Greenhouse gases are transparent to short-wave radiation, but absorb long-wave radiation • This has the same net effect as a greenhouse, which lets in shortwave radiation through the glass, but the glass strongly absorbs and emits infrared radiation. This helps warm the greenhouse. • The earth is kept warm by greenhouse gases • Without greenhouse gases, life as we know it would not exist

36. Greenhouse Gases and Global Climate Change • Recall from Chapter 2 that as the atmosphere developed, much carbon was locked into the Earth in the form of carbonate rocks, and fossil fuels • Burning of fossil fuels and biomass in general releases carbon (in the form of CO2) back into the atmosphere • Recall from Chapter 2 that the Earth was much warmer with more CO2 in the atmosphere • As we burn these fuels and add CO2 to the atmosphere, we are creating conditions which raise global temperatures through an enhanced greenhouse effect • This may have severe and detrimental effects on the Earth and humankind

37. Greenhouse Gases and Global Climate Change

38. Possible Impacts of Global Warming • Climate zones may shift poleward by as much as 550 km (350 mi) • Heat and moisture stress would cut crop production in certain areas • On the plus side, we could farm at higher latitudes • Rising sea levels of 9-88 cm (4-35 in.) from 1990 to 2100) • Inundation of low islands and coastal plains • Many are heavily populated • Decreased snow cover and sea-ice extent

39. Average Annual Temperature Departures from the Long-Term Average

40. What Should We Do? • In spite of scientific uncertainties, many agree that action should be taken to head off possible enhanced greenhouse warming • Many agree that we should: • Sharply reduce oil and coal consumption • Have greater reliance on non-fossil fuel energy sources • Have higher energy efficiencies (e.g, more vehicle miles per gallon) • Massive reforestation, and a halt to deforestation • Even if it were not for enhanced greenhouse warming, doing this would help other problem areas • Example – cutting air pollution