1. Chapter 3 Solar and Terrestrial Radiation
Weather Studies Introduction to Atmospheric Science American 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. Wavelength is Inversely Proportional to Wave Frequency

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
Beginning of Northern Hemisphere Spring

18. Procession of the Earth Around the Sun and 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 Sun at the equator at N
Path of the Sun at the equator at N. Hemisphere midlatitudes at 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