Abstract

During many clear twilights, much of the solar sky is dominated by pastel purples. This purple light’s red component has long been ascribed to transmission through and scattering by stratospheric dust and other aerosols. Clearly the vivid purples of post-volcanic twilights are related to increased stratospheric aerosol loading. Yet our time-series measurements of purple-light spectra, combined with radiative transfer modeling and satellite soundings, indicate that background stratospheric aerosols by themselves do not redden sunlight enough to cause the purple light’s reds. Furthermore, scattering and extinction in both the troposphere and the stratosphere are needed to explain most purple lights.

© 2003 Optical Society of America

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  1. F. A. R. Russell, “Previous analogous glow phenomena, and corresponding eruptions,” in The Eruption of Krakatoa, and Subsequent Phenomena, G. J. Symons, ed. (Trübner, London, 1888), pp. 384–405.
  2. J. E. Clark, “The remarkable sunsets,” Nature 29, 130–131 (1883).
    [CrossRef]
  3. F. A. R. Russell, “Proximate physical cause of the unusual twilight glows in 1883–4,” in The Eruption of Krakatoa, and Subsequent Phenomena, G. J. Symons, ed. (Trübner, London, 1888), pp. 178–196.
  4. A. Riggenbach, Beobachtungen über die Dämmerung insbesondere über das Purpurlicht und seine Beziehungen zum Bishop’schen Sonnennring (Georg, Basel, Switzerland, 1886).
  5. P. Gruner, H. Kleinert, Die Dämmerungserscheinungen (Grand, Hamburg, Germany, 1927), pp. 103–107.
  6. A. Heim, “Explanations of the western purple light and the eastern afterglow (Nachglühen),” Mon. Weather Rev. 44, 624–625 (1916). Although we write “diffraction” here to match the terminology found in Heim’s and earlier accounts, nowadays we use both the language and the physics of single-scattering theories such as Mie theory in explaining the purple light.
    [CrossRef]
  7. J. V. Dave, C. L. Mateer, “The effect of stratospheric dust on the color of the twilight sky,” J. Geophys. Res. 73, 6897–6913 (1968).
    [CrossRef]
  8. F. E. Volz, “Twilights and stratospheric dust before and after the Agung eruption,” Appl. Opt. 8, 2505–2517 (1969).
    [CrossRef] [PubMed]
  9. H. H. Lamb, “Volcanic dust in the atmosphere; with a chronology and assessment of its meteorological significance,” Phil. Trans. R. Soc. London Ser. A 266, 425–533 (1970).
    [CrossRef]
  10. D. Deirmendjian, “On volcanic and other particulate turbidity anomalies,” Adv. Geophys. 16, 267–296 (1973).
    [CrossRef]
  11. G. E. Shaw, “Observations of two stratospheric dust events,” J. Appl. Meteorol. 14, 1619–1620 (1975).
    [CrossRef]
  12. H. Neuberger, Introduction to Physical Meteorology (Pennsylvania State U. Press, University Park, Pa., 1957), pp. 184–185, 190.
  13. J. Walker, The Flying Circus of Physics with Answers (Wiley, New York, 1977), p. 274.
  14. M. G. J. Minnaert, Light and Color in the Outdoors (Springer-Verlag, New York, 1993), pp. 296, 298, 303.
  15. D. K. Lynch, W. Livingston, Color and Light in Nature (Cambridge U. Press, Cambridge, 1995), p. 41.
  16. K. Bullrich, Die farbigen Dämmerungserscheinungen (Birkhäuser, Basel, Switzerland, 1982), pp. 69–70.
  17. T. Simkin, R. S. Fiske, Krakatau 1883: The Volcanic Eruption and Its Effects (Smithsonian Institution Press, Washington, D.C., 1983), p. 418.
  18. T. S. Glickman, ed., Glossary of Meteorology, 2nd ed. (American Meteorological Society, Boston, Mass., 2000), p. 605.
  19. The period during which we measured twilight spectra had no unusual volcanism, so our data are of ordinary rather than volcanic purple lights. Also, our measurements are confined to evening twilights (with one important exception), and strictly speaking so are our conclusions about the purple light.
  20. PR-650 spectroradiometer from Photo Research, Inc., 9731 Topanga Canyon Place, Chatsworth, Calif. 91311. According to Photo Research, at specified radiance levels a properly calibrated PR-650 instrument measures luminance and radiance accurately to within ±4% and has a spectral accuracy of ±2 nm, and its CIE 1931 colorimetric errors are x < 0.001, y < 0.001 for a 2856-K blackbody (CIE standard illuminant A).
  21. G. V. Rozenberg, Twilight: A Study in Atmospheric Optics (Plenum, New York, 1966), p. 17. In purely astronomical terms, civil twilight is the period between sunset or sunrise and h0 = -6°; nautical twilight is the period when -6° > h0 > -12°.
  22. For an overview of the entire CIE 1976 UCS diagram, see R. L. Lee, “Twilight and daytime colors of the clear sky,” Appl. Opt. 33, 4629–4638, 4959 (1994).
  23. G. Wyszecki, W. S. Stiles, Color Science: Concepts and Methods, Quantitative Data and Formulae, 2nd ed. (Wiley, New York, 1982), pp. 306–310. Here we follow convention and set the JND equal to the semimajor axis length of the MacAdam color-matching ellipse at the given chromaticity.
  24. E. L. Deacon, “The second purple light,” Nature 178, 688 (1956).
    [CrossRef]
  25. R. L. Lee, A. B. Fraser, The Rainbow Bridge: Rainbows in Art, Myth, and Science (Pennsylvania State U. Press, University Park, Pa., 2001), pp. 266–267.
  26. During some volcanic twilights, the second purple light is vividly colored (hence its name) because increased concentrations of liquid aerosols and dust in the stratosphere scatter much more sunlight to the surface.
  27. LI-1800 spectroradiometer from LI-COR, Inc., 4421 Superior Street, Lincoln, Neb. 68504-1327. According to LI-COR, at specified radiance levels a properly calibrated LI-1800 instrument measures visible-wavelength spectral radiances accurate to within ±5% and has a spectral accuracy of ±2 nm, and its CIE 1931 colorimetric errors are x < 0.003, y < 0.003 for primaries that are typical of a cathode-ray-tube color monitor.
  28. The × to the right of Fig. 11’s 5-3-00 purity maximum occurred on 6-11-01. Although this × is more purplish than the 5-3-00 maximum, the 6-11-01 maximum is actually closer to the chromaticity of 5-2-00. Thus the largest chromaticity shift that we measured is from 5-2-00 to 5-3-00, even though Fig. 11’s anisotropic scaling suggests otherwise.
  29. J. M. Russell, HALogen Occultation Experiment (HALOE) home page, http://haloedata.larc.nasa.gov/home.html , accessed 7May2002.
  30. Note that 14.4–30.3 km is the largest altitude range for which HALOE data exist on all three dates in Fig. 14. Meteorological soundings on these dates from nearby Dulles International Airport have temperature inversions that place the local tropopause at altitudes of 12.3–15.7 km.
  31. For example, see Ref. 5, Plate 5.
  32. R. W. Fenn, S. A. Clough, W. O. Gallery, R. E. Good, F. X. Kneizys, J. D. Mill, L. S. Rothman, E. P. Shettle, F. E. Volz, “Optical and infrared properties of the atmosphere,” in Handbook of Geophysics and the Space Environment, A. S. Jursa, ed. (U.S. Air Force Geophysics Laboratory, Hanscom Air Force Base, Bedford, Mass., 1985), pp. 18:1–18:80.
  33. F. X. Kneizys, E. P. Shettle, L. W. Abreu, J. H. Chetwynd, G. P. Anderson, W. O. Gallery, J. E. A. Selby, S. A. Clough, “User’s Guide to lowtran 7,” Rep. AFGL-TR-88-0177 (U.S. Air Force Geophysics Laboratory, Hanscom Air Force Base, Bedford, Mass., 1988).
  34. Ref. 18, p. 488.
  35. C. N. Adams, G. N. Plass, G. W. Kattawar, “The influence of ozone and aerosols on the brightness and color of the twilight sky,” J. Atmos. Sci. 31, 1662–1674 (1974).
    [CrossRef]

1994 (1)

For an overview of the entire CIE 1976 UCS diagram, see R. L. Lee, “Twilight and daytime colors of the clear sky,” Appl. Opt. 33, 4629–4638, 4959 (1994).

1975 (1)

G. E. Shaw, “Observations of two stratospheric dust events,” J. Appl. Meteorol. 14, 1619–1620 (1975).
[CrossRef]

1974 (1)

C. N. Adams, G. N. Plass, G. W. Kattawar, “The influence of ozone and aerosols on the brightness and color of the twilight sky,” J. Atmos. Sci. 31, 1662–1674 (1974).
[CrossRef]

1973 (1)

D. Deirmendjian, “On volcanic and other particulate turbidity anomalies,” Adv. Geophys. 16, 267–296 (1973).
[CrossRef]

1970 (1)

H. H. Lamb, “Volcanic dust in the atmosphere; with a chronology and assessment of its meteorological significance,” Phil. Trans. R. Soc. London Ser. A 266, 425–533 (1970).
[CrossRef]

1969 (1)

1968 (1)

J. V. Dave, C. L. Mateer, “The effect of stratospheric dust on the color of the twilight sky,” J. Geophys. Res. 73, 6897–6913 (1968).
[CrossRef]

1956 (1)

E. L. Deacon, “The second purple light,” Nature 178, 688 (1956).
[CrossRef]

1916 (1)

A. Heim, “Explanations of the western purple light and the eastern afterglow (Nachglühen),” Mon. Weather Rev. 44, 624–625 (1916). Although we write “diffraction” here to match the terminology found in Heim’s and earlier accounts, nowadays we use both the language and the physics of single-scattering theories such as Mie theory in explaining the purple light.
[CrossRef]

1883 (1)

J. E. Clark, “The remarkable sunsets,” Nature 29, 130–131 (1883).
[CrossRef]

Abreu, L. W.

F. X. Kneizys, E. P. Shettle, L. W. Abreu, J. H. Chetwynd, G. P. Anderson, W. O. Gallery, J. E. A. Selby, S. A. Clough, “User’s Guide to lowtran 7,” Rep. AFGL-TR-88-0177 (U.S. Air Force Geophysics Laboratory, Hanscom Air Force Base, Bedford, Mass., 1988).

Adams, C. N.

C. N. Adams, G. N. Plass, G. W. Kattawar, “The influence of ozone and aerosols on the brightness and color of the twilight sky,” J. Atmos. Sci. 31, 1662–1674 (1974).
[CrossRef]

Anderson, G. P.

F. X. Kneizys, E. P. Shettle, L. W. Abreu, J. H. Chetwynd, G. P. Anderson, W. O. Gallery, J. E. A. Selby, S. A. Clough, “User’s Guide to lowtran 7,” Rep. AFGL-TR-88-0177 (U.S. Air Force Geophysics Laboratory, Hanscom Air Force Base, Bedford, Mass., 1988).

Bullrich, K.

K. Bullrich, Die farbigen Dämmerungserscheinungen (Birkhäuser, Basel, Switzerland, 1982), pp. 69–70.

Chetwynd, J. H.

F. X. Kneizys, E. P. Shettle, L. W. Abreu, J. H. Chetwynd, G. P. Anderson, W. O. Gallery, J. E. A. Selby, S. A. Clough, “User’s Guide to lowtran 7,” Rep. AFGL-TR-88-0177 (U.S. Air Force Geophysics Laboratory, Hanscom Air Force Base, Bedford, Mass., 1988).

Clark, J. E.

J. E. Clark, “The remarkable sunsets,” Nature 29, 130–131 (1883).
[CrossRef]

Clough, S. A.

R. W. Fenn, S. A. Clough, W. O. Gallery, R. E. Good, F. X. Kneizys, J. D. Mill, L. S. Rothman, E. P. Shettle, F. E. Volz, “Optical and infrared properties of the atmosphere,” in Handbook of Geophysics and the Space Environment, A. S. Jursa, ed. (U.S. Air Force Geophysics Laboratory, Hanscom Air Force Base, Bedford, Mass., 1985), pp. 18:1–18:80.

F. X. Kneizys, E. P. Shettle, L. W. Abreu, J. H. Chetwynd, G. P. Anderson, W. O. Gallery, J. E. A. Selby, S. A. Clough, “User’s Guide to lowtran 7,” Rep. AFGL-TR-88-0177 (U.S. Air Force Geophysics Laboratory, Hanscom Air Force Base, Bedford, Mass., 1988).

Dave, J. V.

J. V. Dave, C. L. Mateer, “The effect of stratospheric dust on the color of the twilight sky,” J. Geophys. Res. 73, 6897–6913 (1968).
[CrossRef]

Deacon, E. L.

E. L. Deacon, “The second purple light,” Nature 178, 688 (1956).
[CrossRef]

Deirmendjian, D.

D. Deirmendjian, “On volcanic and other particulate turbidity anomalies,” Adv. Geophys. 16, 267–296 (1973).
[CrossRef]

Fenn, R. W.

R. W. Fenn, S. A. Clough, W. O. Gallery, R. E. Good, F. X. Kneizys, J. D. Mill, L. S. Rothman, E. P. Shettle, F. E. Volz, “Optical and infrared properties of the atmosphere,” in Handbook of Geophysics and the Space Environment, A. S. Jursa, ed. (U.S. Air Force Geophysics Laboratory, Hanscom Air Force Base, Bedford, Mass., 1985), pp. 18:1–18:80.

Fiske, R. S.

T. Simkin, R. S. Fiske, Krakatau 1883: The Volcanic Eruption and Its Effects (Smithsonian Institution Press, Washington, D.C., 1983), p. 418.

Fraser, A. B.

R. L. Lee, A. B. Fraser, The Rainbow Bridge: Rainbows in Art, Myth, and Science (Pennsylvania State U. Press, University Park, Pa., 2001), pp. 266–267.

Gallery, W. O.

F. X. Kneizys, E. P. Shettle, L. W. Abreu, J. H. Chetwynd, G. P. Anderson, W. O. Gallery, J. E. A. Selby, S. A. Clough, “User’s Guide to lowtran 7,” Rep. AFGL-TR-88-0177 (U.S. Air Force Geophysics Laboratory, Hanscom Air Force Base, Bedford, Mass., 1988).

R. W. Fenn, S. A. Clough, W. O. Gallery, R. E. Good, F. X. Kneizys, J. D. Mill, L. S. Rothman, E. P. Shettle, F. E. Volz, “Optical and infrared properties of the atmosphere,” in Handbook of Geophysics and the Space Environment, A. S. Jursa, ed. (U.S. Air Force Geophysics Laboratory, Hanscom Air Force Base, Bedford, Mass., 1985), pp. 18:1–18:80.

Good, R. E.

R. W. Fenn, S. A. Clough, W. O. Gallery, R. E. Good, F. X. Kneizys, J. D. Mill, L. S. Rothman, E. P. Shettle, F. E. Volz, “Optical and infrared properties of the atmosphere,” in Handbook of Geophysics and the Space Environment, A. S. Jursa, ed. (U.S. Air Force Geophysics Laboratory, Hanscom Air Force Base, Bedford, Mass., 1985), pp. 18:1–18:80.

Gruner, P.

P. Gruner, H. Kleinert, Die Dämmerungserscheinungen (Grand, Hamburg, Germany, 1927), pp. 103–107.

Heim, A.

A. Heim, “Explanations of the western purple light and the eastern afterglow (Nachglühen),” Mon. Weather Rev. 44, 624–625 (1916). Although we write “diffraction” here to match the terminology found in Heim’s and earlier accounts, nowadays we use both the language and the physics of single-scattering theories such as Mie theory in explaining the purple light.
[CrossRef]

Kattawar, G. W.

C. N. Adams, G. N. Plass, G. W. Kattawar, “The influence of ozone and aerosols on the brightness and color of the twilight sky,” J. Atmos. Sci. 31, 1662–1674 (1974).
[CrossRef]

Kleinert, H.

P. Gruner, H. Kleinert, Die Dämmerungserscheinungen (Grand, Hamburg, Germany, 1927), pp. 103–107.

Kneizys, F. X.

R. W. Fenn, S. A. Clough, W. O. Gallery, R. E. Good, F. X. Kneizys, J. D. Mill, L. S. Rothman, E. P. Shettle, F. E. Volz, “Optical and infrared properties of the atmosphere,” in Handbook of Geophysics and the Space Environment, A. S. Jursa, ed. (U.S. Air Force Geophysics Laboratory, Hanscom Air Force Base, Bedford, Mass., 1985), pp. 18:1–18:80.

F. X. Kneizys, E. P. Shettle, L. W. Abreu, J. H. Chetwynd, G. P. Anderson, W. O. Gallery, J. E. A. Selby, S. A. Clough, “User’s Guide to lowtran 7,” Rep. AFGL-TR-88-0177 (U.S. Air Force Geophysics Laboratory, Hanscom Air Force Base, Bedford, Mass., 1988).

Lamb, H. H.

H. H. Lamb, “Volcanic dust in the atmosphere; with a chronology and assessment of its meteorological significance,” Phil. Trans. R. Soc. London Ser. A 266, 425–533 (1970).
[CrossRef]

Lee, R. L.

For an overview of the entire CIE 1976 UCS diagram, see R. L. Lee, “Twilight and daytime colors of the clear sky,” Appl. Opt. 33, 4629–4638, 4959 (1994).

R. L. Lee, A. B. Fraser, The Rainbow Bridge: Rainbows in Art, Myth, and Science (Pennsylvania State U. Press, University Park, Pa., 2001), pp. 266–267.

Livingston, W.

D. K. Lynch, W. Livingston, Color and Light in Nature (Cambridge U. Press, Cambridge, 1995), p. 41.

Lynch, D. K.

D. K. Lynch, W. Livingston, Color and Light in Nature (Cambridge U. Press, Cambridge, 1995), p. 41.

Mateer, C. L.

J. V. Dave, C. L. Mateer, “The effect of stratospheric dust on the color of the twilight sky,” J. Geophys. Res. 73, 6897–6913 (1968).
[CrossRef]

Mill, J. D.

R. W. Fenn, S. A. Clough, W. O. Gallery, R. E. Good, F. X. Kneizys, J. D. Mill, L. S. Rothman, E. P. Shettle, F. E. Volz, “Optical and infrared properties of the atmosphere,” in Handbook of Geophysics and the Space Environment, A. S. Jursa, ed. (U.S. Air Force Geophysics Laboratory, Hanscom Air Force Base, Bedford, Mass., 1985), pp. 18:1–18:80.

Minnaert, M. G. J.

M. G. J. Minnaert, Light and Color in the Outdoors (Springer-Verlag, New York, 1993), pp. 296, 298, 303.

Neuberger, H.

H. Neuberger, Introduction to Physical Meteorology (Pennsylvania State U. Press, University Park, Pa., 1957), pp. 184–185, 190.

Plass, G. N.

C. N. Adams, G. N. Plass, G. W. Kattawar, “The influence of ozone and aerosols on the brightness and color of the twilight sky,” J. Atmos. Sci. 31, 1662–1674 (1974).
[CrossRef]

Riggenbach, A.

A. Riggenbach, Beobachtungen über die Dämmerung insbesondere über das Purpurlicht und seine Beziehungen zum Bishop’schen Sonnennring (Georg, Basel, Switzerland, 1886).

Rothman, L. S.

R. W. Fenn, S. A. Clough, W. O. Gallery, R. E. Good, F. X. Kneizys, J. D. Mill, L. S. Rothman, E. P. Shettle, F. E. Volz, “Optical and infrared properties of the atmosphere,” in Handbook of Geophysics and the Space Environment, A. S. Jursa, ed. (U.S. Air Force Geophysics Laboratory, Hanscom Air Force Base, Bedford, Mass., 1985), pp. 18:1–18:80.

Rozenberg, G. V.

G. V. Rozenberg, Twilight: A Study in Atmospheric Optics (Plenum, New York, 1966), p. 17. In purely astronomical terms, civil twilight is the period between sunset or sunrise and h0 = -6°; nautical twilight is the period when -6° > h0 > -12°.

Russell, F. A. R.

F. A. R. Russell, “Proximate physical cause of the unusual twilight glows in 1883–4,” in The Eruption of Krakatoa, and Subsequent Phenomena, G. J. Symons, ed. (Trübner, London, 1888), pp. 178–196.

F. A. R. Russell, “Previous analogous glow phenomena, and corresponding eruptions,” in The Eruption of Krakatoa, and Subsequent Phenomena, G. J. Symons, ed. (Trübner, London, 1888), pp. 384–405.

Selby, J. E. A.

F. X. Kneizys, E. P. Shettle, L. W. Abreu, J. H. Chetwynd, G. P. Anderson, W. O. Gallery, J. E. A. Selby, S. A. Clough, “User’s Guide to lowtran 7,” Rep. AFGL-TR-88-0177 (U.S. Air Force Geophysics Laboratory, Hanscom Air Force Base, Bedford, Mass., 1988).

Shaw, G. E.

G. E. Shaw, “Observations of two stratospheric dust events,” J. Appl. Meteorol. 14, 1619–1620 (1975).
[CrossRef]

Shettle, E. P.

F. X. Kneizys, E. P. Shettle, L. W. Abreu, J. H. Chetwynd, G. P. Anderson, W. O. Gallery, J. E. A. Selby, S. A. Clough, “User’s Guide to lowtran 7,” Rep. AFGL-TR-88-0177 (U.S. Air Force Geophysics Laboratory, Hanscom Air Force Base, Bedford, Mass., 1988).

R. W. Fenn, S. A. Clough, W. O. Gallery, R. E. Good, F. X. Kneizys, J. D. Mill, L. S. Rothman, E. P. Shettle, F. E. Volz, “Optical and infrared properties of the atmosphere,” in Handbook of Geophysics and the Space Environment, A. S. Jursa, ed. (U.S. Air Force Geophysics Laboratory, Hanscom Air Force Base, Bedford, Mass., 1985), pp. 18:1–18:80.

Simkin, T.

T. Simkin, R. S. Fiske, Krakatau 1883: The Volcanic Eruption and Its Effects (Smithsonian Institution Press, Washington, D.C., 1983), p. 418.

Stiles, W. S.

G. Wyszecki, W. S. Stiles, Color Science: Concepts and Methods, Quantitative Data and Formulae, 2nd ed. (Wiley, New York, 1982), pp. 306–310. Here we follow convention and set the JND equal to the semimajor axis length of the MacAdam color-matching ellipse at the given chromaticity.

Volz, F. E.

F. E. Volz, “Twilights and stratospheric dust before and after the Agung eruption,” Appl. Opt. 8, 2505–2517 (1969).
[CrossRef] [PubMed]

R. W. Fenn, S. A. Clough, W. O. Gallery, R. E. Good, F. X. Kneizys, J. D. Mill, L. S. Rothman, E. P. Shettle, F. E. Volz, “Optical and infrared properties of the atmosphere,” in Handbook of Geophysics and the Space Environment, A. S. Jursa, ed. (U.S. Air Force Geophysics Laboratory, Hanscom Air Force Base, Bedford, Mass., 1985), pp. 18:1–18:80.

Walker, J.

J. Walker, The Flying Circus of Physics with Answers (Wiley, New York, 1977), p. 274.

Wyszecki, G.

G. Wyszecki, W. S. Stiles, Color Science: Concepts and Methods, Quantitative Data and Formulae, 2nd ed. (Wiley, New York, 1982), pp. 306–310. Here we follow convention and set the JND equal to the semimajor axis length of the MacAdam color-matching ellipse at the given chromaticity.

Adv. Geophys. (1)

D. Deirmendjian, “On volcanic and other particulate turbidity anomalies,” Adv. Geophys. 16, 267–296 (1973).
[CrossRef]

Appl. Opt. (2)

For an overview of the entire CIE 1976 UCS diagram, see R. L. Lee, “Twilight and daytime colors of the clear sky,” Appl. Opt. 33, 4629–4638, 4959 (1994).

F. E. Volz, “Twilights and stratospheric dust before and after the Agung eruption,” Appl. Opt. 8, 2505–2517 (1969).
[CrossRef] [PubMed]

J. Appl. Meteorol. (1)

G. E. Shaw, “Observations of two stratospheric dust events,” J. Appl. Meteorol. 14, 1619–1620 (1975).
[CrossRef]

J. Atmos. Sci. (1)

C. N. Adams, G. N. Plass, G. W. Kattawar, “The influence of ozone and aerosols on the brightness and color of the twilight sky,” J. Atmos. Sci. 31, 1662–1674 (1974).
[CrossRef]

J. Geophys. Res. (1)

J. V. Dave, C. L. Mateer, “The effect of stratospheric dust on the color of the twilight sky,” J. Geophys. Res. 73, 6897–6913 (1968).
[CrossRef]

Mon. Weather Rev. (1)

A. Heim, “Explanations of the western purple light and the eastern afterglow (Nachglühen),” Mon. Weather Rev. 44, 624–625 (1916). Although we write “diffraction” here to match the terminology found in Heim’s and earlier accounts, nowadays we use both the language and the physics of single-scattering theories such as Mie theory in explaining the purple light.
[CrossRef]

Nature (2)

J. E. Clark, “The remarkable sunsets,” Nature 29, 130–131 (1883).
[CrossRef]

E. L. Deacon, “The second purple light,” Nature 178, 688 (1956).
[CrossRef]

Phil. Trans. R. Soc. London Ser. A (1)

H. H. Lamb, “Volcanic dust in the atmosphere; with a chronology and assessment of its meteorological significance,” Phil. Trans. R. Soc. London Ser. A 266, 425–533 (1970).
[CrossRef]

Other (25)

F. A. R. Russell, “Proximate physical cause of the unusual twilight glows in 1883–4,” in The Eruption of Krakatoa, and Subsequent Phenomena, G. J. Symons, ed. (Trübner, London, 1888), pp. 178–196.

A. Riggenbach, Beobachtungen über die Dämmerung insbesondere über das Purpurlicht und seine Beziehungen zum Bishop’schen Sonnennring (Georg, Basel, Switzerland, 1886).

P. Gruner, H. Kleinert, Die Dämmerungserscheinungen (Grand, Hamburg, Germany, 1927), pp. 103–107.

F. A. R. Russell, “Previous analogous glow phenomena, and corresponding eruptions,” in The Eruption of Krakatoa, and Subsequent Phenomena, G. J. Symons, ed. (Trübner, London, 1888), pp. 384–405.

G. Wyszecki, W. S. Stiles, Color Science: Concepts and Methods, Quantitative Data and Formulae, 2nd ed. (Wiley, New York, 1982), pp. 306–310. Here we follow convention and set the JND equal to the semimajor axis length of the MacAdam color-matching ellipse at the given chromaticity.

R. L. Lee, A. B. Fraser, The Rainbow Bridge: Rainbows in Art, Myth, and Science (Pennsylvania State U. Press, University Park, Pa., 2001), pp. 266–267.

During some volcanic twilights, the second purple light is vividly colored (hence its name) because increased concentrations of liquid aerosols and dust in the stratosphere scatter much more sunlight to the surface.

LI-1800 spectroradiometer from LI-COR, Inc., 4421 Superior Street, Lincoln, Neb. 68504-1327. According to LI-COR, at specified radiance levels a properly calibrated LI-1800 instrument measures visible-wavelength spectral radiances accurate to within ±5% and has a spectral accuracy of ±2 nm, and its CIE 1931 colorimetric errors are x < 0.003, y < 0.003 for primaries that are typical of a cathode-ray-tube color monitor.

The × to the right of Fig. 11’s 5-3-00 purity maximum occurred on 6-11-01. Although this × is more purplish than the 5-3-00 maximum, the 6-11-01 maximum is actually closer to the chromaticity of 5-2-00. Thus the largest chromaticity shift that we measured is from 5-2-00 to 5-3-00, even though Fig. 11’s anisotropic scaling suggests otherwise.

J. M. Russell, HALogen Occultation Experiment (HALOE) home page, http://haloedata.larc.nasa.gov/home.html , accessed 7May2002.

Note that 14.4–30.3 km is the largest altitude range for which HALOE data exist on all three dates in Fig. 14. Meteorological soundings on these dates from nearby Dulles International Airport have temperature inversions that place the local tropopause at altitudes of 12.3–15.7 km.

For example, see Ref. 5, Plate 5.

R. W. Fenn, S. A. Clough, W. O. Gallery, R. E. Good, F. X. Kneizys, J. D. Mill, L. S. Rothman, E. P. Shettle, F. E. Volz, “Optical and infrared properties of the atmosphere,” in Handbook of Geophysics and the Space Environment, A. S. Jursa, ed. (U.S. Air Force Geophysics Laboratory, Hanscom Air Force Base, Bedford, Mass., 1985), pp. 18:1–18:80.

F. X. Kneizys, E. P. Shettle, L. W. Abreu, J. H. Chetwynd, G. P. Anderson, W. O. Gallery, J. E. A. Selby, S. A. Clough, “User’s Guide to lowtran 7,” Rep. AFGL-TR-88-0177 (U.S. Air Force Geophysics Laboratory, Hanscom Air Force Base, Bedford, Mass., 1988).

Ref. 18, p. 488.

H. Neuberger, Introduction to Physical Meteorology (Pennsylvania State U. Press, University Park, Pa., 1957), pp. 184–185, 190.

J. Walker, The Flying Circus of Physics with Answers (Wiley, New York, 1977), p. 274.

M. G. J. Minnaert, Light and Color in the Outdoors (Springer-Verlag, New York, 1993), pp. 296, 298, 303.

D. K. Lynch, W. Livingston, Color and Light in Nature (Cambridge U. Press, Cambridge, 1995), p. 41.

K. Bullrich, Die farbigen Dämmerungserscheinungen (Birkhäuser, Basel, Switzerland, 1982), pp. 69–70.

T. Simkin, R. S. Fiske, Krakatau 1883: The Volcanic Eruption and Its Effects (Smithsonian Institution Press, Washington, D.C., 1983), p. 418.

T. S. Glickman, ed., Glossary of Meteorology, 2nd ed. (American Meteorological Society, Boston, Mass., 2000), p. 605.

The period during which we measured twilight spectra had no unusual volcanism, so our data are of ordinary rather than volcanic purple lights. Also, our measurements are confined to evening twilights (with one important exception), and strictly speaking so are our conclusions about the purple light.

PR-650 spectroradiometer from Photo Research, Inc., 9731 Topanga Canyon Place, Chatsworth, Calif. 91311. According to Photo Research, at specified radiance levels a properly calibrated PR-650 instrument measures luminance and radiance accurately to within ±4% and has a spectral accuracy of ±2 nm, and its CIE 1931 colorimetric errors are x < 0.001, y < 0.001 for a 2856-K blackbody (CIE standard illuminant A).

G. V. Rozenberg, Twilight: A Study in Atmospheric Optics (Plenum, New York, 1966), p. 17. In purely astronomical terms, civil twilight is the period between sunset or sunrise and h0 = -6°; nautical twilight is the period when -6° > h0 > -12°.

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Figures (21)

Fig. 1
Fig. 1

Photograph of an ordinary purple light taken during evening twilight at Chesapeake Beach, Maryland, on 23 February 1997 at 2250 UTC. Unrefracted solar elevation, h 0 = -0.39°; azimuth relative to the Sun, ϕrel = 0° at image center.

Fig. 2
Fig. 2

Fisheye photograph of an ordinary purple light taken during evening twilight at the USNA in Annapolis, Maryland, on 6 October 1997 at 2257 UTC. Solar elevation, h 0 = -4.07°; the photograph is centered on the zenith.

Fig. 3
Fig. 3

Fisheye photograph taken during evening twilight at the USNA on 2 October 1997 at 2302 UTC. Solar elevation, h 0 = -3.85°; the photograph is centered on the zenith. In contrast to Fig. 2’s twilight, this sky has no discernible purple light. Figures 1 3 were all photographed with the same type of color slide film, Kodak Elite II (ISO 100). Exposure times in Figs. 1 3 ranged from 0.008 to 0.5 s, so reciprocity failure and its possible color shifts are unlikely here.

Fig. 4
Fig. 4

Simplified scattering geometry for the purple light; atmospheric thickness and Sun elevation are exaggerated here. Before they reach the radiometer’s line of sight, parallel rays of sunlight are reddened to varying degrees by transmission through the troposphere or stratosphere.

Fig. 5
Fig. 5

Portion of the CIE 1976 UCS diagram, showing temporal trends in purple-light chromaticities measured during evening twilights at Owings, Maryland (29 December 1999), and Granada, Spain (7 June 2000). Each spectroradiometer’s FOV is 1°, and throughout these measurements its view elevation is h = 20° and its relative azimuth is ϕrel = 0°. The cross labeled “ET sunlight” is the chromaticity of sunlight measured above the atmosphere.

Fig. 6
Fig. 6

Purple-light luminance and integrated radiance as functions of h 0 for the 29 December 1999 Owings evening twilight whose chromaticities are shown in Fig. 5.

Fig. 7
Fig. 7

Temporal trends in purple-light chromaticities measured during adjacent evening (5 September 1998) and morning (6 September 1998) twilights at a rural site near Marion Center, Pennsylvania. Unlike those in Fig. 5, these chromaticities are derived from horizontal irradiances.

Fig. 8
Fig. 8

Temporal trends in evening twilight chromaticities measured at the zenith at Owings, Maryland, on 2, 21, and 23 January 2001. Although the 21 January twilight has a distinct purple light, two days later there is none.

Fig. 9
Fig. 9

Temporal trends in evening purple-light chromaticities at Granada, Spain, on 7 June 2000 for horizontal daylight irradiances and for skylight radiances at h = 20°, 45° and ϕrel = 0°.

Fig. 10
Fig. 10

Comparison of temporal trends in evening twilight chromaticities at Granada on 7–8 June 2000 (h = 20°, ϕrel = 0°). Note that the vivid purple light of 7 June largely disappears by the next day.

Fig. 11
Fig. 11

Variability of 35 purple-light maxima at three of our observing sites. Complete temporal chromaticity curves are drawn for Owings, Maryland, on 2–3 May 2000, two adjacent days with the largest colorimetric difference in purple-light maxima measured from December 1999 to November 2001.

Fig. 12
Fig. 12

Visible-wavelength spectral radiances for a typical purple-light maximum measured at Owings on 24 May 2000 when h 0 = -4.31° compared with those of sunlight above the atmosphere (normalized to have the same sum as the purple-light spectrum). Of the 35 purple-light maxima plotted in Fig. 11, this purple-light spectrum has the chromaticity that is closest to the mean u′, v′.

Fig. 13
Fig. 13

Temporal trends in evening twilight chromaticities at the USNA (12 November 1998) and at Owings (29 and 31 December 1999). ∑β is the total column amount of stratospheric aerosol measured above these sites (see Fig. 14 for details).

Fig. 14
Fig. 14

HALOE limb-sounding satellite measurements of the vertical distribution of normalized aerosol surface area β (square micrometers per cubic centimeter) in the stratosphere, interpolated to the latitudes and longitudes of the USNA (on 12 November 1998) and of Owings (on 29 and 31 December 1999). Compare the total column amounts ∑β of these scatterers with twilight colors on the same dates in Fig. 13.

Fig. 15
Fig. 15

Photograph of a purplish twilight crepuscular ray at Owings on 1 January 2002 at 2222 UTC (h 0 = -5.43°). White lines have been drawn on the sky to help to locate this low-contrast ray.

Fig. 16
Fig. 16

Visible- and infrared-wavelength geostationary satellite images of the United States (a), (b) on the afternoons of 29 December 1999 (visible wavelengths) and 31 December 1999 (visible), respectively, and (c) in the early evening of 12 November 1998 (infrared). Red arrows point from our observing sites toward the Sun’s azimuth at h 0 = -4.5°. Clouds far beyond our sunset horizon on 12 November eliminated the purple light then (see Fig. 13).

Fig. 17
Fig. 17

Radiative transfer model lowtran 7’s calculations of temporal chromaticity trends for ordinary purple light as functions of h 0 and meteorological range V at h = 20° and ϕrel = 0°. For comparison, we also show a chromaticity curve calculated for a volcanic twilight with high levels of fresh volcanic scatterers in the stratosphere. Note the similarities between the V = 23 km curve (our lowtran reference case) and the purple-light chromaticities measured in Figs. 5 and 13.

Fig. 18
Fig. 18

Differential and path-integrated skylight colors predicted by lowtran 7 for Fig. 17’s reference case at h 0 = -4° and along the line of sight h = 20° and ϕrel = 0°. In this figure only, the pluses at which the arrows point denote particular tangent-ray altitudes. For comparison, we also show the entire surface-based twilight curve for the reference case shown in Fig. 17.

Fig. 19
Fig. 19

lowtran 7 calculations of the h 0 dependence of the stratospheric and tropospheric components of skylight radiances received at the Earth’s surface. The model atmosphere here is Fig. 17’s reference case, and the observer’s line of sight is h = 20° and ϕrel = 0°.

Fig. 20
Fig. 20

lowtran 7 calculations of the effects of subvisual cirrus on purple-light chromaticity trends, including the formation of a chromaticity cusp near sunset. The uniform cirrus layer’s base is at 10 km, and its normal optical thickness, τ, is 0.02.

Fig. 21
Fig. 21

Temporal trends in the colors of twilight irradiances illuminating a vertical plane that faces the Sun, measured at the USNA on 21 September and 12 October 2000. When h 0< 1°–2°, the irradiance from skylight begins to exceed that from direct sunlight, thus causing scattering by tropospheric scatterers to become increasingly bluish then.

Tables (2)

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Table 1 Geographic Details of Our Measurement Sites

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Table 2 Summary Statistics for Purple-Light Geometry, 12-29-99 to 11-15-01a

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