Green Thunderstorms

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Green Thunderstorms

• (much of this material is courtesy of Dr. Frank
  Gallagher)

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Introduction

• What is a green thunderstorm?
• A thunderstorm that appears green in color or
  contains hues of green
• What is so unusual about a green thunderstorm?
• Many people associate the greenish color with the
  imminent likelihood of ominous weather
• Green thunderstorms are not an everyday
  occurrence they happen rarely enough to evoke an
  interest in the observer
• When the storm becomes green, observers often
  comment that the storm has become brighter

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History and anecdotal evidence

• Fitzroy (1898)when a sky is of a sickly-looking
  greenish hue, wind or rain may be expected
• Turneramong experienced seamen in the China
  Seas, a green sky is regarded as the forerunner
  of a typhoon
• Smythe (1929)if the dawn is green, one may,
  without any other reason or data, confidently
  forecast bad weather within 24 hours
• Pick (1930)studied 42 non-convective green sky
  events

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History and anecdotal evidence

• John Steuart Curry (1929)Tornado Over Kansas
• Fuller (1987)Tornado Watch 211Then, suddenly,
  the breeze died and a rather ominous still calm
  fell over the landscape. There also was that
  strange yellow sky.
• Ozaukee (Wisconsin) Press (1991)the sky turned
  an omonious shade of green

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History and anecdotal evidence

• Edwards (1995)all around, the gray sky is
  turning a glowing emerald green.
• The Weather Channel Desk Calendat (1994)during
  severe thunderstorms, a definite green tint in
  the dark clouds is often a sign of nearby hail
  and perhaps tornadoes. A satisfactory
  explanation has yet to be agreed upon by
  meteorologists.

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Why might we care?

• This phenomenon has been reported by many, but,
  until recently, there has been no scientific
  explanation
• It is possible that we can improve the
  measurement of storm parameters such as liquid
  water content, hydrometeor size distribution,
  etc., using colorimetric analysis

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red
orange
yellow
green
blue
violet
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red
orange
yellow
green
blue
violet
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red
orange
yellow
green
blue
violet
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red
orange
yellow
green
blue
violet
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Observations

• Green thunderstorms exist but they are not all
  the same color
• Subtle shifts in the spectrum may result in
  dramatic changes in perceived color
• What causes the green color?

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Prevailing hypotheses and anecdotal explanations

• Ground reflection
• Fraser (1978, 1993)
• Bohren (1993)

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Ground reflection (Freier 1992)

• These storm clouds are very tall, dense clouds,
  so that absolutely no direct sunlight gets
  through. Everything gets very dark, but there
  must be some light because you can still see. I
  think that most of this dim light is reflected
  from green foliage on the earths surface to the
  cloudPlants absorb red and blue light, and
  reflect any green light. When the light is dim,
  they also orient their leaves and chloroplasts in
  the leaves to reflect additional green light.
  Reflected green light becomes the primary source
  of illumination for seeing the sky with its dark
  colors.

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Ground reflection issues

• Why are not all thunderstorms green when passing
  over a large region of green vegetation?
• Green thunderstorms have been observed in regions
  where the vegetation was anything but green
  (e.g., west TX), and from aircraft as well.
• Spectral measurements of the color of green
  thunderstorms do not support this hypothesis
  either (the spectral maxima of the green light
  from the ground and the green light from
  thunderstorms do not coincide).

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ordinary blue tstm green foliage green tstm
green pasture green tstm blue tstm
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Frasers hypothesis

• Green light does not emanate from the
  thunderstorm itself
• Instead, the storm produces a dark backdrop
  against which green airlight (light scattered by
  the atmosphere) is seen
• e.g., during the day, a black object at a
  distance doesnt appear black b/c light scattered
  by intervening air will cause the object to
  appear brighter than it actually is
• If we assume that the airlight is sunlight
  scattered by molecules that are small compared to
  the wavelength of light, the color of the
  scattered light will be dominated by shades of
  blue
• if we add to this light another source of light
  that is predominantly red, such as light from the
  setting sun, the airlight can be perceived as
  various shades of green

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Frasers hypothesis

• What if a wide angle of the sky appears green,
  rather than just the sky in the direction of the
  setting sun?
• What if the sun angle is not low?

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Bohren hypothesis

• Green light also does not emanate from the storm
  itself
• Since absorption of light by water (either in
  solid or liquid form) is a minimum in the
  wavelengths of light that correspond to a blue
  color and rises steadily with increasing
  wavelength, any sunlight that passes through a
  significant distance of cloud hydrometeors will
  take on a bluish cast
• Late in the day, sunlight is enriched in the
  longer wavelength (redder) light because a large
  portion of the shorter wavelength (bluer) light
  is scattered out
• When this reddened light illuminates an object
  with an absorption minimum in the blue, the
  result can be perceptually green transmitted
  light

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Bohren hypothesis
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Bohren hypothesis

• What if the sun angle is not low?

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Summary

• Spectral measurements prove that some
  thunderstorms are green indeed.
• When a thunderstorm appears green, it actually
  has a lower radiance, which conflicts with what
  observers describe as a brighter storm.
• Greenness can be observed for periods as long
  as an hour or more.
• Green light can be formed from the mixture of
  reddish light from the sun and the absorptive
  properties of water.

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Summary (continued)

• Not all green thunderstorms have the same
  spectrathere are many spectra that lead to the
  perception of green
• For a given LWC, smaller drops are more likely to
  cause blue-green storms, whereas larger drops
  more likely cause yellow-green storms large
  hydrometeors, such as hail, are not need to
  create green light
• Possibly all cumulonimbus clouds turn green at
  some point in their development, but one needs to
  be in the right place at the right time to see it

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Further reading

• Bohren, C. F., Fraser, A. B. 1993 Green
  thunderstorms. Bull. Amer. Meteor. Soc., 74,
  2185-2194.
• Gallagher, F., W. H. Beasley, and C. F. Bohren,
  1996 Green thunderstorms observed. Bull. Amer.
  Meteor. Soc., 77, 2889-2897.
• Gallagher, F. W., 2000 Distant green
  thunderstorms Frasers theory revisited. J.
  Appl. Meteor., 39, 1754- 1761.
• Gallagher, F. W., 2001 Ground reflections and
  green thunderstorms. J. Appl. Meteor., 40,
  776-782.