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Noctilucent Clouds

Observations, Frequency & Linkage to Climate Change. Noctilucent Clouds. Noctilucent Clouds taken from 36,000 feet above the South of Nunivak Island, Alaska on June 20, 2006. Image credit: John Boardman. http://www.nasa.gov/mission_pages/aim/multimedia/nocticulant_clouds.html.

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Noctilucent Clouds

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  1. Observations, Frequency & Linkage to Climate Change Noctilucent Clouds Noctilucent Clouds taken from 36,000 feet above the South of Nunivak Island, Alaska on June 20, 2006. Image credit: John Boardman. http://www.nasa.gov/mission_pages/aim/multimedia/nocticulant_clouds.html Jenell Walsh-Thomas CSI 655 Atmospheric Physics Monday, May 16, 2011

  2. Presentation Outline • Introduction • What are noctilucent clouds? • Observations • Location, formation & properties • Observation techniques • Occurrence & observation frequency • Linkage to Climate Change • Conclusion

  3. Introduction • Purpose: develop an understanding of noctilucent clouds (NLC) • First reported June 8, 1885 • Timeframe when industrial revolution began…coincidence? • Increased frequency of occurrence/observation • Indicator of global change in the middle atmosphere? • Linkage to climatic changes?

  4. What are noctilucent clouds? • Clouds that form at very high altitudes: 82 km • Primarily composed of microscopic ice crystals • Seen at twilight & glow a white to bluish color • NLC vs. (high) Cirrus clouds • Peak observations around summer solstice • NLCs/PMCs; “night shining clouds” • Atmospheric historical importance

  5. Formation & Properties • Formation conditions • Atmospheric environment: extremely low T at mesopause, very low P, high amount of water vapor, presence of condensation nuclei • Time of day: twilight • Size & shape of particles • Average radius 150 nm • Actual shape speculated to be non-spherical, possibly needle or cubic • Cloud shape & form: veil, bands, waves & whirls • Bluish-white in color with occasional red edge Summary of average properties of NLCs (Kokhanovsky 2005).

  6. Observation Techniques • Ground-based (NLCs) • Direct & photographic • Space-based (PMCs) • 2 important first observations • Conclusions from space: • Asymmetrical in N & S hemispheres • S hemisphere contain smaller particles • AIM Geometry of observing NLCs (Avaste 1993).

  7. Occurrence & Observation Frequency • Typical occurrence: around the summer solstice at twilight • First observed in 1885 • Provide insight into conditions of mesosphere • Asymmetrical observation numbers between N & S hemisphere • Determining frequency • Increase in frequency & brightness

  8. Linkage to Climate Change • Two major greenhouse gases: CO2 & CH4 • CO2: causes cooling • CH4: dissociation occurs, adds H2O vapor • Also influencing increase in brightness • Indicators of change in upper atmosphere  indicator of future change in lower layers & at mid-latitudes? • July 2009 mid-latitude outbreak event

  9. Conclusion • NLC/PMC rare & exquisite site for observers • Much more to be learned about their properties, formation & possible foreshadowing for future changes • Limited time & geographic range • Both ground & space-based improving • AIM satellite mission • Standard need for observations

  10. Questions? Comments? Noctilucent cloud (NLC), polar mesospheric cloud (PMC) or "night-shining" cloud (Russell et al 2009). (Photographed by Tom Eklund, July 28, 2001, Valkeakoski, Finland.)

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