1. Observations, Frequency & Linkage to Climate Change Jenell Walsh-Thomas CSI 655 Atmospheric Physics Monday, May 16, 2011 1 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

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

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? 3

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 4

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 5 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 6 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 7

8. Linkage to Climate Change  Two major greenhouse gases: CO 2 & CH 4  CO 2 : causes cooling  CH 4 : dissociation occurs, adds H 2 O 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 8

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 9

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