The Naked Scientists

The Naked Scientists Forum

Author Topic: Why are Noctilucent Clouds being more frequently seen in Northern latitudes?  (Read 6401 times)

Offline FuzzyUK

  • Sr. Member
  • ****
  • Posts: 206
    • View Profile
    • http://www.fusniak.co.uk
I took these pictures in the City of Cambridge, UK at 11pm on the evening of 9 July 2010, and
the following morning at 3 am (i.e. 2 hours after sunset and 2 hours before sunrise) of a
magnificent display of Noctilucent clouds. The orange tint on houses is from sodium street lights.
[Canon 400D, ISO 200, F5.6, 4 second exposures]





There is speculation using a new forecasting method that such clouds may be visible again
tonight (19 July) in Northern latitudes.

Enjoy the BBC presentation 'Audio slideshow: Noctilucent clouds':
http://www.bbc.co.uk/news/science-environment-10635796
« Last Edit: 20/07/2010 16:09:12 by FuzzyUK »


 

Offline RD

  • Neilep Level Member
  • ******
  • Posts: 8128
  • Thanked: 53 times
    • View Profile
I don't know, but I know of a song about them ...
:)
 

Offline frethack

  • Sr. Member
  • ****
  • Posts: 394
    • View Profile
Ive just returned from out of town (my honeymoon  ;D ) and am ready for bed, so Ill have to do a little more research on this tomorrow.

My understanding is that they are a relatively recent phenomenon...maybe the late 19th century?  They coincide with the Modern Thermal Maximum (and I would venture speculation that they were also seen during the Medieval Warm Period), and are mesospheric clouds that are illuminated as the sun dips below the horizon.  As the atmosphere increases in its capacity to hold water vapor, the mesosphere gains enough vapor to form ice crystals and clouds.  Im not sure why this effect is amplified in the northern latitudes, but Ill see what I can find on the subject tomorrow.

 

Offline imatfaal

  • Neilep Level Member
  • ******
  • Posts: 2787
  • rouge moderator
    • View Profile
Congratulations Mr and Mrs Frethack. 

On the question - could it just be that this is viewed from a UK perspective and that it is easier to see any night time phenomena away from the enormous light pollution that is london, the south east and the midlands (although thats a great photo from Cambridge).  The man observing for Material World does it from Anglesey - far away from the great metropolitan light sources (I guess nearest big city is Dublin, perhaps Liverpool).  I have been keeping an eye out ever since I heard the first broadcast with no luck - but then I can almost read in my back garden on a cloudy night due to the reflected street lights.

Matthew
 

Offline Mazurka

  • Hero Member
  • *****
  • Posts: 510
    • View Profile
Orientation of the Earths axis.

Noctilucent clouds are the highest clouds observed and are normally invisible - so poorly understood.  They become visible when light from below (i.e. whent he sun is just below the horizon)which (over the UK) occurs around the summer solstice.

They were first described backend of the 19thC, when they became more obvious.  This is thought to relate to volcanic aerosol from Krakatoa.  Although they are probably more common now due to mankinds activities there are no physical reasons to assume they did not exisit prior to this.  Given how hard they are to spot (even when there are good conditions for observation they are (frustratingly) not always there) it is that much of a suprise that they were not described earlier.  For a bit of context, the describtions of clouds we routinely use was devised at the begining of the 19thC.
 

Offline frethack

  • Sr. Member
  • ****
  • Posts: 394
    • View Profile
Sorry it has taken so long for me to reply.

From Robert et al. 2010
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, D00I12, doi:10.1029/2009JD012359, 2010

If you would like to read the entire paper, and cant download it, I can post a link to the PDF here.

Quote
1. Introduction

[2]   Noctilucent clouds (NLC), also known as polar mesospheric clouds (PMC), are atmospheric phenomena usually occurring poleward of 50° during the summer season. The extremely low temperatures (~130 K) and enhanced water vapor content (~3 ppm) near the summer mesopause lead to large saturation ratios (~100) and consequently to the formation of water ice aerosols [Hervig et al., 2001]. It is still not clear what preexisting cores act as nuclei for the formation of NLC [Rapp and Thomas, 2006; Gumbel and Megner, 2009], if indeed heterogenous nucleation is required [Zasetsky et al., 2009]. The NLC particles usually grow up to typical sizes of several tens of nanometers [Gumbel et al., 2001; Karlsson and Rapp, 2006; Robert et al., 2009] by direct deposition of water vapor on their surface. An ensemble of such particles forms a layer of roughly 1 km in vertical extent at altitudes of about 83 km [Fiedler et al., 2003] and has an ice particle density on the order of 102 cm−3 at the cloud peak brightness [Baumgarten et al., 2007]. NLC particles are transported by winds and settle under the action of gravity, and eventually sublimate when reaching subsaturated regions. A typical NLC season lasts from beginning of June until the end of August in the Northern Hemisphere and from beginning of December until the end of February in the Southern Hemisphere [Thomas and Olivero, 1989]. NLC are considered to be indicators of the state of the mesosphere [Thomas and Olivero, 2001], and observing their temporal and spatial variation can tell us more about phenomena taking place in this remote region of the Earth's atmosphere.

[3]   NLC are affected by many different atmospheric processes such as gravity waves [e.g., Gerrard et al., 2004; Chandran et al., 2009], planetary waves [e.g., Merkel et al., 2003; von Savigny et al., 2007; Merkel et al., 2008] and interhemispheric coupling [Karlsson et al., 2007, 2009]. The SBUV instruments on board various NOAA satellites have been an invaluable source of information, providing NLC properties since 1979. As a result of this lengthy data record, it was possible to assess the impact of the 11 year solar cycle on NLC activity, showing significant anticorrelation between the occurrence frequency and the Lyman-α irradiance in both hemispheres [DeLand et al., 2003], with a stronger anticorrelation in the Northern Hemisphere. Hervig and Siskind [2006] confirmed these findings using the HALOE instrument data set and showed the variation in NLC properties to be a consequence of temperature and water vapor changes in the upper mesosphere. Moreover, once the effect of the solar activity is removed from the SBUV time series, a positive secular trend of up to 20% was observed over the last 27 years in both NLC occurrence and albedo [Shettle et al., 2009; DeLand et al., 2007]. It has been argued that this long-term change in NLC properties could be caused by an enhanced radiative cooling of the upper atmosphere due to a rise in greenhouse gas concentration as well as an increase in mesospheric water vapor concentrations [Olivero and Thomas, 2001; Grygalashvyly and Sonnemann, 2006], although most measurements do not support the hypothesis of a significant long-term temperature decrease near the mesopause [Lübken, 2000; Beig et al., 2003].

[4]   While the effect of the 11 year solar cycle on NLC is fairly well established, there is no peer-reviewed publication on the consequences of the quasi 27 day variation of the solar irradiance on NLC properties. This short-term UV flux variability, the result of the Sun's differential rotation, has a mean amplitude at 121.6 nm which corresponds to about 25% that of the 11 year solar cycle [Woods et al., 2000], and varies considerably with solar activity itself so that it is larger during solar maximum and vice versa. Because the magnitude of this signal is not negligible compared to that of the 11 year solar cycle, it could conceivably affect the state of the upper mesosphere and consequently, the formation of NLC.

[5]   The difficulty of the task lies in the detection of this signal during the short NLC season. If one conserves only the season's core containing approximately 90% of the total NLC detections, about 70 days are left to detect a 27 day signal. Moreover, as described before, many other processes with no direct link to the solar irradiance impact NLC, and so the search for a connection between NLC and solar activity will invariably be affected by these. The proxy for NLC activity must therefore be chosen with care so as to reflect best the effect that the solar radiation could have on NLC and maximize the population sampled. Among the many alternatives of possible NLC properties, the daily occurrence frequency, averaged zonally and over a latitude range of 60°–80° was chosen as a good indicator of NLC activity. It has the advantage of being simple, easily retrieved with few assumptions made and available from both SCIAMACHY and SBUV. The NLC albedo, used in many SBUV studies [DeLand et al., 2007] could be employed as well, but because it is strongly correlated to the occurrence frequency for SBUV data, the conclusions drawn from a cross-correlation analysis based on either parameter should be similar. Moreover, SCIAMACHY albedo computation would require making assumptions on the particle size distribution of the NLC and would be quite sensitive to these assumptions because of the limb-viewing geometry. Other possible proxies of NLC activity would be the brightness peak altitude, the particle size and ice water content, which have the advantage of being real physical properties of NLC but usually require more assumptions to be made or cannot be measured accurately by either SCIAMACHY or SBUV.

[6]   In this work, we investigate the effect of the 27 day solar flux variation on NLC occurrence frequency in both hemispheres using NLC data sets from the SCIAMACHY and SBUV instruments. Cross-correlation plots of solar Lyman-α irradiance and NLC occurrence frequency anomalies are presented for years 2002–2009 for SCIAMACHY and 1979–2006 for SBUV. Results obtained through a superposed epoch analysis of the solar forcing on NLC occurrence frequency are also shown in order to substantiate the relationship between these geophysical parameters. In connection with this analysis, we also examine MLS Aura mesospheric temperature and H2O volume mixing ratios for the summer seasons 2005–2007.
 

The Naked Scientists Forum


 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums