Noctilucent Clouds: What Are They and Where Do They Come From

Noctilucent clouds appear after sunset and are frozen fingers of water crystals and dust--clouds--that creep across the upper atmosphere of both geophysical poles during polar summers. But, the mesosphere, where noctilucent clouds form, is 100,000 times drier than the Sahara Desert and is colder than -250 Celsius. And there is no naturally occurring dust or water in the mesosphere.

Well, at least there wasn't until 1883 when Krakatoa erupted sinking islands, covering the earth with ash, and spewing gasses and dust all the up to the edge of space into the mesosphere. The first noctilucent cloud, sometimes referred as a NLC and more recently as a PMC (Polar Mesospheric Cloud), was seen two years after the eruption by a German named T. W. Backhouse. He was the first to notice that about 30 minutes after darkness fell a bright blue cloud could be seen shinning above the western horizon.

Scientists had been content to attribute the "night shinning" or noctilucent clouds to an atmospheric effect of Krakatoa's eruption especially because sunsets the world over were spectacularly enhanced by Krakatoa's ash for years afterward. As the debris from Krakatoa died down so did the sunsets, but the NLCs continued on.

Every year since then, the NLCs, which are currently scientifically referred to as PMCs, spread further south and grow in brightness. Today, they are seen in the northern hemisphere as far down as Colorado, Utah, and Virginia. In the southern hemisphere they are routinely seen during the southern summer over Australia and similar latitudes. In the 20th century, you had to go to northern Russia or Scandinavia to see PMCs.

And as they have spread southward, they have grown amazingly bright and thickly spread. This Russian Web site shows a gallery of PMCs that are really astounding; the noctilucent clouds look to me exactly like heavy frost growing up a window pane: http://tunguska.ru/fotog/nc/ [Note: I believe the next to the last may actually be a painting, but the others are photographs.] The images span many decades, with images taken as early as 1889 and as late as 2000 [some show no date]. Click on the images to see how amazing the NLCs are. The fifth one, which was taken in 2000, is my favorite.

So why did PMCs continue to exist? And why are they spreading and brightening? Well, that is what no one knows. Although in a few years there may be some concrete answers to these questions, as well as some others. On April 25, 2007 NASA and its partner Hamilton University launched the Aeronomy of Ice in the Mesosphere (AIM) to collect data from the mesosphere and from PMCs. AIM will collect and measure such things as cosmic dust, methane gas, and water molecules.

Earlier scientific studies provided some insights into the current sources of water and dust that are needed to form clouds. In 1999 researchers used a lidar system (radar-like laser) to obtain a temperature profile of the mesosphere by measuring the amount of laser light reflected from mesospheric air molecules. You can read about that study here: http://www.agu.org/sci_soc/prrl/prrl0108.html.

Another study used the Middle Atmosphere High Resolution Spectrograph Investigation instrument, called MAHRSI, to determine that one current source of water in the mesosphere is the exhaust, which is 97% water, from the NASA Space Shuttle. The Office of Naval Research used MAHRSI to track a plume of shuttle exhaust. The plume took a little over a day to reach the north pole from the launch latitude. You can read about this study here: http://www.nrl.navy.mil/pressRelease.php?Y=2003&R=35-03r, and here:http://www.nasa.gov/centers/goddard/news/topstory/2003/0522shuttleshine.html.

But this certainly doesn't explain the existence of noctilucent clouds between 1885 and the first rocket or shuttle launches. So, the night shinning noctilucent clouds remain a mystery. Many scientists are coming to believe that noctilucent clouds are a manifestation of how the planetary and atmospheric systems have been changed by industrial wastes, like methane and others. It is a fact that over the last 50 years noctilucent clouds have become farther spread and brighter. As you contemplate this, you can hear the AIM mission theme song by the Chromatics here: http://aim.hamptonu.edu/outreach/3-4audio.html. It is GREAT! Give it a listen.

You can see noctilucent clouds for yourself over the summer if you live at or north of 40 degrees North, abreast of Colorado, or if you live down-under around Australia. Go out after sunset, when the Sun is 6 degrees to 16 degrees below the horizon and look west. Look for wispy electric blue tendrils that look like they are in space. regular clouds tend to get blurry when magnified in binoculars, by noctilucent clouds get more distinctly detailed. Enjoy and after the two year AIM mission, maybe we'll know about NLCs/PMCs: night shinning noctilucent clouds.

For further reading:

http://www.nasa.gov/centers/goddard/news/topstory/2003/0522shuttleshine.html

http://www.nrl.navy.mil/pressRelease.php?Y=2003&R=35-03r

http://www.agu.org/sci_soc/prrl/prrl0108.html

http://science.nasa.gov/headlines/y2003/19feb_nlc.htm