Q. On a couple of recent nights I’ve photographed noctilucent clouds. I have a couple of questions about them:
Why are they sporadic, i.e. why don’t we see them every night?
Why/when do they extend into the lower latitudes?
Noctilucent clouds (“night shining clouds”) are the highest clouds in the Earth’s atmosphere, located at an altitude of about 50 miles. Most of the time, they are too faint to be seen, or are blocked by other clouds. These clouds are visible only when they are illuminated by sunlight some time after sunset. At these times, the lower layers of the atmosphere are in shadow and only the high altitudes experience the sun’s illumination. We cannot see them every night because the conditions that allow them to be seen are very specific and rather uncommon.
Noctilucent clouds are composed of tiny crystals of water ice, with water collecting on the surface of dust particles. The dust is believed to come from tiny meteors, although volcanoes and dust from the troposphere may also contribute. The moisture probably originates from lower elevations, but may also be a result of chemical reactions involving methane. Manmade sources are a possibility as well: water vapor from the exhaust of the Space Shuttle has been known to generate small high-altitude clouds.
The ice crystals which comprise noctilucent clouds can only form at temperatures below about 180 degrees F below zero. This means that they form primarily during summer when, surprisingly, the upper atmosphere is coldest. Noctilucent clouds form mostly near the poles where the upper atmosphere is coldest; only rarely are temperatures at low latitudes cold enough for them to form.
The first known sighting of noctilucent clouds dates to 1885, two years after the 1883 eruption of Krakatoa. No one knows whether this resulted from unique effects of the volcano, or whether the discovery was due to more people observing the legendary sunsets caused by the volcanic dust in the atmosphere. It was assumed that the clouds were simply a result of volcanic ash; however, after the dust had settled out of the atmosphere, the noctilucent clouds remained.
The first scientist to study these clouds extensively was Otto Jesse of Germany, who was the first to photograph them (in 1887), and seems to have been the one to coin the term “noctilucent cloud.” After making detailed observations of the sunsets for several years following the Krakatoa eruption, Jesse concluded that the clouds were a new phenomenon. During this research the height of the clouds was first determined, via triangulation. The project was continued until 1896.
On April 25, 2007, the AIM satellite (Aeronomy of Ice in the Mesosphere) was launched. AIM is the first satellite dedicated to studying noctilucent clouds; its first observations on May 25, 2007. Images taken by the satellite show shapes in the clouds that are similar to shapes in lower atmosphere clouds, hinting at similarities in their formation and composition.
On August 28, 2006, scientists with the Mars Express mission announced that they found clouds of carbon dioxide crystals over Mars that extended more than 60 miles above the surface. They are the highest clouds discovered over the surface of a planet. Like noctilucent clouds on Earth, they can only be observed when the Sun is below the horizon.
Some beautiful images of noctilucent clouds are available online. I suggest you visit Google Images (images.google.com) and search for “noctilucent clouds.” Wikipedia also has a nice writeup, from which some of the information here was obtained.