Noctilucent Clouds

The first Noctilucent Clouds of 2009 have been seen over northern Europe. On May 29th, photographers recorded wispy electric-blue tendrils spreading across the twilight skies of Denmark, Northern Ireland and Scotland. This follows a similar display over Russia on May 27th. These sightings signal the beginning of the 2009 Noctilucent Cloud season, which is expected to last until late July. Early-season clouds are usually feeble, but these were fairly bright and vibrant, suggesting that even better displays are in the offing.

Noctilucent Clouds are still an unsolved puzzle. They float 83 km above Earth's surface at the edge of space itself. People first noticed NLCs in the late 19th century. In those days you had to travel to high northern latitudes to see them. In recent years, however, the clouds have been sighted in the United States as far south as Oregon, Washington and even Colorado. Climate change, space dust, and rocket launches have all been cited as possible explanations for the phenomenon. Interestingly, low solar activity seems to promote the clouds, so the ongoing deep solar minimum could set the stage for a good season in 2009.

The best time to look for Noctilucent Clouds is just after sunset or just before sunrise when the sun is between 6 and 16 degrees below the horizon. That's when the geometry is just right for sunlight to illuminate the tiny ice crystals that make up the clouds.

To spot them, look west 30 to 60 minutes after sunset when the Sun has dipped 6 to 16 degrees below the horizon. If you see luminous blue-white tendrils spreading across the sky, you've probably spotted a noctilucent cloud.


The optimum viewing geometry for noctilucent clouds. Sunlight scattered by tiny ice crystals in NLCs is what gives the clouds their characteristic blue colour.

Noctilucent Cloud Primer:

Noctilucent clouds are tenuous cloud-like phenomena that are the "ragged-edge" of a much brighter and pervasive polar cloud layer called polar mesospheric clouds in the upper atmosphere, visible in a deep twilight. They are made of crystals of water ice. The name means roughly night shining in Latin. They are most commonly observed in the summer months at latitudes between 50 and 70 degrees north and south of the equator.

These are the highest clouds in the Earth's atmosphere, located in the mesosphere at altitudes of around 76 to 85 kilometers (47 to 53 miles). The clouds are composed of tiny crystals of water ice 40 to 100 nanometers in diameter. Much like the more familiar lower altitude clouds, noctilucent clouds are formed from water collecting on the surface of dust particles. The origins of both the dust and the water vapour in the upper atmosphere are not known with certainty. The dust is believed to come from micrometeors, although volcanoes and dust from the troposphere are also possibilities. The moisture could be lifted through gaps in the tropopause, as well as forming from the reaction of methane with hydroxyl radicals in the stratosphere.

Normally, Noctilucent clouds are too faint to be seen, but are visible when illuminated by sunlight from below the horizon while the lower layers of the atmosphere are in the Earth's shadow. Noctilucent clouds are not fully understood and are a recently discovered meteorological phenomenon; were first observed in 1885, two years after the 1883 eruption of Krakatoa. It's not clear whether their appearance had anything to do with the volcano, or whether their discovery was due to more people watching the spectacular sunsets that resulted from the volcanic dist thrown into the atmosphere.

Noctilucent clouds can form only under very restrictive conditions; their occurrence can be used as a sensitive guide to changes in the upper atmosphere. Since their discovery the occurrence of noctilucent clouds has been increasing in frequency, brightness and extent. It is theorised that this increase is connected to climate change.

The clouds are generally colourless or pale blue but, occasionally, other colours including red and green can be seen. The characteristic blue colour is the result of ozone absorbing the other colours from the sunlight falling on the cloud. Clouds can appear as featureless bands but frequently show distinctive patterns such as streaks, wave-like undulations and whirls.

Noctilucent clouds can be confused with cirrus clouds. However, they appear sharper under magnification. Clouds caused by rocket exhausts tend to show colours other than silver or blue because of iridescence caused by the uniform size of the water droplets produced.

While the clouds are generally seen only in latitudes between 50 and 70 degrees north or south, they appear from mid-May to mid-August in the northern hemisphere and between mid-November and mid-February in the southern hemisphere. They are also best seen when the sun is between 6 and 16 degrees below the horizon. Although noctilucent clouds occur in both hemispheres, they have been observed thousands of times in the northern hemisphere but less than 100 times in the southern. This may be because the southern hemisphere has less land mass and a smaller population to make observations.

More Information

Strange Clouds from the Edge of Space,
Noctilucent Clouds Videos
Noctilucent Cloud Observers' Homepage
Observing Noctilucent Clouds [PDF File]

Next page: Arietid Meteor Shower


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