CLIMATE CHANGE: Noctilucent Clouds Indicate Something is Changing in the Atmosphere

By Henry M. Smith

Noctilucent clouds are thin, ice clouds that occur in the mesosphere. The mesosphere is the part of our atmosphere that is high above the Earth, and is just under the exosphere where our atmosphere slowly blends with space. They are called noctilucent because they appear bright at night, being brightened by the sun well after it has set. They are too thin to be noticed in daylight.

Originally, these clouds were associated with the eruption of the volcano Krakatoa. In 1883, Krakatoa had a massive, upward explosion, and the appearances of noctilucent clouds began shortly thereafter. This led to the natural connection between the two events. It was believed dust and water were ejected to great heights, and did not settle immediately. This was believed to have served as the source of the clouds, since noctilucent clouds first appeared just a few years after the volcanic event.

Today, this idea has lost its appeal, since over the years the materials placed in the upper atmosphere should have eventually settled. Even more perplexing is that these clouds are increasing in coverage.

Noctilucent clouds were initially thought to be two distinct varieties, one called noctilucent and the other called mother of pearl, based on their appearances. Noctilucent clouds covered a significant portion of the sky, but mother of pearl clouds might be smaller. Today they are all regarded as noctilucent clouds, or mesospheric clouds.

Normally, these clouds remain far from the equator in both hemispheres. This situation is slowly changing. They are now appearing farther south in the Northern Hemisphere, and farther north in the Southern Hemisphere. Simply go to http://Spaceweather.com and view the photographs, paying attention to the locations from which the photographs originate. While most places mentioned are far north, a few are much closer to the equator than would have been thought possible a few decades ago.

This raises the question of why is this happening. Apparently, temperature conditions are changing in the upper atmosphere. Water vapor condenses into ice clouds only if the air is supersaturated. Since there is no apparent increase in water vapor available to make these clouds, the temperature in the mesosphere must be falling to cause this supersaturated condition to become more prevalent.

Is there a connection with changes in the mesosphere and our climate? According to Nasa, there might be an association between these clouds increasing and global warming, albeit a controversial connection. A few years ago Nasa in Strange Clouds at http://science.nasa.gov/headlines/y2003/19feb_nlc.htm was reporting the cooling of the mesosphere might be associated with global warming. Notice the word "might." Whether there is or not remains uncertain, but there is some certainty that something is changing in the mesosphere.

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