Noctilucent Clouds
Mysterious Night-Shining Clouds May Signal Global Warming
Noctilucent clouds, also called polar mesospheric clouds or night-shining clouds, are found in the mesosphere, higher than any other clouds in the atmosphere. Most clouds and weather on earth are confined to the troposphere, or layer of air closest to the ground. The mesosphere is located 80 kilometers above our planet's surface.
Noctilucent clouds were only first reported by observers in 1885. The clouds are made up of tiny ice crystals. One mystery is how these ice-crystal clouds could exist in the mesosphere, a region that is extremely cold and dry. For the clouds to form, they need water vapor and dust particles for that water vapor to stick to.
From Krakatoa to Global Warming
Up in the mesosphere, the original source of the dust was probably the volcanic eruption of Krakatoa. Krakatoa spewed fine ash particles throughout Earth's atmosphere in 1883, including as high as 80 kilometers into the mesosphere. This was just before noctilucent clouds were first noticed.
The dust from Krakatoa has long since disappeared, yet the clouds have not. Sources of the dust could even be from outer space, as micrometeoroids rain down onto our atmosphere. The source of the water vapor is related to summertime weather and the upwelling of moisture into the mesosphere. This is why noctilucent clouds appear more often in the summer.
These night-shining clouds have normally been seen at far northern and southern latitudes, such as Scandinanvia and Siberia, but now they are appearing in lower latitudes also, such as the northern tier of the United States.
Global warming may be responsible for the spread of these clouds. In order for clouds to form in such a dry environment, extreme cold is required. Greenhouse gases that are warming the air near Earth's surface actually lower the temperature of the air high in the atmosphere. It may be telling that these clouds were first spotted during the Industrial Revolution.
How to Observe Noctilucent Clouds
In order to see NLCs, as they are sometimes called, location and time of year are important. NLCs generally appear between 70 to 40 degrees latitude, but the closer to the poles you are in that range, the better. Summer is considered the NLC season.
The clouds are seen after sunset when the sky has grown dark because these thin clouds cannot compete with bright daylight. After sunset when the sun is at least six degrees below the horizon, the light from the sun still reaches high into the upper edges of the atmosphere and causes the clouds to glow. This is similar to how satellites shine far above earth from sunlight that is below the horizon.
The clouds often are described as "electric blue". They look similar in shape to cirrus clouds except that they are so high up they seem to be in outer space. Their eerie and awesome spectacle should not be confused with the aurora borealis, which can appear at any time of night and are sheets of undulating particles that can appear in colors from blue to red to green.
Sources: NASA, Spaceweather.com
