Natural Phenomenon: Noctilucent Clouds

Noctilucent clouds are the highest clouds in the Earth’s atmosphere, 83 km (50 miles) and are observed slightly below the mesopause in the polar summertime.  These clouds are of special interest, as they are sensitive to both global climate change and to solar/terrestrial influences. The first recorded sightings of noctilucent clouds were reported in 1885 and both satellite and ground-based observations over the past four decades have indicated that the presence of these clouds has been increasing in both frequency and brightness. Scientists now realize that these clouds are very sensitive indicators for what is going on in the atmosphere at higher altitudes as small changes in the atmospheric environment can lead to large changes in the properties of these clouds. Further, since these clouds form on condensation nuclei through cold temperatures and the presence of water vapor – and these properties of the mesosphere are tied to carbon dioxide and methane, the anthropogenic causes of climatic change may be directly related to the presence of noctilucent clouds.

Noctilucent clouds are of significant interest within the aeronomy and climate science communities as noctilucent clouds are very sensitive indicators for what is going on in the atmosphere at higher altitudes.  The extreme cold temperatures and very low density of the mesosphere creates an environment where very small changes in the atmosphere can drive large changes in observed noctilucent cloud properties.  So by observing noctilucent clouds, we can learn a lot about the atmosphere as a whole (e.g. dynamical and thermal state, solar-terrestrial influences).

Since first observed in 1885, both satellite and ground observations throughout the last 40 years have indicated that 1) noctilucent clouds have increased in brightness over recent decades, 2) noctilucent clouds have extended to lower latitudes over recent decades, and 3) noctilucent clouds have been observed with increasing frequency over recent decades.  

In general, clouds need two things to form: 1) low temperatures relative to the dew point, and 2) water vapor.  We know that carbon dioxide acts as a net cooling agent in the upper atmosphere (though it warms the lower atmosphere, it radiates more heat away than it traps in the upper atmosphere, causing it to cool).  Further, we know that methane photo-disassociates in the upper atmosphere and a major by-product is water vapor.  Further, the introduction of chlorofluorocarbons (CFCs) in the lower atmosphere depletes the ozone, causing the stratosphere to shrink, and driving the mesosphere to be colder. As carbon dioxide, methane, and CFCs are all largely industrial by-products, they are believed to be man-made contributors to global climate change.  So if we see more noctilucent clouds, colder temperatures and/or increased concentrations of water vapor must exist in the mesosphere, which may be caused by the introduction and upward convection of carbon dioxide, methane, and CFCs in the lower atmosphere.

Noctilucent clouds reside near a poorly understood yet critical region to reentry vehicles.  The ground-track of space vehicles traveling to the International Space Station (ISS) missions reach latitudes where the low-latitude noctilucent clouds may be present.  Since we don’t know enough about the mesosphere, manned space missions have chosen to act conservatively, enacting constraints for manned space missions that may be irrelevant. Through a better understanding of noctilucent clouds and the mesosphere, we may be able to design more operable space architecture.

Source: projectpossum.org