Illumination on Noctilucent Clouds

The article "Dusty Plasmas in the Laboratory, Industry, and Space," by Robert Merlino and John Goree (Physics Today, July 2004, page 32) was interesting, informative, and well written. However, allow me to correct three statements. Although they are admittedly not central to the point of the article, it would be a shame to misinform readers with outdated information.

Recent results by Michael Gadsden¹ and by Sheila Kirkwood and Kerstin Stebel² show that the apparent increase in the observed frequency of noctilucent clouds over the past 30 years does not hold up to rigorous reanalysis of the observations. I concede that the belief in the increased frequency was widespread until August 2002, but it is wrong. The occurrence of these clouds does exhibit an apparent anticorrelation with the solar cycle, but their minimum occurs before the sunspot maximum. Whether the correlation is real or coincidental is still under study. There is no linear trend in the occurrence of noctilucent clouds.

Among hundreds of published temperature measurements taken near the high-latitude mesopause in summer, only one has been as cold as 100 K. To my knowledge, even the author who published that measurement no longer quotes it. Consensus among scientists working in the field is that noctilucent clouds occur when the temperature at an altitude between 80 and 88 km drops below 150 K, which occurs often at high latitudes in summer. The average mesopause temperature at high latitudes is near 128 K. Even that value is rather sensational, the coldest temperature on or near Earth.

It is completely new that water vapor released at high altitudes tends to collect near the poles, and such a statement should not be published without proof or reference. The global circulation in the summer stratosphere and mesosphere is upward and slightly poleward.³ The large-scale, long-term average upward motion is a consequence of acceleration by breaking buoyancy waves. Through adiabatic cooling, the updraft leads to the cold temperatures that create the noctilucent clouds. The water vapor content in the mesosphere is difficult to measure, but appears to be near 5 parts per million by volume in summer at high latitudes.⁴ The source of the vapor is still under study. Michael H. Stevens and coauthors have published one slightly elevated water vapor observation in the high- latitude mesosphere,⁵ which may be a plume from a space shuttle launch. It is unlikely that any significant portion of observed noctilucent clouds should be due to shuttle launches, as those clouds have been observed since 1884.