Noctilucent clouds are the highest clouds on Earth and are quite rarely seen in the US as they are primarily visible at high latitudes above 55°N. For the past several weeks, however, they have been seen much more often than normal around the world and at unusually low latitudes. Two possible explanations for the on-going extraordinary viewing season revolve around the current (low) activity on the sun and the high water vapor levels way up in the atmosphere. Specifically, noctilucent clouds tend to be more prevalent during solar minimums and we are now entering what may be the deepest solar minimum in more than a century. In addition, unusually high levels of water vapor have been detected in the mesosphere creating favorable conditions for the formation of noctilucent clouds.
Noctilucent clouds are the highest clouds on Earth forming in the mesosphere nearly 50 miles above the ground and generally in high latitude polar regions. These clouds form when molecules of H2O adhere to specks of meteor smoke forming ice crystals way up in the atmosphere. They are best viewed at dusk and dawn when the sun is about 6 to 16 degrees below the horizon and when the sunbeams hit those ice crystals, they tend to glow electric-blue. The typical “season” for viewing noctilucent clouds spans from early June through late July when summertime water vapor rises to the top of the Earth’s atmosphere. The past several weeks have featured sightings in many mid-latitude locations around the world including here in the US and as far south as New Mexico and southern California.
While noctilucent clouds may look like cirrus, they form in a different part of the atmosphere and are best viewed just after sunset or just before sunrise whereas cirrus are often seen during the daytime hours. Cirrus clouds form in the highest portions of the troposphere, where temperatures can drop to about minus 22 degrees Fahrenheit. At that temperature, enough water vapor is available in the atmosphere to support the development of ice crystals, producing cirrus clouds. In contrast, noctilucent clouds form in the mesosphere, nearly 50 miles above the Earth's surface. At this height in the atmosphere, so few air molecules exist that it becomes extremely difficult to produce ice crystals. The temperature must drop below minus 207 degrees Fahrenheit for ice crystals to form, and so little water vapor is present that the mesospheric air is a thousand times drier than air from the middle of the Sahara Desert.
Impact of solar minimum on noctilucent clouds
Studies have shown evidence of a solar cycle dependence with the appearance of polar mesospheric clouds. One study in particular suggests there is an anticorrelation with noctilucent cloud sightings and solar activity (i.e., the less the solar activity, the more noctilucent clouds become visible). The sun has been blank for 61% of the year in what may very well be the beginning of the next solar minimum phase and all indications are that this one may be quieter than the last which was the deepest in nearly a century.
During a solar minimum, noctilucent clouds tend to become more prevalent as this period favors the frosting of meteor smoke high above the Earth. Water molecules stick to specks of meteor smoke, gathering into icy clouds that glow electric blue when they are hit by high altitude sunlight. Extreme ultraviolet radiation (EUV) radiation can destroy those water molecules before they freeze. Less EUV during solar minimum could therefore give us more noctilucent clouds. The solar cycle is currently entering into perhaps one of the deepest solar minima of the past century. Extreme ultraviolet radiation from the sun is at its lowest level in a decade--a deficit that can lead directly to more noctilucent clouds. Coincidentally, the 2019 season for noctilucent clouds began in late May just as the sun entered a period of sustained spotlessness (34 days in a row).
Impact of high-altitude water vapor on noctilucent clouds
The mesosphere is unusually wet this year and this provides favorable conditions for the formation of noctilucent clouds. The surplus in water vapor is notable at mid-latitudes where a lot of unusual sightings have taken place in the past few weeks. New data from NASA’s Microwave Limb Sounder (MLS) show that water vapor concentrations in the mesosphere between 35N and 45N remain near their highest levels in at least 12 years.
Meteorologist Paul Dorian