1:20 PM | *Rare noctilucent clouds – the highest clouds on Earth - have been unusually prevalent in the US in recent days*

"I have never seen clouds like this before!" says Tucker Shannon, who took this picture from Corvallis, Oregon during the past weekend (courtesy spaceweather.com).

Overview

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.  This weekend, however, noctilucent clouds were seen across many spots in the US including as far south as Freedom, Oklahoma (+36°N) which according to spaceweather.com, may be the lowest latitude sighting ever. Research studies have shown that these clouds tend to become more prevalent during solar minimums and we are now entering into what is likely to be a deep and perhaps historic solar minimum.   

The best way to view noctilucent clouds is through a video of several frames. This video was recorded by Keven Lapp outside Edmonton, Alberta, show their hypnotic fine-structured rippling motions (courtesy spaceweather.com).

Discussion

Noctilucent clouds are the highest clouds on Earth forming in the mesosphere nearly 50 miles above the ground.  These clouds are very cold, filled with ice crystals and seeded by meteoroids and they float at the edge of space.  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 and generally at the high latitude polar regions. This past weekend, however, there were noctilucent clouds seen in the following states: California, Utah, Minnesota, Wisconsin, Washington, Montana, Iowa, Oregon, Oklahoma and Maine.  In addition, these same types of clouds were seen during in many parts of Europe this weekend including the United Kingdom and Poland. 

This image illustrates the layers of the Earth's atmosphere. Noctilucent clouds photographed by the crew of the International Space Station are visible in the mesosphere, the layer of the atmosphere between the stratosphere and space. (Coutesy NASA, Weather Channel)

While noctilucent clouds may look like cirrus, they form in a different part of the atmosphere and are best viewed after the sun has dropped below the horizon in the evening or before it rises above the horizon in the morning 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 (Source: Weather Channel).

The sun is blank for the 23rd day in a row which very well may be the beginning of the next solar minimum phase of the solar cycle (courtesy spaceweather.com)

Impact of solar minimum on noctilucent clouds

Prior 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 23 days in a row in what may very well be the beginning of the next solar minimum phase and all indications are that this may be even quieter than the last one 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.

Meteorologist Paul Dorian
Perspecta, Inc.
perspectaweather.com