Every summer since the late 19th century, Earth's polar skies have lit up with eerie blue-white glowing clouds, slowly twisting and undulating in the twilight sky.
These mystifying clouds are referred to as "night-shining" clouds, or noctilucent clouds.
Volcano drew attention
A series of massive eruptions from the Krakatau volcano (also spelled Krakatoa) in late August 1883 may have serendipitously helped to draw attention to the phenomenon of noctilucent clouds.
Dust and ash injected high into the atmosphere from the Indonesian volcano caused spectacular and colorful sunsets worldwide for several years.
On the evening of June 8, 1885, T. W. Backhouse was admiring one such beautiful sunset from Kissingen, Germany, when he noticed something rather strange: as darkness deepened and the ruddy glows faded, he noticed wispy bluish-white filaments seemingly glowing in the north and northwest sky. At that time, scientists dismissed this effect as some curious manifestation caused by the volcanic ash.
But after a few more years, the ash settled and the vivid sunsets induced by Krakatoa faded.
And yet the noctilucent clouds persisted.
Interestingly, there is some debate that Backhouse possibly was not the first to describe them, since in a report dated from 1854, Thomas Romney Robinson, situated at Armagh, Ireland, communicated his personal observation of the " . . . phosphorescent properties of ordinary clouds." So it might be that Robinson was making a reference to noctilucent clouds 31 years before Backhouse.
What causes them
Noctilucent clouds can form only under very restrictive conditions. They are the highest clouds in our atmosphere, located in that layer known as the mesosphere at altitudes between 47 to 53 miles (76 to 85 kilometers). They are normally too faint to be seen, and are visible only when illuminated by sunlight from below the horizon while the lower layers of the atmosphere are in the Earth's shadow.
Ice crystals in clouds need two things to grow: water molecules and something for those molecules to stick to dust, for example. Water gathering on dust to form droplets or ice crystals is a process called nucleation. It happens all the time in ordinary clouds. Ordinary clouds, which generally appear at altitudes of up to 50,000 feet, get their dust from sources like desert wind storms.
But it's all but impossible to push wind-blown dust all the way up into the mesosphere. So scientists speculate that the dust associated with noctilucent clouds originates from outer space. Every day, our Earth encounters countless millions of meteoroids that have been shed by comets. While some of this material rams into our atmosphere in a flash to produce the effect of a shooting star, other tiny particles remain aloft. As for the source of the water vapor necessary to produce clouds at such extreme altitudes, upwelling winds during the summertime are capable of carrying water droplets from the moist lower atmosphere toward the mesosphere.
That's why noctilucent clouds only appear during the warm summer months. The clouds consist of tiny ice crystals about the size of the particles in cigarette smoke.
How to see them
In order for you to have a good chance to see noctilucent clouds, four criteria must be met:
1 - The sky must be free of tropospheric ("ordinary") clouds.
2 - The region of the atmosphere where they form must be sunlit. This means that the sun must be no more than 16 degrees below the horizon.
3 - The background sky must be adequately dark enough for the clouds to stand out. This final requirement means that the sun must be at least 6 degrees below the horizon, what astronomers refer to as the end of civil twilight.
4 - Your viewing location should be at a latitude north of 45 degrees (about the latitude of Minneapolis, Milan, Italy, and Budapest, Hungary, although as you will soon see, the clouds have been sighted at more southerly latitudes in recent years.
Timetable for viewing noctilucent clouds
In the table below, we indicate the "observing windows of opportunity" for making a possible sighting of noctilucent clouds for different dates and at different latitudes.
We indicate the number of minutes after local sunset that it becomes dark enough to sight the clouds (first number) and when the sun has dropped to 16-degrees below the horizon (second number), when the clouds are no longer being illuminated by reflected sunlight and hence can no longer be seen. (Note that for 55 and 60 degrees latitude, the words "All Night" are provided in place of the second number. That's because at these high latitudes, twilight lasts all night long in the summer and the sky never gets completely dark. So from these locations, it is possible to see noctilucent clouds all through the night!)
Example: On Aug. 1 at 45 degrees latitude north (the approximate latitude of Minneapolis-St. Paul or Bangor, Maine), you can look for noctilucent clouds from 33 to 108 minutes after local sunset. Your viewing window of opportunity is 75 minutes.
For latitudes down to 40 degrees north (where the clouds have occasionally been sighted in recent years), try looking from about 30 to 90 minutes after local sunset.
More viewing tips
Interestingly, while reports of noctilucent clouds from Europe and Russia date back to the late 19th century, the first observation from North America did not come until 1933, probably because most were not specifically looking for them, or if they did see them, they didn't realize what they were looking at.
From North American observations of the clouds over the past three-quarters of a century, we have been able to deduce some interesting facts, namely that the earliest and latest sightings were (respectively) Apr. 1 and Sep. 28. Peak activity comes around July 20 about one month after the summer solstice. Ninety two percent of the displays are observed during the months of June, July and August and 82 percent are observed after the summer solstice . Before the solstice, the clouds tend to be faint and cover small areas of the sky, whereas after the solstice they are usually brighter and more extensive.
In general, it would seem that the best times to look for them are during July and August.
As to what you're looking for: gossamer, electric-blue clouds, resembling luminous tendrils, spreading across the northern to northwestern sky and slowly twisting and rippling in the twilight.
Case for global warming?
Over the last few decades, the occurrence of noctilucent clouds seems to have been increasing in frequency, brightness and extent.
A century ago, for instance, the clouds were confined to latitudes above 50 degrees north; you had to go to places such as the United Kingdom, Scandinavia and Russia to see them. But in recent years, they've been glimpsed as far south as Colorado, Utah and Virginia.
It is theorized that this increase is connected to climate change. Gary Thomas, a professor at the University of Colorado has noted that "extreme cold is required to form ice in a dry environment like the mesosphere." Ironically, global warming helps. While greenhouse gases warm Earth's surface, they actually lower temperatures in the high levels of our atmosphere.
Studies from above
Satellites that have been launched to help study these clouds include Sweden's Odin and NASA's AIM (Aeronomy of Ice in the Mesosphere).
Last September, the United States Naval Research Laboratory (NRL) and the United States Department of Defense Space Test Program (STP) conducted the Charged Aerosol Release Experiment (CARE) using exhaust particles from a Black Brant XII suborbital sounding rocket launched from NASA's Wallops Flight Facility to create an artificial noctilucent cloud.
The rocket's exhaust plume was widely observed and reported from New Jersey to Massachusetts.
Recent evidence indicates that at least some noctilucent clouds result from freezing water exhaust from space shuttles. In fact, the clouds have been observed and photographed by astronauts from orbiting shuttles; from space they are called polar mesospheric clouds.
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.