- Rare noctilucent clouds have been visible in the northern U.S. the past few days.
- These clouds form in the mesosphere, nearly 50 miles above the ground.
- Noctilucent clouds are best viewed at dusk and dawn when the sun is 6 to 16 degrees below the horizon.
- The "season" for viewing noctilucent clouds spans from early June through late July.
Noctilucent clouds, the highest clouds on Earth, have been visible in portions of the northern United States over the past few days.
The word almost sounds like it is from space, and indeed, noctilucent clouds are different than their more familiar, terrestrial cousins.
If you've never heard of noctilucent clouds, that's because they occur under strict atmospheric conditions and are visible only during a handful of weeks each year, which happens to be right now.
Above is a photo showing an example of noctilucent clouds from Roseburg, Oregon, Sunday evening. They've also been seen in several other locations the past few days, including Nevada, Minnesota, and across the Atlantic Ocean in the United Kingdom.
Noctilucent clouds were first seen in 1885 after the Indonesian volcano Krakatoa erupted and hurled plumes of volcanic ash miles into the Earth's atmosphere, Gary Thomas, a professor at the University of Colorado, tells NASA.
While they may look like cirrus clouds, noctilucent clouds form differently than the more common, icy, whispy clouds. For example, noctilucent clouds develop in a different section of the atmosphere than cirrus, cumulus and even thunderstorm (cumulonimbus) clouds.
Additionally, cirrus clouds are often seen during the day, while noctilucent clouds develop only in the summer and are best viewed during twilight, after the sun has dropped below the horizon in the evening or before it rises above the horizon in the morning.
The image below illustrates the layers of Earth's atmosphere. The majority of our weather occurs in the lowest layer, called the troposphere, which spans from the Earth's surface to as high as 12 miles into the sky.
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.
Despite the harsh conditions, tiny cubic ice crystals do form in the mesosphere during the summer north of 40 degrees latitude. In the United States, this includes Washington, Oregon, Idaho, Montana, Wyoming, North Dakota, South Dakota, Nebraska, Minnesota, Iowa, Wisconsin, northern Illinois, Michigan, northern Indiana, northern Ohio, Pennsylvania north of the Pennsylvania Turnpike, northern New Jersey, New York and all of New England.
The "season" for viewing noctilucent clouds in the Northern Hemisphere begins in early June and lasts through late July, according to meteorwatch.org. While they are present during daylight, the best time to view noctilucent clouds is at dusk and dawn.
"Look west perhaps 30 minutes to an hour after sunset when the sun has dipped 6 to 16 degrees below the horizon," Thomas said, according to NASA. "If you see luminous blue-white tendrils spreading across the sky, you've probably spotted a noctilucent cloud."