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Night Clouds in Roseburg, Oregon on Sunday, June 9, 2019.
(Photo of Gail Horak in Roseburg, Oregon.)
- Rare nocturnal clouds have been observed in the northern United States in recent days.
- These clouds form in the mesosphere, about 20 km from the ground.
- Nocturnal clouds are optimized at dusk and dawn when the sun is between 6 and 16 degrees below the horizon.
- The "season" for observing noctilucent clouds extends from early June to late July.
The night clouds, the highest in the world, have been visible in parts of the northern United States in recent days.
The word sounds almost like it came from space, and indeed, the noctilucent clouds are different from their more familiar terrestrial cousins.
If you've never heard of nocturnal clouds, that's because they happen under harsh weather conditions and are only visible a few weeks a year, which is the case right now .
Above, a photo showing an example of nocturnal clouds from Roseburg, Oregon on Sunday night. They have also been seen in several other places in recent days, including Nevada, Minnesota, and across the Atlantic Ocean in the United Kingdom.
Nocturnal clouds were observed for the first time in 1885 after the eruption of the Indonesian volcano Krakatoa and its jet of volcanic ash plume into the earth's atmosphere.Gary Thomas, a professor at the University of Colorado, told NASA.
Although they may look like cirrus, noctilucent clouds form differently from more common, icy, glaucous clouds. For example, nocturnal clouds develop in a section of the atmosphere different from that of cirrus clouds, cumulus clouds and even storm clouds (cumulonimbus clouds).
In addition, cirrus clouds are often observed during the day, while nocturnal clouds develop only in the summer and are best seen at dusk, after the sun has sunk below the horizon. in the evening or before getting up over the horizon in the morning.
The picture below illustrates the layers of the Earth's atmosphere. Most of our time is spent in the lowest layer, called the troposphere, which extends from the surface of the Earth up to 12 miles into the sky.
This image illustrates the layers of the earth's atmosphere. Very nuanced clouds photographed by the crew of the International Space Station are visible in the mesosphere, the layer of atmosphere between the stratosphere and space.
(NASA)
Cirrus clouds form in the highest parts of the troposphere, where temperatures can drop at around minus 22 degrees Fahrenheit. At this temperature, there is sufficient water vapor in the atmosphere to support the development of ice crystals, producing cirrus clouds.
In contrast, nocturnal clouds form in the mesosphere, nearly 20 km above the surface of the Earth. At this height in the atmosphere, there are so few molecules of air 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 that of the middle of the Sahara Desert..
Despite the harsh conditions, tiny cubic ice crystals 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, the Wisconsin, northern Illinois, Michigan, northern Ohio, Pennsylvania and northern Pennsylvania Turnpike, northern New Jersey, New York and all of New England.
The "season" for observing night clouds in the northern hemisphere begins in early June and ends until late July., according to meteorwatch.org. Although they are present during the day, the best time to observe the nocturnal clouds is at dusk and dawn.
"Look west maybe 30 minutes to an hour after sunset when the sun has plunged 6 to 16 degrees under the horizon, "Thomas said, according to NASA." If you see bright blue-white tendrils spilling into the sky, you have probably already spotted a noctilucent cloud. "
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