Astro Bob: Noctilucent clouds — what are they, where are they and when to see them
Silky and blue, clouds made of meteor dust glow at the edge of outer space.
DULUTH — Recently, I had the opportunity the see a couple displays of noctilucent clouds. Now that we're approaching the summer solstice, this is "high season" for observing these elusive silver-blue wisps of water vapor and space dust.
Noctilucent or night-shining clouds make their appearance long after sunset when the rest of the clouds have gone dark. On a summer day, fluffy mounds of cumulus float about 5,000 feet (1,525 meters) above the ground and are composed of tiny water droplets. Higher up at 20,000 to 40,000 feet (6100 -12,200 meters), feathery cirrus tickle the top of the troposphere, the lowest layer of the atmosphere.
When the sun set for ground dwellers its reddened light still shines on clouds like cirrus because they're high enough to catch the light a little longer. Similarly, a tall mountaintop will continue to glow in sunlight even after the sun has set for those who live in the valley.
Noctilucent clouds are the highest of all, with altitudes between 249,000 and 279,000 feet (76,000 - 85,000 meters) or around 50 miles. They're so lofty they form far above the troposphere in a layer called the mesosphere. NASA, the FAA and the U.S. military consider 50 miles the beginning of outer space, placing these night-glowing clouds at its very edge.
We can't see noctilucent clouds in the daytime because they're too thin and tenuous. But at dusk, when the lower layers of the atmosphere succumb to shadow, sunlight still touches the clouds at their tremendous altitude, setting them off against the darkening sky. They glow pale blue because the light they reflect has to pass through the ozone layer in the stratosphere on its way to our eyes. Ozone acts like a sponge, absorbing the warmer yellows and reds in sunlight and letting the blues pass through.
NLCs are more common in the polar regions and high latitudes because the mesosphere is colder there. That cold is critical to their formation as is a source of water vapor and dust. Scientists still don't know exactly how water gets up there but theorize it filters through gaps at the top of the troposphere. Methane from human-based activities may also be a factor, bubbling up into mesosphere and chemically combining with other molecules to make water. Dust from meteors and possibly volcanoes act as "seeds" on which water can condense into ice.
They're only visible during the summer months at dusk and dawn from about June through August (December through February in the southern hemisphere) from the northern U.S., Canada and northern Europe. Why summer? Paradoxically, the mesosphere is colder in summer than in winter which makes for better cloud-forming conditions.
In recent years, NLCs seem to have become more common and widespread, no longer restricted to just the highest latitudes. I watch for them every clear night in June and July from northern Minnesota, but they've been sighted as far south as Las Vegas and Los Angeles.
Crucial to spotting the clouds is having a place where you can see close down to the northwestern horizon. We're talking at least as low as the bright star Capella, which often marks their maximum elevation when viewed from the northern states. I've seen a few more expansive displays that reached as high as 20-25° or two fists above the horizon, but those are rare at least here in Minnesota.
Start watching an hour after sunset (your sunset time at timeanddate.com/sun ). This is when the higher, brighter displays could show, otherwise an hour and a half after sunset is generally the peak time. For Duluth, 10:30 p.m. is the sweet spot for the remainder of June. This past week, I've seen two back-to-back displays low in the northwestern sky between 10:30 and about 11:15 p.m. local time. NLCs are fickle. They may show one night and then be absent for a week.
Take a camera and tripod to record their color and forms. The clouds have a lacy, webby appearance and move very slowly as befitting their high altitude. Binoculars will reveal amazing textures better than with the naked eye. Remember that you can also watch for NLCs starting about 2 hours before sunrise. Share your observations and photos on Astro Bob's Astronomy for Everyone Facebook page .
Happy cloud hunting!
"Astro" Bob King is a freelance writer for the Duluth News Tribune.