WHAT’S UP IN THE MESOSPHERE? Last month, people around the world who had never heard of noctilucent clouds (NLCs) suddenly found themselves seeing NLCs on a regular basis. The electric-blue clouds rippled to record low latitudes, with sightings as far south as New Mexico and southern California. As July unfolds, however, the clouds have subsided.
What’s happening up there? Lynn Harvey of the Laboratory for Atmospheric and Space Physics at the University of Colorado has taken a look at NASA satellite data, and here are her findings:
“At mid-latitudes, the mesosphere remains quite wet, but temperatures have been rising this month,” says Harvey. “The rising temperatures could be suppressing the formation of NLCs.”
Noctilucent clouds form when summertime wisps of water vapor rise to the edge of space, frosting specks of meteor smoke. When sunlight hits those tiny ice crystals, they glow electric-blue. Moisture boosts NLCs, but warmth destroys them. The recent uptick in temperature may be responsible for their retreat.
Although fewer people are seeing NLCs as July unfolds, the season is not over. NASA’s AIM satellite continues to see a bright ring of electric-blue circling Earth’s north pole, and only two days ago Bertrand Kulik witnessed a display of NLCs over Paris, France:
“Noctilucent clouds were visible over the Eiffel tower around 4 o’clock in the morning on July 10th,” says Kulik. “It was not a strong display, but still very nice.”
The northern summer season for noctilucent clouds typically continues through mid-August. Observing tips: Look west 30 to 60 minutes after sunset (or before sunrise) when the sun is just below the horizon. If you see luminous blue-white tendrils spreading across the sky, you may have spotted a noctilucent cloud.