In recent nights, noctilucent clouds (NLCs) have rippled across Europe from Scandinavia to the south of France. “We have been observing NLCs every night here in N. Ireland,” reports Martin McKenna of Maghera in Co. Derry. “Their brightness and complexity have been getting more advanced since the solstice, with whirls and knots glowing electric blue above a yellow midnight sunset horizon.” He observed this ‘noctilucent tornado’ on June 25th:
“It was amazing to watch,” he says. “This area then morphed into an succession of dynamic shapes–a wedge, a funnel, angel wings, an electrified smoke ring, then a long rope tornado which reached towards the horizon.”
What creates these forms? The answer is “gravity waves.”
Gravity waves are, essentially, waves of pressure and temperature spawned by powerful storm systems. Gravity does not vary inside the waves; they get their name from the fact that gravity acts as a restoring force that tries to restore equilibrium to up-and-down moving air. Gravity waves can propagate all the way from Earth’s surface up to the mesosphere, where they imprint themselves on the the forms of noctilucent clouds. When a sufficient number of gravity waves meet, they can interfere to produce all of the structures McKenna saw–plus many more.
Summer is the season for NLCs. Summertime wisps of water vapor rise to the top of Earth’s atmosphere where they wrap themselves around specks of meteor smoke. Mesospheric winds gather the resulting ice crystals into noctilucent clouds.
Via NASA Spaceweather.com