2019 northern hemisphere noctilucent clouds season starts unusually strong

The 2019 northern hemisphere noctilucent cloud season has begun unusually strong. Electric-blue clouds were first observed by NASA's Aeronomy of Ice in the Mesosphere (AIM) spacecraft on May 20, marking the official start of the season.

​"For the past 2 weeks, a huge blue noctilucent cloud has been pinwheeling around the Arctic Circle, shaped and controlled by a 5-day planetary wave," SpaceWeather's Dr. Tony Phillips reports.

Noctilucent clouds (NLCs) in May are nothing unusual as they form every year around this time but these NLCs are different, Phillips noted. "They are unusually strong and congregated in a coherent spinning mass, instead of spreading as usual all across the polar cap."

"This is most likely a sign of planetary wave activity,” Cora Randall of the AIM science team at the University of Colorado said.

Planetary waves are enormous ripples of temperature and pressure that form in Earth’s atmosphere in response to Coriolis forces. They are responsible in part for undulations in the jet stream and can have a major influence on global weather. All rotating planets with atmospheres have these kind of waves, Phillips said.

Image credit: MLS

NLCs have already made an unusually early appearance in the USA and in Europe this season.

The first known observation of noctilucent clouds dates back to 1885, two years after the 1883 eruption of Krakatoa. At first, they were only seen at higher latitudes, but have recently started appearing ever lower in latitudes. 

Featured image credit: NASA/AIM