Sky watchers in the San Francisco Bay Area, California witnessed an extraordinary display of noctilucent clouds (NLCs) on June 9, 2019. As these clouds are typically confined to latitudes above +55 N and San Francisco is at +38 N, this marks one of the lowest-latitude sightings of NLCs, Dr. Tony Phillips of SpaceWeather reports.
A huge outbreak of NLCs occurred over Europe and the USA over the past weekend, with electric-blue night-shining clouds seen as far south as Oregon, Utah and California.
The 2019 northern hemisphere noctilucent cloud season begun unusually strong on May 20.
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 kinds of waves, Phillips said.
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.
Yesterday's sightings over Bay Area mark one of the lowest-latitudes they were ever seen.
Check out SpaceWeather's NLC Real-time Gallery here.
Featured image: Noctilucent clouds of San Francisco Bay Area on June 9, 2019. Credit: Ed Brooks via SpaceWeather