.
.
Noctilucent clouds form when molecules from summertime water vapour stick to the microscopic debris of disintegrated meteoroids.

May 21, 2019: Every summer since the late 1970s, radars probing Earth’s upper atmosphere have detected strong echoes from altitudes between 80 km and 90 km. The signals come from noctilucent clouds (NLCs).  NASA’s AIM spacecraft is still waiting to spot the first NLCS of the 2019 season, but the echoes have already begun. Rob Stammes of the Polarlightcenter in Lofoten, Norway, detected them on May 19th and 20th:

“I detected these VHF signals from Eastern Europe,” he explains. “They reflected from the mesosphere back down to my receiver in Norway. The wave patterns were recognizable and very strong.”

Researchers call them “Polar Mesospheric Summer Echoes” or “PMSEs.” They occur over the Arctic during the months of May through August, and over the Antarctic during the months of November through February. These are the same months that NLCs appear.

The underlying physics of these echoes is still uncertain.