NOCTILUCENT CLOUD ACTIVITY: Sky watchers in Europe are reporting bright displays of noctilucent clouds this weekend. “Strands of electric-blue started showing very early, just after midnight BST on July 1st,” says Andy Stables, who sends this picture from Milovaig on Isle of Skye, Scotland:
“They gradually grew brighter and more extensive as our twilight arc shifted into the north, reaching peak intensity around 2am,” he says.
This and other displays like it are telling us something about the weather in Earth’s mesosphere: It’s very cold up there. Noctilucent clouds form when summertime wisps of water vapor rise to the top of the atmosphere and wrap specks of meteor smoke with fragile crystals of ice. This process requires temperatures as low as 145 K (-128 C). In early June, an unexpected heat wave in the mesosphere temporarily wiped out noctilucent clouds. Their return proves that the heat wave is over.