This morning a vast bank of electric-blue noctilucent clouds rippled across northern Europe. "It was like water in the sky," says Barbara Grudzinska, who photographed the display from Warsaw, Poland:
Noctilucent Clouds
© Barbara Grudzinska
"These are the first noctilucent clouds this year so clearly visible at our latitude in Warsaw (52 N)," says Grudzinska.

When NLCs first appeared in the 19th century, the mysterious clouds were confined to the Arctic, most often seen in the same places as Northern Lights. In recent years, however, their "habitat" has been expanding, rippling as far south as Colorado, Virginia, Kansas, and Utah. (Here are some examples of sightings in the lower United States.) There is growing evidence that the expansion is a sign of climate change, although this remains controversial.

Whatever the reason, noctilucent clouds aren't just at high latitudes anymore, so sky watchers everywhere should be alert for them.

Observing tips:

Look west 30 to 60 minutes after sunset when the sun has dipped 6o to 16o below the horizon. If you see luminous blue-white tendrils spreading across the sky, you've probably spotted a noctilucent cloud.