London ? As volcanic ash cast a shadow over millions of lives, Londoners and other city dwellers across Europe were treated to a rare spectacle of nature: Pristine, blue skies brighter than any in recent memory.

The remarkable sight happened in part because mass flight groundings prevented busy airspace from being crisscrossed with plumes of jet exhaust that create a semi-permanent haze — and other effects beyond the white contrails themselves.

Just as city lights make it necessary for us to go to the desert to appreciate the true glitter of stars, so has modern aviation dulled us to what the noontime sky can really look like — until the erupting volcano in Iceland offered a reminder.

Britain’s poet laureate, Carol Ann Duffy, was inspired to write verses about the unusually clear skies above London: “Five miles up the hush and shush of ash/Yet the sky is as clean as a white slate/I could write my childhood there.”

Scientists cast the phenomenon in more prosaic terms. Without aircraft contrails, “the skies have been particularly blue,” said meteorology professor Chris Merchant of the University of Edinburgh.

The clearer skies are primarily due to a high pressure system in the region, but Merchant said the blue tone has been deeper than normal because of the lack of vapor from aircraft engines. Depending on weather conditions, the vapor trials can expand into thin cirrus clouds.

It’s as if somebody suddenly ripped a veil away, exposing the true colors of the heavens. Amid frustration at the travel disruptions caused by the volcano, some European urbanites have also found something eerily pleasant in the sight of a sky without planes.

In fact, part of the surreal quality of the whole affair has been the illusion of going back to a calmer, less complicated age in which the air was cleaner, life was less harried (no cross-planet shuttles for one-day meetings in Hong Kong), and jets didn’t rumble constantly in our ears.

“It’s definitely quieter without the planes,” said Margaret Mellard, a 63-year-old retiree in London’s Regent’s Park. “You really do see the difference. It’s been really pleasant.”

The crisis has caused some to reflect, perhaps nostalgically, on the age when people spent weeks or months en route to their destination. Hopping on a plane, popping an Ambien and waking up 10 hours later in a different time zone and culture seems somehow less romantic.

There was also introspection in the notion of humankind’s vulnerability to the whims — or is it laws? — of nature. Would even the climate be affected? In an era of unprecedented concern about the environment, that, too, captured attention.

For skygazers, the ash cloud produced another fringe benefit: spectacular fiery sunsets caused by dusk light filtering through ash.