February 2024

Stratus Nebulosus: a Cosy Cloud Blanket

The lowest of the ten main cloud types, Stratus is a layer cloud with a base that is typically no higher than about 1,500 feet (500 metres). Viewed from below, it covers the low sky like a thick, warm blanket. Some find a duvet of Stratus cloud feels too close – almost smothering. But any Cloudspotters willing to forsake their bed and head up a mountain for a view of the cloud from above will see Stratus in a whole new light.

It doesn’t actually need to be a mountain. Often a hill will get you high enough to look down onto Stratus, for this is such a low cloud it can form right down at ground level (or water level), which is when it’s known as fog. Paolo Bardelli (Member 45,963) was up on Campo dei Fiori, a mountain in the Varese region of Lombardy, in northern Italy, when he spotted this Stratus at night, lit by the moonlight as it filled the valleys below.

Stratus like this is known as nebulosus. This is the most common species of Stratus, appearing as an extended and fairly featureless layer. It looks quite different from the alternative species, known as Stratus fractus, that appears as scraps or fragments of low cloud rather than an extensive layer. The fractus form is found on the slopes of hills and mountains, like misty shreds formed as moist air drifts up the inclines. Stratus nebulosus, by contrast, tends to extend over much of the sky or, when you are looking down onto it, much of the ground below.

You can think of Paolo’s Stratus nebulosus as valley Stratus. On long winter nights when the sky is relatively clear, the ground on mountain and hill slopes gets cold as it radiates away the Sun’s warmth from the previous day up into the starry sky above. The cold ground cools the air in contact with it, encouraging some of the air’s moisture to condense into droplets that appear as cloud. Since cold air is more dense than warm, it starts to sink down the sloped terrain to displace the valley air that had been above the ground. And as it sinks it carries its droplets with it, filling the valley with a thick, fleecy layer of cloud.

From the chill of the mountaintop, Paolo watched the bright moonlight illuminate the quilted undulations, as everyone down in the valley prepared for bed beneath their cosy blanket of Stratus.

Stratus nebulosus spotted at night from Campo dei Fiori, Varese, Italy by Paolo Bardelli (Member 45,963). View this image in the photo gallery.

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