Valley Stratus

Low overcast clouds that fill the lower elevations of mountainous regions are sometimes referred to as valley stratus.  They can occur any time of year, but are most common during the cool-season when the energy input from the sun is low, enabling the stratus to be more persistent.  

We are experiencing valley stratus in the Salt Lake Valley today, with mostly cloudy skies.

However, the highest elevations of the Wasatch are above these clouds.  Note the contrast below between the lower and upper mountain at Snowbird.

Source: Snowbird

Source: Snowbird

Satellite imagery shows an interesting transition in the valley stratus today.  At around noon, the stratus was confined to near the Great Salt Lake, which was fully covered by clouds. 

Visible satellite imagery at 18:56 UTC (11:56 AM MST). Source: College of Dupage

An hour later, stratus was developing along western slope of the Wasatch Range and within the Salt Lake and Utah Valleys immediately upstream.  

Visible satellite imagery at 18:56 UTC (11:56 AM MST). Source: College of Dupage.

This morning's sounding from the airport is typical of valley stratus situations.  At low levels, there's relatively cool, moist air, capped by a strong inversion that in this case is based at about 725 mb (about 9000 feet above sea level).  

Source: SPC

The stratus forms near the top of the cool, moist layer, just beneath the inversion.  Conditions at. higher elevations are spectacular due to the dry air aloft, but the valley remains cool. 

If this were summer, the energy input from the sun would be sufficient to warm the valley airmass enough to break up or completely dissipate the clouds.  This time of year, however, they often persist unless other atmospheric changes contribute to their demise.  

This situation differs from pollution inversion events because the inversion is based quite high.  Thus, pollution in the valley, while still trapped to some degree, is able to mix to about 9000 feet, so air quality today is good.  Over time, however, the inversion will lower, which will dissipate the clouds, but also trap the pollution in a shallower layer.  I would expect to see more haze tomorrow, for example. 

Valley stratus events are even more common in wetter environments, such as the Coast and Columbia Mountains of British Columbia and the European Alps.  When I taught at the University of Innsbruck, the students were required to forecast the likelihood of valley stratus.  

The ability to recognize valley stratus events is valuable for mountain adventures.  If you woke up in Innsbruck this morning, you were greeted by grey, depressing skies.

Source: foto-webcam.eu

It might also be cloudy at upper elevations, but in the Inn Valley, it is always worth a look at the web cams.  Indeed, the mountains near town were above the clouds, a short funicular and tram ride away.  

Source: foto-webcam.eu

Just like Utah, at least meteorologically.