Widespread valley fog developed this morning in the complex terrain of western Montana and Idaho.
The visible satellite clearly shows how widespread this was:
Let’s take a look at Helena, MT, circled here:
Helena is on the southern end of a bowl shaped valley around 3900 feet. The valley low point is around 3600 feet and cool katabatic flows off the higher terrain after sunset flow in from all directions. The only location for air to “drain” out of the valley is along the NE end at the opening of Gates of the Mountains Wilderness along the Missouri River–in essence, the entire valley is enclosed.
A Google Earth view of the only outlet on the NE end of the Valley.
In the winter (mainly under the influence of high pressure), this region is notorious for long-lasting inversions as the snow-pack reflects the weak insolation, supporting long lasting inversions and decreased air quality as stagnant air is essentially “trapped” until a synoptic scale system can mix the air mass out.
A few things worth mentioning in the surface observations. Note, that by the 8:53 observation, the temperatures are on the increase with morning insolation. Pressure slowly decreases (seen on the right) as rising thermals slowly develop. Dew point slowly increases as well, but nearly saturated conditions and low stratus hang on until noon. The depth of the fog is obvious here. A common misnomer in the world of meteorology is that fog “burns off”. It is too bad meteorologists insist on using this term as it can be confusing to the public. Of course what is occuring is the development of a convective boundary layer which promotes mixing and drying of the saturated layer as insolation (even through the deep fog) will develop thermals with time. Note the rapid decrease in surface pressure (boxed on the right) after the stratus layer mixes out–a good example of the “mass” of air that can subside into valleys at night.