Temperature Inversion and Fog
March 15, 2018
This photo of valley fog during a temperature inversion was taken from Bompart Ridge on the southern edge of Helena, Montana, on Sunday, December 10, 2017. Cold air was sitting in the valley below. The temperature on the valley floor was about 15 F (-9.5 C), and it was about 20 F (-7 C) on this ridge.
It's a valley thing; during winter months, the mountain valleys of western Montana are prone to inversions – called this because they really are upside-down situations. Normally, the atmosphere gets colder as you get farther away from the surface because the air is warmed from the bottom up by heat given off by the Earth. However, during inversions, air at the surface layer is cooler (sometimes much colder) than the air above. Local hikers know this means that it can be 5 to 10 F (3 to 5.5 C) degrees warmer on top of the mountain than it is down at the trailhead.
The recipe for a temperature inversion is clear, calm and cold. Mountain valleys serve as sinks where cold, dense air may sit for several days, especially in December and January. On clear nights heat escapes quickly out to space, via radiational cooling, and air at the surface becomes cold (and heavy). The low angle of the Sun in December and January prevents the valley air from heating up during the day. In addition, snow cover, which reflects sunlight, and the shortness of winter days also help to keep the valley air cooler. If the inversion persists for several days, air quality worsens as the stagnant, cold air fills with pollutants such as smoke from wood-burning stoves or emissions from automobiles. Note the smoke rising in the valley at far left. For more about this (blog with photo tour) click here.
Photo Details: Camera: Panasonic DMC-ZS60; Exposure Time: 0.0031s (1/320); Aperture: ƒ/3.3; ISO equivalent: 80; Focal Length (35mm): 26.