Cold season temperature inversions

From about November to February temperature inversions accompanied by low clouds and fog are a common weather feature in the Columbia Basin. These are not unique to the Columbia Basin with many valleys throughout the Western United States and the world also experiencing strong inversions, including the Salt Lake City area in Utah and Almaty, Kazakhstan.

Typically, temperatures in the lowest 5.5 to 11 miles (9 to 17 km) of the atmosphere decrease with height. This is why under normal conditions high elevation locations are cooler than low elevation locations nearby. We use the term inversion to refer to a reversal of this – temperatures rising with altitude instead of decreasing.

Temperature inversions are common globally throughout the year at night. During the day, heat from the sun warms the ground which in turn warms the air close to the ground. Because warm air is lighter than cold air, the warmed air will start to move upward allowing the atmosphere in that location to be well mixed.

At night heat from the sun is removed and cold air sinks toward the ground. Low clouds and fog can develop during the overnight, or nocturnal, inversion as temperatures cool to reach the dew point allowing water to condense. On nights that are clear and have calm winds this the inversion can be especially remarkable.

During the 2021 heat wave in the Pacific Northwest, the WSU AgWeatherNet station at Vancouver, Washington observed a difference of about 9°F (5°C) across a vertical distance of just 23 feet (7 meters) in the early morning hours.

Most of the year the nocturnal inversion will clear shortly after sunrise thanks to daytime heating. From about November to February, however, the sun is too low in the sky over the Columbia Basin to heat the ground enough to generate the vertical mixing necessary to clear the low clouds and fog when they develop as part of an inversion. Inversion season is longer further away from the equator and shorter as one gets closer to it.

An inversion will often be enhanced by high pressure aloft effectively smashing the cold air into a basin or valley keeping it trapped and by cold air descending from nearby mountains into the lower elevations.

Once an inversion like this is in place it typically takes something like a cold front or incoming low pressure system generating a decent amount of wind to clear out the inversion. In arid regions like the Western United States several days to a couple weeks can pass before such a pattern change occurs.

Inversions don’t only trap cold air and clouds but also pollutants. Over time this creates a notable drop in air quality. Pollutants can be both sourced locally as well as from distant locations where air is sinking from.

Large cities like Salt Lake and Almaty have a lot of local pollutants. In the Tri-Cities, pollution is sourced locally as well as from places like Spokane as cold air sinks from nearby mountains, moves across the city, and continues down into the lower elevations of the Columbia Basin.

Long-lasting cold season inversions are generally limited to valleys and basins because places like the Great Plains are flat enough to not efficiently trap air in any one location. There are no good ways for air to leave the Salt Lake area without being pushed over the surrounding mountains whereas there isn’t a big mountain range blocking the air somewhere like Omaha.

There is only one location for air to leave the Columbia Basin without moving over mountains – the Columbia River Gorge. When a strong inversion is in place in the Columbia Basin, extreme winds can be experienced for long periods of time along the Columbia River from about Boardman to the Portland metro area.

The wintertime east winds are so common in and downstream of the Columbia Gorge that several indigenous tribes had legends attempting to explain their origin and existence. A few times a year the Columbia Basin inversion will get deep enough to engulf Spokane in fog and produce strong east winds across mountain passes like Snoqualmie Pass east of Seattle.

The featured image is a NOAA satellite image of Washington showing clouds produced by a strong inversion in the Columbia Basin.