Rare Weather Phenomenon Creates River of Clouds Over Grand Canyon

A rare weather phenomenon that affects the area only about once a decade, filled the Grand Canyon in the U.S. with a dense, white fog at the end of November. The phenomenon, known as "temperature inversion," happens when the temperature profile of the atmosphere is inverted from its usual state, and cooler air is trapped at the earth's surface by warmer air above.

Typically, the temperature of air in the atmosphere falls the higher up in altitude you go. This is because most of the suns energy is converted into heat at the ground, which in turn warms the air at the surface. The warm air rises in the atmosphere, where it expands and cools. When temperature inversion occurs, the temperature of air actually increases with height. The warm air above cooler air acts like a lid, trapping the cooler air and fog at the surface and preventing it from rising.

Temperature inversions happen once or twice a year, typically in the winter months. However, these inversions are partial and cover only few parts of the Grand Canyon. The most recent inversion happens only once every 10 years, because the fog filled up the entire canyon and it happened on a cloudless day.

AccuWeather.com Meteorologist Andy Mussoline explained the factors that contributed to the event.

"First, there was higher-than-normal moisture in the canyon," he said. "There was 0.75 of an inch of liquid precipitation that fell between Nov. 20 and Nov. 24 at the Grand Canyon National Park Airport [both snow and rain]. Normal precipitation during that time is only 0.19 of an inch, which converts to nearly 400 percent of normal precipitation within about a week of the event."

“Additionally, the average high temperature for this time of year is about 48 degrees Fahrenheit, which means there would be less evaporation of that precipitation than there would be in the summer months. This allowed more moisture to stay in the air inside the canyon.”

"A high pressure system settled into the region late last week and allowed for clear skies and calm winds, two important weather conditions that allow the air near the ground to cool rapidly," Mussoline said. "The rapid cooling of the ground allowed a temperature inversion to form."