Cool, crisp, clean mountain air has long been an important reason why people move to Colorado. Unfortunately, the air in the city of Denver isn’t quite so clean. By the 1970s, the pollution hanging over the city had a name – the brown cloud. Denver’s location at the foot of the Rocky Mountains makes it prone to temperature inversions in which warm air traps cooler air near the ground, preventing pollutants from rising into the atmosphere. Learn more about temperature inversions with this eye-catching, hands-on science experiment.