Why does Jackson County have poor ventilation during the winter?

The Rogue Valley has some of the worst ventilation in the country. One of the reasons for this is winter temperature inversions. Because of our unique weather and topography, sometimes cold air becomes trapped in the valley beneath warmer air. The cooler air, because of its greater density, settles close to the ground, and the warmer air forms a blanket above it in a temperature inversion. Because the winter days are short, often the sun can't warm the cold lower air enough for it to rise out of the valley. Pollutants such as wood smoke and automobile exhaust are trapped in this cooler air.