During winter, temperature inversions are common, causing air pollution in the valley. They trap pollutants, moisture, and cold temperatures in the valley while the surrounding mountains enjoy warm temperatures and clear skies. This can cause some melting snow in the mountains and unhealthy air quality and low visibility in the valley. This weather event lasts from a few days to up to a month in extreme cases, and is caused when an area of high pressure settlens over the Great Basin.