A visibility alert has been issued for the Pittsburgh area due to dense fog conditions, significantly impacting travel and outdoor activities. The National Weather Service has advised residents to exercise caution while driving, emphasizing the need to be aware of rapidly changing visibility conditions. The fog, expected to persist through the morning, results from moist air near the ground cooling overnight, causing water vapor to condense into tiny droplets. This process is enhanced by light winds and clear skies, leading to widespread dense fog across the Pittsburgh metropolitan area.