 This causes air stagnation and thick smog in the valley from several days to weeks at a time and can result in the worst air-pollution levels in the U.S., reducing air quality to unhealthy levels.Aside from occasional heavy snows in winter, severe weather is very rare.According to the 2010 United States Census, in the city of Clearfield there were 30,112 people living in 9,595 households, of which 76.5% were families.