STORY: New York City's skyscrapers, which can be seen for miles away on a clear day, were rendered nearly invisible.

People were instructed to limit time outdoors, while those with respiratory issues were advised to consider wearing a mask.

The smoke is crossing the U.S. northern border from Canada, where wildfire season got off to an unusually early and intense start due to persistent warm and dry conditions.

Canada is on track for its worst-ever wildfire season.