Hazy skies from wildfires resulted in another long day for travelers at Denver International Airport as hundreds of flights were delayed Tuesday, officials said.

As of 4:45 p.m. Tuesday, 348 flights have been delayed at the airport, according to FlightAware. This comes after 638 flights were delayed at the airport Monday.

The flight delays were caused by reduced visibility caused by smoke and haze in the air, the airport said.

Because of the haze, the Federal Aviation Administration created a flight plan spacing out the number of planes landing in Denver, which meant fewer planes could land at the same time, said Alex Renteria, a spokeswoman for the airport.

Planes departing from were not impacted by the flight plan, Renteria said. 

Tuesday marks the 16th day in a row that Denver has been under an air quality alert as the area experiences a combination of regular pollution and wildfire smoke, according to the Colorado Department of Public Health and Environment.

Moderate to heavy smoke was observed Tuesday morning near the Morgan Creek wildfire in Routt County. Colorado is also getting smoke from the Oregon Bootleg Fire, the nation’s largest active wildfire which had burned 364,000 as of Tuesday afternoon.

On Monday, Denver’s air quality was similar to that of cities like Mexico City and Beijing, known for their smog, according to the National Center for Atmospheric Research.

Haze can be such an issue for flights because it lowers visibility for pilots trying to land, officials said.

“Cruising at low altitude, you decide that haze is a minor annoyance only. But climb higher and the ground may fade to a vague splotchy outline, even if you are flying hundreds of miles from the nearest cloud,” said Dan Namowitz with the Aircraft Owners and Pilots Association.

Namowitz said haze’s ceiling can be “upwards to 15,000 feet,” which is why it impacted flights landing, but not departing from Denver Monday and Tuesday.

Slight relief might be on the way, as the Department of Public Health and Environment expects the smoke from Colorado’s wildfires to begin to drain to lower elevations Wednesday. However, the smoke from Oregon’s wildfire isn’t going anywhere soon.

In the meantime, Renteria encouraged travelers to check their flight status before leaving for the airport. To view flight delays, click here.