ST. LOUIS — A dense fog bank lined the Mississippi riverfront at St. Louis Wednesday morning. Bommarito Automotive Group SkyFOX Helicopter captured epic images of the Gateway Arch before a sea of clouds. The FOX 2 photographer in the helicopter, Doug Larson, says that this is a rare sight to see. The weather phenomenon only happens a few times a year, in the spring and in the fall.

Meteorologist John Fuller explains that what we saw this morning is called radiation fog. It is also known as ground fog or valley fog. It happens when there is a clear sky, little wind, and high humidity. The fog is produced when the air near the ground is cooled to saturation.

The conditions were right along the Mississippi River at St. Louis to create this epic fog bank Wednesday. Larson says it is not something he often sees on his daily flights over St. Louis.

In this case, the chilly river water was cooling the humid air. The fog dissipates as temperatures rise during the day. That is why you usually see this sort of fog in the early morning or at night.

The pictures of the fog at sunrise are a hit on social media, with hundreds of likes and comments. One FOX 2 fan writes on Facebook, “STL would look good as a bay city.” Another fan replied, “Fog is beautiful unless you have to drive in it.”