UPDATED 07/19/11 9:03 p.m.

CHICAGO (CBS) — All but one of Chicago’s beaches were under a swim ban most of Tuesday afternoon after dense fog rolled in off the lake, making it difficult for lifeguards to keep watch for swimmers in trouble.

As CBS 2’s Mai Martinez reports, although the beaches were open to visitors, no one was being allowed to go swimming in the lake because lifeguards could not keep an eye on swimmers through the dense fog. The only beach without a swim ban was 12th Street Beach, officials said.

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“A temporary swim ban has been posted due to dense fog and low visibility as a safety precaution,” the Chicago Park District said in a statement on its website.

The swim bans started just before noon Tuesday at Oak Street beach and North Avenue Beach, where lifeguards and police told people the beach was closed.

“They said you got to get off the beach and come to the sidewalk,” said Frank Pedota.

A short time later, folks were let back on the sand, but the water was still off limits.

“I’m ready to go in and I just got yelled at by a life guard,” said Venus Encarnacion.

By 2:30 p.m., it was the same story at every beach in the city, except 12th Street. The Chicago Park District said because of the poor visibility it was just too dangerous to let people in the water. Still, some tried. One man was even taken into custody after ignoring warnings by police to get out of the water at North Avenue Beach, but most of the beachgoers listened.

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Hundreds of beach-goers decided to stick it out, despite not being allowed in the water. Alex Berg and his friends were among them. “Water is closed for now just throw the ball around figure something out hopefully,” he said.

Lifeguards at each individual city beach to determine if there is enough visibility for them to see swimmers in the water and, if there’s not, the lifeguards can impose a temporary swim ban.

As CBS 2’s Mike Parker reports, once the fog lifted Tuesday evening and the North Avenue Beach lifeguards went home for the day around 7 p.m., the swimmers were back in the water.

One of those swimmers, Victor Cuellar said swimmers went in when the lifeguards left, “but we had to come out because it’s freezing.”

It was the same scene at Olive Park Beach, just down the lakefront – no fog, no lifeguards, folks were swimming again.

But that was a far cry from the Chicago beach scene at midday, when police used bullhorns to tell swimmers that they had to get out of the water because lifeguards had called for a swim ban due to the fog.

The fog was so heavy that lifeguards couldn’t see swimmers in the water.

“As disappointed as we are that we can’t go in, it is probably a good safety precaution,” Jessica Perlaiza said. “You can’t see anything out there so it’s probably best.”

Navy Pier was socked in with fog nearly all day. The Ferris wheel was partially obscured and the east end of the pier seemed to have vanished.

But tourists weren’t complaining.

“I was kind of surprised, everyone seemed like they were enjoying things,” Kevin ford said. “It was kinda cool.”

“Oh yes, a lot of fog. We couldn’t even see the boats,” Greg Macias said, adding that the fog probably cooled things off quite a bit along the lakefront. “Much better for us,” he added.

AccuWeather Meteorologist Bob Larsen said it’s not unusual for fog to develop on a hot day in the middle of Lake Michigan, but it rarely reaches the shore.

Dangerous heat was hovering on the Chicago area Tuesday, with temperatures at 91 degrees as of midday, and heat indices as high as 110 in some areas.

The heat is expected to stick around for the rest of the week, and the air temperature will climb to a dangerous 97 degrees on Wednesday and Thursday.