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Nothing says a warm day quite like the beach, but beyond the shore lies a number of dangers, from rip currents and strong waves to shark attacks and bobbing jellyfish. Onshore, however, you will likely find a flag warning you of potential dangers, and whether it's purple, yellow, red or blue can tell you which hazard could be lurking in the waters. 

A day on the beach can be full of fun, but when swimming in the ocean, be aware of its conditions and what other forms of life could be present beneath the surface. According to the U.S. Lifesaving Association, beach flags and their designated meanings were created to help inform people of just that. Many of these flags are also used around the world.

So what exactly are beach flags and what do they mean? Here are the flags approved by the association for use on beaches where lifeguards are trained to its standards. 

Green beach flag

When ocean conditions are calm or mild, the beach may not have a flag up, as the International Life Saving Federation decided not to officially adopt the color. But some localities will fly a green flag to indicate that the water is safe.

The international body didn't adopt the green flag because "there is always a potential hazard present," the USLA says on its website, "and the view that it is best to notify people when conditions are unusually challenging, rather than suggesting that they are ever completely safe."

Yellow beach flag

Yellow beach flags indicate that there is a "medium hazard" at the beach, the association says, with the ocean exhibiting moderate surf conditions and/or currents. When this flag is up, weak swimmers should refrain from going into the water, while others should exercise "enhanced care and caution." 

Beachgoers should be aware of the potential for rip currents, which can form anywhere where waves are present in any kind of weather or conditions. These currents can be deadly and are most often seen at low tide when waves are at least 2 feet tall.

Red beach flag

As with many things, red indicates hazards on the beach. The U.S. Lifesaving Association says that when this bold red flag is up, it means that there are rough conditions in the water, like strong surf and currents. At this point, the association said, "all swimmers are discouraged from entering the water," and those who choose to do so anyway, "should take great care." 

Double red beach flag

If you see double red — or two red flags stacked on top of each other — it means the water is closed to the public. 

Purple beach flag 

At the beach, purple means pests — but don't worry, that doesn't necessarily include sharks. Purple flags will be posted when marine animals that can cause minor injuries, such as jellyfish and stingrays, are in the area. The association says that the flag "is not intended to indicate the presence of sharks," although some destinations may use purple to indicate they are present. Always look at your destination's specific warning system before going to the beach. 

Red and yellow beach flag

If you see a single flag that's half red and half yellow, the area you're in is among the safest on the beach. The flags indicate an area is protected by lifeguards and is being closely supervised. According to the association, a single flag can be posted to show an area where swimming is permitted and is being guarded, or could be used in pairs spaced apart to more clearly designate a supervised zone. 

Red and white beach flag 

Flags that are quartered into red and white mean one thing — it's time to leave the water. If one of these flags is up, swimmers should immediately evacuate the surf because of an emergency, such as water contamination, a water rescue or even marine creatures that have the potential to cause major harm, like sharks, are present. 

Sometimes, however, purple flags or double red flags are used when sharks are present. These guidelines set forth by the association are guidelines, and localities can use flags as they see fit. 

Black and white beach flag

Surfers will want to keep an eye out for black-and-white flags. These flags, which have those colors quartered, are used when an area is designated only for surfboards and other nonpowered watercraft. Swimmers need to stay out of the areas to prevent injuries or other issues. 

Yellow beach flag with central black ball 

If surfers and paddleboarders see this flag — a yellow rectangle with a large black circle in its center — it means that surfboards and other nonpowered watercraft can't be used at all. 

Orange windsock

Windsocks are a long-used scientific tool, but at the beach, they can be life-saving. If an orange windsock is up, not only can you see the direction the wind is going and see how forceful it is, but it also indicates that it's not safe for inflatable objects to be used in the water, the association says. This can be particularly important for those who use inflatable rafts or swimming rings for their children. 

A beach warning reminder

While these are the flags used by the association, some areas may use flags slightly differently. Florida, for example, has a uniform statewide system that only uses purple, green, yellow, red and double red flags. Before going to the beach, it's always best to search for the beach flag system for that specific location, or even ask lifeguards you find on the sand and check the local weather for any storms or currents that are expected. 

In:
• Storm
• Science of Weather
• Shark
• Shark Attack

Li Cohen

Li Cohen is a senior social media producer at CBS News. She previously wrote for amNewYork and The Seminole Tribune. She mainly covers climate, environmental and weather news.

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