Science, Tech, Math › Science Understanding Weather Warning Flags Share Flipboard Email Print Science Weather & Climate Understanding Your Forecast Storms & Other Phenomena Chemistry Biology Physics Geology Astronomy By Tiffany Means Meteorology Expert B.S., Atmospheric Sciences and Meteorology, University of North Carolina Tiffany Means is a meteorologist and member of the American Meteorological Society who has worked for CNN, the National Oceanic and Atmospheric Administration, and more. our editorial process Tiffany Means Updated September 01, 2018 Have you ever visited the coast or lake shore and noticed red flags posted along the beach or waterfront? These flags are weather warnings. Their shape and color indicate a unique weather hazard. The next time you visit the coast, make sure you know what each of the following flags mean: Rectangular Red Flags Lyn Holly Coorg / Getty Images A red flag means that high surf or strong currents, such as rip currents, are present. Notice double red flags? If so, you'll have little choice but to avoid the beach altogether, since this means the water is closed to the public. Red Pennants David H. Lewis / Getty Images A single red triangle (pennant) symbolizes a small craft advisory. It is flown whenever winds of up to 38 mph (33 knots) are expected to be a danger to your sailboat, yacht, or other small vessels. Small craft advisories are also issued when sea or lake ice exists that could be hazardous for small boats. Double Red Pennants Bryan Mullennix / Getty Images Whenever a double pennant flag is hoisted, be warned that gale-force wind (winds of 39-54 mph (34-47 knots)) are forecast. Gale warnings often precede or accompany a hurricane watch but can be issued even when there's no threat of a tropical cyclone. Rectangular Red and Black Flags Logan Mock-Bunting/Getty Images A single red flag with a black square center signifies a tropical storm warning. Whenever this flag is raised, be on the lookout for sustained winds of 55-73 mph (48-63 knots). Double Rectangular Red and Black Flags Joel Auerbach / Getty Images University of Miami sports fans will no doubt recognize this next flag. Double red-and-black-square flags indicate hurricane-force winds of 74 mph (63 knots) or higher are expected to impact your forecast area. You should take precautionary measures to protect your coastal property and your life! Beach Warning Flags Matt Cardy/Getty Images In addition to flying weather flags, beaches follow a similar practice that makes visitors aware of water conditions and advises guests whether or not to enter the ocean based on those conditions. The color code for beach flags includes: Green flags are an "all-clear" and symbolize that the risk of hazards is low and it's safe to swim. Yellow flags indicate moderate surf. You'll typically see these when ocean conditions are rough, but not life-threatening. Purple flags are flown when dangerous marine life (jellyfish, sharks, etc.) have been spotted. They indicate you should exercise caution while in the water.Red flags are the most serious of all beach flags. They signal a serious hazard. Unlike weather flags, the shape of beach flags doesn't matter — just the color. They may be triangular in shape or in the classic rectangular shape.