The U.S. Coast Guard called off its search Wednesday for three missing men, including two NFL players, whose boat capsized off the coast of Florida on Saturday. How does the Coast Guard decide when to give up a search?
When there's no chance that the missing person is still alive. If someone vanishes at sea, the Coast Guard appoints a mission coordinator to run the search and rescue. In addition to organizing planes, helicopters, and boats, he has to keep tabs on the missing person's statistical odds of survival. If days pass and the person is still lost—and the search team has looked everywhere—it's his job to call it off. (They never officially give up until they find a body—they just downgrade the case's status to "Active Search Suspended, Pending Further Developments.")
To determine someone's odds of survival, the Coast Guard uses software developed by the Canadian Defense Department called the Cold Exposure Survivability Model. Plug in various factors—water temperature, air temperature, the person's height and weight, garments worn, time of disappearance, access to flotation devices—and the program tells you how long the person is likely to stay alive. In general, someone floating in 50 degree water can only survive four hours (PDF). In 65 degree water, which was the temperature off the Florida coast this week, it takes the average person as many as seven hours to lose consciousness. If the water temperature is above 70 degrees, he could survive for days—that is, if he doesn't die of thirst or exposure first. (The CESM can also calculate the odds of survival if someone is lost on land in cold weather.)
Mission coordinators must also evaluate the thoroughness of a search before instructing would-be rescuers to give up. To maximize the probability of success, the Coast Guard uses complex search methods to cover as much area as possible—for example, planes might zigzag across a rectangular area or cut V-shapes over a circular area. To search underwater, a diver will often trace a spiral around a single buoy; or if there's more than one diver, they trace concentric circles. In general, the wider the sweep and the more times the pilots and divers cover the area, the higher the chance of detection. (If you want to get deep into the math, click here.)
Once the coordinator decides to give up, the team usually waits another day before the official suspension, since they like to notify the next of kin a day in advance.
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Explainer thanks Jack Frost of the U.S. Coast Guard.
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