Afterdrop: “A further cooling of core temperature [that] occurs after the victim is removed from the cold environment.” (Source: Hypothermia.org.) Also after-drop.
From the Open Water Chicago website:
When you first enter the water and begin your swim, your body reacts by constricting the peripheral blood vessels in your arms and legs. This helps prevent heat loss by consolidating your body heat into your core. And as long as you continue with your physical activity, you will easily preserve a stable temperature.
However, once you end your swim and exit the water, your body sends your blood back to the skin to “warm up.” Because your skin is very cold at this point, your blood actually gets colder and is then recirculated back to your core. In essence, your core body temperature actually decreases during this rewarming period in a phenomenon known as the “afterdrop.”
Afterdrop is part of the specialized lingo of physicians and emergency workers, and it’s also familiar to cold-water swimmers like me and other members of San Francisco’s Dolphin and South End clubs, where we swim year round, without wetsuits, in San Francisco Bay. Water temperature in our part of the bay ranges between 50°F/10°C (January) and 66°F/19°C (September, if ever). To prevent afterdrop, swimmers and boaters are advised to go horizontal; warm up gradually, preferably in a sauna or swaddled in blankets; and avoid drinking alcoholic beverages. (Some hypothermia resources recommend continued exercise. The truth, according to a physician I consulted, is “No one knows.”)
Dolphin Club member Dr. Thomas J. Nuckton, a specialist in pulmonary and critical care medicine, conducted a study of 11 Dolphin Club members who had swum from Alcatraz Island to San Francisco’s Aquatic Park on New Year’s Day in 1998. (It’s an annual tradition.) His findings, published in The American Journal of Emergency Medicine in 2000, included the observation that 10 of the 11 swimmers had experienced core afterdrop of at least 1.8°F. Nuckton also tells the story of 16 Danish fishermen who were rescued after spending an hour in open water; although they were able to climb aboard the rescue vessel, all of them later died of hypothermia.
The OED’s earliest citation for afterdrop (spelled after-drop in the dictionary) is a 1945 British report: “Rewarming must be rapid to cushion the ‘after-drop’ in temperature, which continues to fall for 10 to 15 minutes after the subject is removed from the cold water.” (That is, rewarming must commence quickly; it’s now considered risky to rewarm a patient too rapidly.)
The English word has a grim history, according to the OED: “Originally after German Nachsinken (1942 in a typescript document reporting the results of experiments on live subjects at the Dachau concentration camp, later used as part of the Nuremberg trials).”