“Afterdrop” is common after swimming in cold water; you get out and feel fine, and then you start to get colder, sometimes growing faint, shivering violently and feeling unwell.
Afterdrop is the phenomenon of your body temperature continue to drop even after you get out of cold water and into a warmer environment – so that you feel colder 10 or 40 minutes after you exit than you did in the water.
When you swim, your body shuts down circulation to your skin, pooling warm blood in your core. This process helps you stay in the water longer: with reduced circulation to your peripheries skin and sub-cutaneous fat is turned into a thermal layer, similar to a natural wetsuit – hence the wild swimmers’ term bioprene for fat.
But when you exit the water, the cooling process does not stop straight away. Even dry on the banks, this cold layer of skin and muscle continues to cool your core. You can lose up to 4.5°C from your core temperature (according to Golden and Tipton, Essentials of Sea Survival), bringing on shivering, hypothermia, or feeling faint and unwell.
For a while there was a popular urban myth that afterdrop happened as blood returning to the skin as you warmed up, and cooling as it travelled. In Winter 2021 Mike Tipton from the Extreme Environments Lab worked to educate the winter swimming community that that this is not what is happening: continued cooling on the banks is down to “conductive cooling” not blood flow.
The key to warming up and staying well is to warm up slowly and gradually.