A wetsuit is a neoprene insulation suit made for warmth and protection in watersports. It functions by trapping a thin layer of water between your body and the suit. This layer of water is warmed by your body which prevents you from losing too much heat while in the water. Water molecules conduct energy (heat) 25-40 times faster compared to air molecules. For example, on a 60 degree day you probably feel comfortable outdoors with jeans and a shirt whereas you will probably start to shiver within minutes while swimming in the same temperature water.
Wetsuits are not meant to keep you entirely dry. Neoprene is made of small closed cells that are filled with air which provide insulation against cold water by trapping heat in. The thicker the suit's neoprene, the warmer the suit will be because it has more heat trapping insulation. It is important to research the water temperature (keeping in mind the different seasons and swells) in the region where you will primarily use your wetsuit. If the temperatures are cold enough to make your extremities go numb, think about using boots, gloves and hoods too.
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