The Hypothermia Bar (also know as Body Temperature - or Exposure - Bar/Meter) is a feature introduced in Tomb Raider III. It's is one of the Status Bars. Its first occurrence is in the Antarctica Level. This light blue coloured bar appears when Lara jumps into Icy Water. It will start running down as long as Lara remains in the water. She will take heavy damage, ultimately leading to her death, if she does not leave the water quickly, once it's depleted.
There is a popular bug which will reset the hypothermia bar when you save and reload while in the water.