Algor mortis

Algor mortis is the fall in body temperature that begins once death halts the metabolism that normally produces heat. The body gives up heat to its surroundings by conduction, convection, radiation and evaporation until it reaches the temperature of the environment; in a very hot place it may hardly cool at all. Core temperature is usually measured in the rectum and compared with the temperature of the scene.

Cooling does not follow a straight line. Core temperature often stays nearly level for a while after death, a lag called the postmortem plateau, then drops more quickly, and finally slows as it nears the surrounding temperature, tracing an S-shaped curve. Simple rules that assume a steady loss of about 1.5 °F (0.8 °C) per hour are therefore unreliable. The rate also depends on body size and fat, clothing and coverings, air movement, contact with cold surfaces, immersion in water, and whether the person had a fever or hypothermia when they died.

Even so, body temperature is one of the more useful clues during roughly the first day, before the body approaches room temperature. Henssge's nomogram, the most widely taught method, combines body weight, ambient temperature and correction factors to give an estimate with a range of several hours, which is then checked against livor mortis, rigor mortis and other findings.