The Glaister equation is a straightforward, historical formula used in forensic science to estimate the time elapsed since an individual died, a measurement formally known as the Post Mortem Interval (PMI). This calculation is one of the earliest methods investigators employed to provide an approximate time of death. The equation relies on the cooling of the body after all metabolic processes cease. By comparing the measured body temperature to a known standard, the formula converts the heat loss into a duration of time.
The Scientific Basis of Algor Mortis
The Glaister equation utilizes the physical phenomenon called Algor Mortis, a Latin term meaning “coldness of death.” Algor mortis begins immediately after death because the body loses its ability to regulate its own temperature, halting all heat generation. The deceased body then loses heat to its surroundings through various mechanisms, including conduction, convection, and radiation. This heat loss continues until the body’s temperature reaches equilibrium with the ambient temperature. Rectal temperature is typically measured because it is the most accurate representation of the body’s core temperature. Algor mortis is one of the three primary post-mortem changes, alongside livor mortis (discoloration) and rigor mortis (stiffening), that forensic investigators use to help narrow the window of death.
Decoding the Glaister Equation
The Glaister equation is a linear model that mathematically translates the body’s temperature drop into hours since death. The formula assumes a constant rate of cooling and is stated as: (98.4°F – Rectal Temperature) / 1.5 = Approximate Hours Since Death.
The baseline starting temperature used in the formula is 98.4 degrees Fahrenheit, which is the historical average considered normal for a healthy living human body. The measured rectal temperature is subtracted from this standard to find the total temperature drop, representing the total heat lost since death.
The constant divisor, 1.5 degrees Fahrenheit per hour, is the assumed rate at which a body will cool under typical conditions. Dividing the total temperature drop by this constant rate yields the estimated number of hours passed. This provides a quick estimate of the post-mortem interval, generally within the first 12 hours after death.
Why the Equation is Not Always Reliable
The accuracy of the Glaister equation is fundamentally limited by its assumption of a fixed, linear cooling rate of 1.5°F per hour. In reality, a body’s cooling curve is not a straight line but follows a non-linear, sigmoid-shaped curve. The cooling process often includes an initial plateau phase where the temperature drops slowly, followed by a period of rapid cooling, and finally a slow approach to the ambient temperature.
Environmental factors introduce significant variability that the simple equation cannot account for, causing the estimated time to become unreliable. The ambient temperature of the surrounding air is a major factor; a body in a cold, windy environment will cool much faster than one in a warm, insulated room. Furthermore, the presence of clothing or blankets acts as insulation, dramatically slowing the rate of heat loss.
Physiological factors related to the deceased person also affect the cooling process and compromise the formula’s accuracy. The initial body temperature at the moment of death may not have been the standard 98.4°F, especially if the person had a fever or hypothermia. Body mass is another influence, as individuals with more body fat have greater insulation, causing them to cool more slowly.
The equation completely fails to provide a meaningful result once the body temperature has reached equilibrium with the ambient temperature. Because of these many variables, modern forensic science rarely relies solely on the Glaister equation, instead using more complex nomograms and multiple indicators like rigor mortis and forensic entomology for a comprehensive time of death estimation.