Ambient Air Thermometer
Most ghost hunting groups use a digital IR thermometer to track "cold" spots. The theory is that paranormal phenomena are somehow able to extract energy from the air around them, by doing so they cause a "cold" spot to occur. By removing energy from around them, they effectively reduce the air temperature. In some instances, it is thought that negative entities (demons for example) cause the air temperature to go up. In both of these cases, a device is needed to measure the ambient air around the "cold" or "hot" spot. Standard IR (Infrared) thermometers focus on a solid surface to gain the temperature. In most cases, this would be a wall or some other inanimate object, not the air! An ambient air thermometer uses a probe to get the air temperature from a specific air space, thus allowing the investigator to determine the low temperature of the "cold" spot or the high temperature of the "hot" spot.
CORPS has found that the ambient air thermometer is the most accurate means of finding and documenting a reported "cold" spot while also getting a baseline of the regular room temperature.
Ambient Air thermometers are available online at places like TE Equipment and usually cost around $130. You'll most likely need to purchase the air probe as a separate attachment for around $30.