Thermometers are a very common tool among paranormal investigators. Most ghost hunters associate cold or warm spots to be associated with the presence of a ghost or spirit. It is important to note that this feeling in a great difference in temperature is not proof positive of a ghostly encounter. It is simply a piece of information about the environment that is up for interpretation by the investigator. Temperature drops don't automatically mean that a ghost or spirit might be present, but they can be used as an indicator that "something" is causing a significant and notable difference in temperature in a specific spot. There are two main types of thermometers used on ghost hunting expeditions.
First and most popular are infrared thermometers. These work off of the same principles that the infrared cameras use. The infrared thermometer simply emits a beam of infrared light that will measure the surface temperature of the object or other nearest surface it is pointed at. The beam bounces back a measurement to the handheld device and displays the temperature reading.
The caution here is that infrared thermometers only measure the temperature of "surfaces". Things like high humidity, a dense fog, or other conglomerations in the air have been known to affect or alter a measurement. This is where the application to ghost hunting comes in. Most professional ghost hunters theorize that infrared thermometers can show drastically different temperatures when a spirit or ghost is in the way of the thermometer and the surface it is pointed at. While ghosts or spirits are one possible explanation for such variations from the target surface temperature, it is always important to consider all potential causes. Infrared thermometers might also measure abnormal temperature spots on surfaces for other explainable reasons such as AC or Heating ducts blowing on an area of the wall or a person or other object leaning against the object being measured, thereby affecting its temperature even after the person is no longer in contact with the surface itself.
Open Air Thermometers
The other type of thermometer used on ghost hunting trips is your standard, open air reading devices. These thermometers measure the temperature of the immediate air. These types of thermometers make great portable tools to get a temperature reading in an immediate and specific location. Unlike the infrared thermometers, these will not be aimed at or attempting to read the temperature of a surface or physical object.
Ghost Hunting Store - Thermometers
TheGhostHunterStore.com carries a wide variety of both infrared and open air thermometers for use in ghost hunting.