Cold spots and hot spots
Warm spots ....
In the picture above the arrows show the flow of radiative heat, which, unlike convection, travels in straight lines. At the top of the picture there is a hot surface radiating heat and causing a 'warm spot'; anyone standing there would feel heat coming from the direction of the hot surface.
Below there is a cold surface (such as an uncurtained window on a cold night). Because it is cold, any nearby warm body (like a human) will radiate extra heat towards it. The effect is a feeling of cold in that direction. It operates like a radiative heater in reverse. The effect could be interpreted, by someone standing in that area, as a 'cold spot' since there would be no obvious cause for it.
It is not a particularly obvious concept. The best way to think of it might be like a heater working in reverse. It is drawing heat away from the room. Luckily, it doesn't operate over a huge distance.
What is a cold spot?
People often report 'cold spots' in haunted locations. These are small areas (usually a lot smaller than a room) that feel significantly colder than the surrounding area. They are considered by some to be a sign of a ghost in the area. Some cold spots are always felt in the same place while others seem to appear and disappear at different locations.
When thermometers are placed in rooms where cold spots are reported they generally fail to register any drop in temperature. What is more, only certain people seem to feel these cold spots. Others can stand in the same place and feel nothing. Some people see this as a sign of a paranormal origin with some people more 'sensitive' than others. Others see it as a sign that cold spots are purely subjective. However, there is another possibility! There are natural phenomena that have a real physiological effect on someone without a change in the air temperature. Draughts are the obvious example but there are others.
Draughts and convection
Air is almost always on the move in a room, even with the door and windows closed. This is because the surfaces of some objects are at different temperatures to others. Heat will be exchanged between the objects in an attempt to equalise the temperature. This is done mainly through convection. This is an air flow whereby warm air (which is less dense) rises towards the ceiling, cools and drops back towards the floor.
Humid air is lighter than dry air. So, a draught may be generated in a humid room (particularly if it is also hot) if there is a way for air from a dry (and/or cool) room to reach it, maybe around the edges of a closed door (and even more so if a door or window is actually opened). This could apply particularly to kitchens or bathrooms.
There is obviously even more scope for draughts where the room connects to other areas eg. through an open door, window, hatch, fireplace or gaps in draught-proofing.
When moving air is in contact with human skin it will generally feel cool (windchill). This is because the moving air removes heat from the skin. It also cools by causing evaporation from the skin. This cooling will NOT show up as a temperature drop with a conventional thermometer.
There is another way where you might feel cold without any measurable change in temperature. Curiously, you might feel cold and damp in warm air conditions with a low relative humidity. In this situation moisture evaporates from your skin making you feel cool and clammy.
Never underestimate the power of suggestion. Tell someone that they are standing in the 'cold spot' and they may start feeling it! It is best not to tell anyone where the cold spots are supposed to be and then see if anyone reports one.
Some people suggest that the coldness of a cold spot indicates that heat has been abstracted for some paranormal process. If so, it is curious because heat is about the worst source of energy you could choose.
Radiative heat loss
Though it is less obvious than convection, cold spots can also be created by radiative heat loss.
When you stand directly in front of an electric fire or radiator, you will feel heat. Less well known is that people can LOSE heat in the same way. If you stand directly in front of a cold object, such as an uncurtained window on a cold night, you will feel colder. Your body is radiating heat in all directions. However, it will radiate more, to maintain its temperature, in the direction of cold objects. This additional loss of heat will be felt as cooling. Generally, you need to be quite close to a cool object to get the radiative loss. Like a heater, if there is anything between you and cool object, you may not feel the effect. Like convection, a conventional thermometer will not register this apparent temperature drop.
Measuring a cold spot
The most obvious cause of a cold spot is a drop in air temperature. So the first thing you need on a vigil is a thermometer. The ideal set up would be an array of small thermal sensors placed in a grid to precisely locate any cold spot.
To spot convection you'll need an anemometer to measure wind speed. Some instruments can measure windchill directly.
To measure the temperature of cold surfaces you can use a 'point and shoot' infra-red thermometer. However, be aware that there are various issues with these instruments that can cause inaccurate readings. For instance, different surfaces can appear to have different temperatures purely due to the colour they are painted. This is called emissivity. Also, most IR thermometers indicate the surface they are measuring with a laser dot. However, the area measured is wider than this and varies according to the distance of the surface.
For more information on the use of IR (laser) thermometers, see here.
You might also try using thermal imaging to measure cold surfaces.
© Maurice Townsend 2007, 2011