The Bernoulli Principle in a chimney
In the picture above, wind across a roof lowers the air pressure in a chimney. This, in turn, lowers the pressure in the room with the fire place. This might be enough to open a door that was not secured properly. The arrows indicate air movement.
Testing opening doors
You should look out for the following things when checking doors that open by themselves:
- was the witness falling asleep or waking at the time?
- did the witness actually see the door open or just find it open or hear it open?
- can the door be closed so that it appears closed but is not securely shut?
- if the door is not closed properly, how much pressure is required to open it?
- if there is a fireplace in the room, does the air pressure drop with the door closed?
Mysterious door opening
One of the phenomena reported quite frequently in haunting cases is doors opening by themselves. In some cases doors are found open having been left shut, though no one actually saw them open. In other cases, witnesses have claimed to hear the door opening (in the dark or from afar) or even caught sight of it moving. There are even reports of door handles being seen to move prior to a door being opened.
What is one to make of such reports? One obvious answer might be some form of hallucination, particularly if the witness is in bed and close to sleep. It could, for instance, be hypnagogic or hypnopompic hallucinations or possibly sleep paralysis.
But is there a possibility that, at least in some cases, the phenomenon is entirely objectively physical (see right)?
The Bernoulli Principle
As the velocity of a fluid increases, the pressure decreases. If that reminds you uncomfortably of school, Bernoulli himself, a Swiss mathematician and physicist, expressed it as an equation, which is even worse. The Bernoulli Principle is part of the reason why aircraft can fly.
If there is a flow of air (wind) across the top of a chimney, it will induce a reduction in pressure in the room below. This applies even if there is no fire alight in the grate. Thus on windy days there will be lower air pressure in rooms with fireplaces and this could open doors that are not properly closed (see right).
If there IS a fire alight in the grate, there will be an even bigger updraft caused by the hot gases from the fire, which have a lower density than the surrounding air (and so float upwards).
You should note what the weather was doing when such incidents of door opening were reported. Are they mostly on windy days or cold days (when the fire might have been lit)?
Draughts, caused by the Bernoulli principle or otherwise, can obviously cause doors to rattle. This can give the impression that someone, or something, is trying to open a door. You can imagine what report such a phenomenon might produce if it was heard by a witness lying in bed in a darkened room in the middle of the night. You should try rattling the door yourself to see what it sounds like.
I was recently helping investigate reports of a door that apparently opened by itself in a haunted room in a castle. The ill-fitting door could be closed to a point where it appeared firmly shut. Indeed, I did this myself before I even realised it was the 'haunted' door! Minutes later it opened again, by itself!
It was obvious, on inspection, that a draught had opened the door. The door touched the sides of the surrounding frame, without leaving any gap for air to flow around it, so that it jammed shut. This meant that air could not flow freely into the room, so obstructing the draught, which then opened the door. I repeated the process as an experiment with the same result. Only when I closed the door more firmly (firmer that was apparently necessary) did the latch actually click, after which the door no longer opened.
So why was a draught trying so hard to enter the room? A clue lay in a massive fireplace and huge chimney (unusually large compared to others in the building) in the room. Could it be the Bernoulli effect?
The wind blowing over the chimney pots on the roof above could have caused the air pressure to lower in the room below. This would cause a pressure difference between the (effectively) 'sealed' room below and the rest of the building. Air pressure could then have opened the door.
The difference in air pressure produced by the chimney was measured in that room and others nearby. The biggest pressure difference was found in the haunted room.
Rattling doors on video
In the Dover Castle investigation, a large closed door was observed, on video, apparently being rattled violently. Unfortunately, there was no video camera on the other side of the door though the investigators said the area was locked off at the time. A flag is seen to move just before the door is rattled though it seems unlikely a draught could have caused such violent rattling.
© Maurice Townsend 2006