# The Naked Scientists Forum

### Author Topic: Why does the water level in my toilet fall during storms?  (Read 12276 times)

#### Keitho

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##### Why does the water level in my toilet fall during storms?
« on: 12/03/2009 16:30:02 »
Keitho asked the Naked Scientists:

I would like to know why the level of water in my toilet bowel is lower during
storms and periods of high winds? Is this the result of low pressure? If it is,
is the pressure differential enough to lower the water by one inch?
What do you think?

#### Don_1

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##### Why does the water level in my toilet fall during storms?
« Reply #1 on: 12/03/2009 14:37:11 »
It's likely to be the result of low pressure beyond the 'U' bend caused by strong winds having a sucking effect on the outside end of the downpipe. The amount of suction will depend on wind speed and direction and the cavity into which your downpipe opens.

#### paul.fr

• Guest
##### Why does the water level in my toilet fall during storms?
« Reply #2 on: 12/03/2009 14:41:22 »
Aside from air pressure, it could be due to the venturi effect
http://en.wikipedia.org/wiki/Venturi_effect

Oh, and I think you mean toilet bowl

Quote
The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section of pipe. The fluid velocity must increase through the constriction to satisfy the equation of continuity, while its pressure must decrease due to conservation of energy: the gain in kinetic energy is balanced by a drop in pressure or a pressure gradient force. An equation for the drop in pressure due to venturi effect may be derived from a combination of Bernoulli's principle and the equation of continuity.

The limiting case of the Venturi effect is when a fluid reaches the state of choked flow, where the fluid velocity approaches the local speed of sound. In choked flow the mass flow rate will not increase with a further decrease in the downstream pressure environment.

However, mass flow rate for a compressible fluid can increase with increased upstream pressure, which will increase the density of the fluid through the constriction (though the velocity will remain constant). This is the principle of operation of a convergent-divergent nozzle.

Referring to the diagram to the right, using Bernoulli's equation in the special case of incompressible flows (such as the flow of water or low speed flow of gas), the theoretical pressure drop (p1 − p2) at the constriction would be given by \rho(v_2^2 - v_1^2)/2.

The Venturi effect is named after Giovanni Battista Venturi, (1746–1822), an Italian physicist.

#### wolfekeeper

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##### Why does the water level in my toilet fall during storms?
« Reply #3 on: 12/03/2009 14:59:08 »
I asked the guy in a bathroom shop this once and he told me the answer:

"It just does."

About 1 second later I worked it out, based on that.

Basically, what's happening is that water is sloshing down mostly the sides of the bowl and viscosity between the flowing water and the largely stationary water that is in the center of the outlet pulls the centre down with it. Once it goes down a bit however, there's a head from the U-bend that pushes it back up again, so it reaches a rough equilibrium, depressed from its normal position.

If the u-bend was too shallow then the center would be carried over the u-bend by the viscosity and the airlock would break which would let sewer gases out; so that sets how high the u-bend must be, so it depends a bit on the flush rate as well.

There may also be reduction in the pressure on the down pipe due to the flow of water as well, but I think those effects are much smaller, the flow is mostly down the sides and the air is able to rush up the middle and keep the pressure largely at ambient (air has much lower viscosity that water, so there's little pressure drop).
« Last Edit: 12/03/2009 15:29:32 by wolfekeeper »

#### Don_1

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##### Why does the water level in my toilet fall during storms?
« Reply #4 on: 12/03/2009 15:09:34 »

Oh, and I think you mean toilet bowl

Oh Bugger, I missed that, but then maybe he knows what he/she meant & said it, perhaps he/she is having trouble with low pressure in the bowel during storms!!! If this is the case and it is due to external winds, I would suggest a strong pair of rubber undies might be of some assistance.

#### lyner

• Guest
##### Why does the water level in my toilet fall during storms?
« Reply #5 on: 12/03/2009 16:52:32 »
The reason is disgusting.
I mean the reason is dis.  - Gusting wind causes  bursts of low pressure on the top of the (delightfully named) stench pipe. The water sloshes over the back (weir) and is lost for ever so the level remains lower than it was before even when the pressure returns. You can get a resonance with repeated gusts which will take even more water out than the actual pressure difference might predict.
The stench pipe is there to stop the siphon effect which you would get with a closed system - not only  would you take too much water our of the toilet U bend but you could take the water out of a number of other U bends in the system too.

#### Keitho

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##### Why does the water level in my toilet fall during storms?
« Reply #6 on: 14/03/2009 21:41:28 »
Thanks for the spelling correction Don_1, if I did have a low pressure in my bowel during storms I would certainly need a stockpile of Depends and rubber undies.

#### Keitho

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##### Why does the water level in my toilet fall during storms?
« Reply #7 on: 14/03/2009 21:45:58 »
Thank you sophiecentaur, I like your simple and clear explanation best, but Don_1 has you beat with his joke about my bad spelling.

#### lyner

• Guest
##### Why does the water level in my toilet fall during storms?
« Reply #8 on: 14/03/2009 22:35:04 »
Your bowels are clearly very dear to you. You can't keep them out of your mind.

#### The Naked Scientists Forum

##### Why does the water level in my toilet fall during storms?
« Reply #8 on: 14/03/2009 22:35:04 »