Tuesday, February 21, 2006

Why does static electricity increase in cold weather?

I'm hoping you could give me some information about why static electricity increases in the cold weather?

The reason static is more problematic in cold weather is because of low humidity of the air. "Relative humidity" is the percentage of moisture held in the air compared to the maximum it could hold at that temperature. So 50% rh means the air has only half the amount of moisture it could hold. It turns out that static is promoted if rh drops below about 30%.

Cold air can hold less moisture than warm air. So cold outside air at say 0oC and 100% rh is taken into a building and heated up to make it comfortable. The relative humidity drops by one half for every 10oC rise in temperature - so if no moisture is added, the air will be 50%rh at 10oC and 25%rh if heated to 20oC. 25% is certainly dry enough to promote static electricity!.

The main factors conributing to static electricity indoors are floor covering and shoe sole materials, and furniture covers materials, and dry air conditions.

15 comments:

Anonymous said...

Thank you, doctor.

I have often wondered why static electricity increases so much in cold weather, because I get shocked much more often.

I can't believe I just typed my question into the internet, hit the search button, and you had already posted the answer!

J. Piel
Elkridge, MD

Anonymous said...

This doesn't completely answer the question. What is the relationship between humidity and static electricity?

S. Mallory
Auburn, WA

Anonymous said...

I have been in England for over a year but this is the first time in my life I have experienced an increase in static electricity in/on my body and also itchiness and spots that look like insect bites. Is there anything I can do to stop or lessen this.

Static Doctor said...

I have heard several reports that itchy spots like insect bites can be related to static electricity, but I have never seen such a case myself. It's likely that the static and the itchyness could be related to dry air conditions also. If you have some way of increasing humidity, try this. Having house plants can help.
Static levels are also usually caused by the floor covering material, so you could try using a rug or other floor cover of different material.

Static Doctor said...

To understand why static electricity is encouraged by low humidity, we need to understand that water conducts electricity very well. If the humidity is above about 30%, many materials will either have absorbed some moisture, or will have a very thin surface layer of moisture. The thickness of this layer increases with the amount of moisture present, and at low humidity is nearly absent. his moisture layer tends to conduct away static electricity as it is generated. So if the moisture layer is absent, static electricity is more free to build up.

C. Ochoa said...

Thank you Doctor. Are you truly like a Doctor with a degree in Physics and Electricity or is it just...how did you know this??
I would have thought that, using your own argument, since water conducts electrity very well, then a higher humidity would cause a higher conductivity because there's more water particles, thus more electricity would be conducted...I had the concept all backwards.

Static Doctor said...

Yes, I am a Doctor of Engineering. My first Degree was in Electronics and I worked as an electronics designer for some time, before doing a PhD in studies of ignition of sensitive materials by electrostatic discharges (ESD). (See my bio)

Just as water tries to find its own level, static electricity tries to move to neutralise its peaks and troughs (positive and negative high voltages) and return to earth potential. It can only do that if it can move - conductive materials allow the charges to move, and insulating materials prevent the charges from moving. So conductive materials tend to discourage build-up of static electricity and insulating materials (like plastics) encourage build-up of static electricity.

Anonymous said...

Great info! Thanks!

Nickname unavailable said...

I thought that it was just me! every time I stroke Julio ( my delinquent tabby cat ) I get a shock, and my hair is standing on end. I thought it was my shampoo.
It is -5 outside and warm as toast indoors. I am glad that it has been explained, and I am not abnormally electrically charged its just a weather phenomenon.
Jan

Anonymous said...

How about using a humidifier?

Anonymous said...

Can you show your experimentation with static electricity with its relationship yo humidity and dry air..

Anonymous said...

Anonymous,

I recently had shoulder surger for a spur and some burtsitis . I have been shocking my self even more than before Closing car doors, opening doors with keys, even basically touching someone. I feels like a burning going through my shoulder that brings me to my knees. I get weak and my arm goes num.

KB said...

Thanks for the explanation about humidity helping to conduct static.

I recently got a burr-style coffee grinder. I'd noticed a build up of static electricity among ground coffee beans with my old blade-style coffee grinder, but the burr-style produced far more. So much static, in fact, that dumping the ground beans into the coffee filter lead to an sort of airborne cloud of coffee particles that stuck to me, to the exterior of the coffee maker, and eventually landed on the counter.

The beans fall through the grinder and the grounds are caught by a little plastic box (with a hole in the lid).

I'd known all my life-- as far as I can remember-- that humidity would eliminate static, so I started exhaling gently into the box through the hole in the lid, filling the box with humidity, and waiting a bit until the static dissipated.

This morning I finally got around to trying to understand why this works!

I was very happy to find your explanation in the comments of this blog post!

Thanks again.

KB

Anonymous said...

Doc,

We are having a problem with the microwave breaker tripping. It is on a dedicated circuit. The microwave is brand new and even has been tested by multiple people and we have narrowed the problem down to maybe static electricity in the building? Could this be the issue?

Thanks,

Darin - SLC, UT

Static Doctor said...

Sorry Darin, I can't comment on this without knowing a lot more.