Why does your hair rise when you take off a sweater?

On humid days, a very thin layer of moisture covers the sweater and hair, allowing accumulated charge to leak away gradually. When air is dry in autumn and winter, there is no easy escape route, so the charge remains. Even with the same sweater, it happens much more often in a heated winter room than during the rainy season. A simple rule of thumb: the better a day is for drying laundry, the more static you get.

Each hair acquires the same kind of charge. Like charges repel, so the strands spread in all directions to get away from one another. They are pulled toward the sweater while repelling neighboring strands, making your hair float up. This differs from a magnet and does not attract metal objects. The sting at your fingertip when you touch a doorknob after taking off clothes is the same phenomenon: charge accumulated on your body escapes all at once.

Static voltages are higher than you might think. According to figures used in the electronics industry, simply walking across a carpet on a dry day with 10 to 25% humidity can build up 35,000 volts on your body. Above 65% humidity, the same steps produce only about 1,500 volts. The number sounds frightening, but the amount of charge is tiny and flows away in an instant, so for a person it ends with a little sting. In fact, you usually need several thousand volts before you feel anything, so charge often builds up and drains away without you noticing.

Even a small increase in indoor humidity helps a lot. You can use a humidifier or hang laundry in the room. A little water or conditioner on your hair makes it settle immediately, and clothes rinsed with fabric softener also accumulate less charge. Anti-static sprays create a thin, electrically conductive layer on fabric so charge can flow away. Wearing a cotton layer under your sweater also helps, because less yarn rubs against your hair. At a gas station, though, where fuel vapor hangs in the air, even a small spark deserves caution. When filling up at a self-service pump, first touch the anti-static pad on the pump with your hand before opening the fuel cap.