Electricity is all around us. We use it in domesticated form to give us light and power, and we see it in the wild quite often in the guise of lightning. We also feel it now and then as a painful dart of static electricity.
Every atom comprises a nucleus at the centre, with a number of negatively charged particles, called electrons, whizzing around it. The electrons of some materials are capable of transferring their allegiance to another atom when stimulated by heat or friction. Atoms which gain electrons in this way end up with a surfeit of negative charge; atoms which lose electrons are left with a net positive charge.
This separation of electric charge by friction occurs all the time - by the normal movement of people, or as a result of vehicles travelling rapidly along the road. As a charge builds up, it often leaks away to earth unnoticed, or it may be gradually neutralised by contact with oppositely charged particles in the air. If, however, the material in question is an insulating material - one not capable of conducting electricity - the dissipation of the charge tends to be slow; each part of the object must be discharged independently, because electricity cannot flow from one part of it to another.
In recent years, synthetic materials have come to predominate over natural fibres and leathers, and synthetics are nearly always good insulators. So the charge which builds up as we slide across the car seat, perhaps, or shuffle on the office carpet to the filing cabinet, is unable to escape to earth; it discharges, often painfully with spark and crackle, when we touch a piece of metal, as two opposite charges try to neutralise each other.
The weather is important in determining the extent to which the static charge accumulates. The most important variable is relative humidity. Water is a good conductor of electricity, so in a moist atmosphere it is relatively easy for the electrical charge to leak away into the air. When the air is dry, however, the charge builds up, ready to follow the easiest direct path to earth when given half an opportunity.
People prone to such unpleasant surprises can alleviate the problem by wearing clothes and shoes of natural materials. Another ploy is to have a metal object like a key in your hand before you attempt to touch anything made of metal; as you stretch out towards the car door, you will see the spark jump from the key to the adjoining metal.