KentuckyFC writes "Many types of small spider release threads into the air which then lift and carry them significant distances. Biologists have found them at altitudes of up to 4 km. The conventional thinking is that the threads catch thermal air currents which then carry them away but this does not explain how spiders perform their trick even when there is little or no wind. Now one physicist says the explanation is the atmosphere's natural electric field which has an average downward-pointing magnitude of 120 Volts per metre. He calculates that a strand of silk need only gain a negative charge of around 30 nanoCoulombs to lift a spider. That explains how the spiders take off on windless days, how they reach such great heights and how several strands can lift heavier spiders of up to 100 milligrams."