They sense and manipulate Earth’s electric fields:
When their silk leaves their bodies, it typically picks up a negative charge. This repels the similar negative charges on the surfaces on which the spiders sit, creating enough force to lift them into the air. And spiders can increase those forces by climbing onto twigs, leaves, or blades of grass. Plants, being earthed, have the same negative charge as the ground that they grow upon, but they protrude into the positively charged air. This creates substantial electric fields between the air around them and the tips of their leaves and branches—and the spiders ballooning from those tips.
Ed Yong, “Spiders Can Fly Hundreds of Miles Using Electricity” at The Atlantic
Hat tip: Ken Francis, co-author with Theodore Dalrymple of The Terror of Existence: From Ecclesiastes to Theatre of the Absurd