http://www.slate.com/articles/health_and_science/science/2015/05/_spider_rain_australia_ballooning_event_mystery_causes_arachnid_sky_video.html

In the Southern Tablelands region of Australia, a “ballooning” event has seen millions of spiders riding the wind and creating their own arachnid storm. For reasons that are not entirely clear, the spiders in question crawl to the highest point they can find, release strands of silk, then launch themselves into the air.