Humanity spends an increasing proportion of its time interacting online, yet-given the importance of social media to human welfare-the external factors that regularly shape online behavior remain markedly understudied. Do environmental factors alter rates of online social activity? We conducted two large natural experiments to investigate how worse weather conditions affect social-media use in the United States, analyzing over 3.5 billion posts from Facebook and Twitter (now X) between 2009 and 2016. We found that extreme temperatures and added precipitation each independently amplified social-media activity, effects that persisted within individuals. Compounded weather extremes produced markedly larger increases in social-media activity. Days colder than -5 °C with 1.5 to 2 cm of precipitation elevated social-media activity by 35%, nearly triple the surge seen on New Year's Eve in New York City. Our study highlights that environmental conditions play a critical-but overlooked-role in shaping digital social interaction.