A run of cloudy days in Paris led to the discovery of radioactivity
In 1896, Henri Becquerel was testing whether sunlight made phosphorescent uranium salts emit X-rays. Overcast skies forced him to store his crystals and wrapped photographic plates in a dark drawer for days. When he developed the plates anyway, they'd fogged clearly — the uranium was emitting radiation with no sunlight involved, revealing a wholly new phenomenon.