MORE THAN 12,000 public weather stations dot the US, which seems like a lot until you realize more than 180,000 people run their own weather-tracking devices from home. Linked together, these personal weather stations—each one a toaster-sized assembly of sensors mounted on a pole—provide the most accurate weather forecasts in the country. "Weather data at your house is usually different from the nearest airport or even from your neighbor's weather station down the street," says Kari Strenfel, a meteorologist at Weather Underground, a website that aggregates this citizen-collected data and presents it as hyper-localized forecasts anyone can access. "The more data we have feeding our weather models, the more accurate our forecasts can be."