Weather apps are generally accurate for simple forecasts over the next few days; their accuracy can decrease when hazardous weather is expected. Most weather app forecasts are automatically generated from government weather models, which predict the weather by feeding current observations into mathematical equations that represent the physics of the atmosphere. Some apps systematically adjust model forecasts to account for known model errors, and a few have human meteorologists occasionally tweaking the forecast. For many U.S. cities, the top weather apps achieve an accuracy of 80 percent or higher according to Forecast Advisor, a website that grades the accuracy of temperature and precipitation forecasts for the next one to three days. Yet many weather apps provide forecasts 7 to 14 days in the future, even though specific weather forecasts have very little skill beyond 10 days or so. According to meteorologist Jack Sillin, the “illusion of precision” is the “most dangerous aspect of weather apps.”