Ever wonder how weather forecasts are so accurate? How predictions are made over days and weeks? How hurricanes and blizzards are forecasted? General circulation models (GCMs) are instrumental in weather forecasting. They are highly detailed grid-based simulations of weather that use atmospheric physics to predict events over hours, days and even further into the future. These models are commonly used to predict climate change over years, decades and centuries. GCMs have become more and more accurate as the physics of the atmosphere has become better understood. As computers have become more capable computationally, the models have become more accessible to the general public. Before they required a mainframe computer. You can now run them on laptops! In this module, we explore how GCMs work.