ABSTRACT: Modern weather forecasting is a multistage process. Data flow electronically into the National Center for Environmental Prediction where computers simulate the weather using the old familiar laws of physics and chemistry taught in junior and senior high school science courses. Meteorologists make these laws into forecast equations which use the data to make a forecast of the atmospheric flows for a time step of a few minutes. We then use these forecasts as data for the next forecast. Repeating the steps many times, the simulations produce useful results which are guidance for the forecasters to use in making the final weather forecasts. The cooling of a coffee cup is a useful classroom or laboratory exercise which can show both how accuracy in a very simple forecast equation is affected by the length of the time step and some of the potential calculation errors which can occur.