Dear All, We are delighted to present the MaPSS Seminar topic of Monday 01/05; please see the abstract below. **This Semester the Seminar will always run on Monday, at 5:00pm in 535A** Following the talk, there will be pizza on offer. Speaker: Brent Giggins (Sydney University) Title: How to Predict the Weather - An introduction to Chaos, Data Assimilation and Ensemble Forecasting Abstract: In 1961, Edward Lorenz discovered that the atmosphere has a finite limit of predictability, even if we have a perfect model of the atmosphere and the initial conditions are known almost perfectly. This was a catalyst for the fields of numerical weather prediction and chaos theory, which is the study of dynamical systems that exhibit sensitive dependence to small perturbations in the initial conditions - often referred to in popular culture as the "butterfly effect". In this talk, we will examine what it means for a dynamical system to be "chaotic" and look at ways to characterise chaotic behaviour both globally and locally. We will look at this through the context of weather and climate forecasting - the main example of chaotic behaviour in natural systems - and summarise the basic components needed for numerical weather prediction. In particular, we will examine the topics of Data Assimilation and Ensemble Forecasting in generating optimal initial conditions for a weather or climate model and consider the practical problems that arise. Finally, we will look multi-scale dynamical systems and illustrate the challenges of forecasting over multiple time and length scales. Supervisors, please encourage your students to attend. Thanks, MaPSS Organizers