Since the Icelandic volcano erupted the Met Office has been using its models to forecast how the ash cloud will move. It starts with observations of the ash from many sources including the state of the volcano, the amount and height of the ash being emitted from the Icelandic Meteorological Office, scientific balloons, research aircraft, satellite and ground-based radar systems. Met Office observers across the UK are also providing visual observations of the ash cloud. The Met Office model then disperses the ash by tracking model particles through a simulated atmosphere. The particle sizes vary depending on observations and are moved by 3D forecast winds that vary with time and space as well as random motion to represent atmospheric turbulence. The particles are only removed from the atmosphere and therefore the model by one of a number of physical processes, fall out due to gravity, settling on the surface, washout where the pollutant is 'swept out' by precipitation and rainout where the pollutant is absorbed directly into cloud droplets. From this forecast the Met Office produces charts using concentrations of ash deemed safe by the CAA. The forecasts are subject to the usual uncertainty as the weather, as well as the uncertainty of future behaviour of the volcano. So those homeward bound at the weekend need to pay homage to the Greek god of heavenly and earthly fire, Hephaestus who is also the Greek god of volcanic activity.