The interaction of aerosol with clouds leads to the so called aerosol indirect effect. As the size distribution and chemical composition of aerosol entering clouds change, the structure and lifetime of cloud systems changes. Some of these changes in aerosol properties are man made as the amount and chemical composition of emissions change. These effects are the largest uncertainty in quantifying future climate change. A particular gap in our knowledge is to understand the way in which aerosol particles affect the properties of ice, and mixed ice and liquid water, clouds. Among other substances such as dust and organic material, it is thought that black carbon-containing aerosol (soot) maybe an important ice nucleus in some conditions. Black carbon is also very important in affecting the radiative properties of cloud as it acts as an absorbing material for solar radiation. In determining its effectiveness both as an ice nucleus and as an absorber it is very important as to whether it is incorporated into cloud particles or not. In this project we aim to address the issues of the effectiveness of a wide range of aerosol types as the centres on which liquid droplets form and also their ability to act as centres to initiate the formation of ice crystals in clouds below 0C by using a mixture of techniques. Airborne insitu measurements of the type number and size of ice particles and water droplets in clouds will be made and the aircraft will also make measurements of the size distribution and chemical composition of aerosol, particles below and above the cloud layers. Remote sensing using the Chilbolton Radar and Lidar, whilst the aircraft is flying in the vicinity will give a simultaneous view of the larger scale cloud structure and aerosol properties below cloud. Ground based measurements of aerosol properties will give a more detailed view of the aerosol properties that can be achieved on the aircraft. Studies in a cold chamber in Germany will enable us to measure the ice nucleating properties of a very wide range of particles that occur in our detailed studies and more generally world-wide. In order to get a more general view of the cloud structures that occur longer-term measurements will be made with the Chilbolton radars and Lidars coupled with ground based measurements of the aerosol properties. Detailed modelling of the cloud formation on the aerosols, particularly the ability of the aerosols to produce water droplets and ice crystals will link all these observations and the cold chamber studies. The ability of the model to reproduce the detailed structure of the cloud, the ice crystals and the liquid water together with the precipitation will be tested by comparing the model predictions with the observations from the aircraft and radar. The model input will be the atmospheric structure (vertical profiles of temperature, wind and humidity) and the aerosol measured below cloud. Having incorporated our new understanding of the links between aerosol and cloud properties in our detailed field programme into our cloud model, this model will be used, in conjunction with satellite data, to better understand the influence of aerosol on mixed phase (ice and water clouds) globally. In this way we will establish the first global treatment of ice nuclei based on aerosol properties. We will establish remaining uncertainties in this issue. This will be a step towards a fuller treatment of the role of aerosol particles on ice and mixed phase clouds.