The SCAR Action Group on Antarctic Clouds and Aerosols (ACA) aims to organise an international large scale campaign to investigate clouds and aerosols in Antarctica through a series of special observing periods when intensive ground-based measurements would be made at the same time as in-situ measurements using instrumented aircraft.

The Intergovernmental Panel on Climate Change (IPCC) has identified the indirect aerosol effect as the biggest uncertainty in the Earth's climate system. For this reason in many regions worldwide great efforts are being made to measure aerosols and cloud microphysical properties and the associated effect on the climate. However, in the Antarctic very few measurements have been made of cloud microphysical properties. Climate model representations of the Antarctic climate rely on parameterisations of clouds and cloud properties that based on mid latitude observations and these are likely to be wrong -for example the number nuclei available for the formation of cloud particles will be very different in the pristine air over the Antarctic.