Rain is certainly one of the meteorological quantities that most directly affects human life, much more than the other key quantities that characterize the weather from a physical point of view, e.g. pressure, humidity, temperature. On the one hand rain is important for every day life, in agriculture for example, but on the other, it is not well known, as it is so difficult to measure and to forecast quantitatively. The rain field is much more complex and more variable in time and space than, for instance, pressure or temperature, which, instead, can be measured with a much higher accuracy. The multi-fractal nature of the rain field is certainly very poorly characterized either by conventional instruments (rain gauges), by long-range weather radar or by new spaceborne radar like TRMM Precipitation Radar. These are the reasons why the EC is promoting projects related to rain aspects in its various forms, from the lack of it (droughts) to the excess of it (floods, erosions). In this context, the VOLTAIRE project is probably the most comprehensive up to date research and technological development project that has specifically been related to a systematic study of various representations of rain fields and rain-related topics. This special issue presents the results that have been obtained with VOLTAIRE. VOLTAIRE is the acronym of 'Validation of multisensors precipitation fields and numerical modelling in Mediterranean test sites'. It is a research and technological development project within the EU V Framework Programme. The papers deal with: - rain field characterization in complex environments such as mountainous terrain and the Mediterranean area, - comparisons between rain forecasting and rain observations, - comparison of rain cell characteristics, - comparison of data quality assurance schemes for ground and space radars, - algorithms that have been adopted or developed during the VOLTAIRE project for radar data quality control, - methods to generate three-dimensional high-resolution rainfall fields, - interpolation methods and data evaluation,

The need for better forecasting of the weather, especially for nowcasting extreme events, has driven the scientific community to continuously improve the spatial resolution of numerical weather prediction models. Although surface information is relatively well represented in time and space, free atmosphere information is still lacking.

For a long time, radiosoundings were the only source of information on the vertical variation of meteorological parameters like temperature, humidity and wind. More recently data from satellites and commercial airplanes have increased the amount of information available for assimilation by models. Additionally, important data is now provided by ground-based remote sensing systems.

The international COST 720 TUC experiment was organized to test the ability of ground-based remote sensing systems for measure under cold and stable weather conditions, including situations where fog was present. COST is an intergovernmental European framework for international cooperation between nationally funded research activities.

This special issue focuses on advantages and limitations of such automatic systems for measuring temperature and humidity profiles within the planetary boundary layer, as well as cloud characteristics. Data from two microwave radiometers, one cloud radar, one ceilometer and one wind profiler were analyzed and validated using radiosonde data, or compared with GPS and satellite measurements. A first attempt to integrate information from various systems to improve the overall accuracy of temperature and humidity profiles is then presented in this special issue.