Increasing temperatures in the Arctic over recent decades have been interpreted as 'Arctic amplification' of the warming due to increasing concentrations of greenhouse gases. Understanding the mechanisms for such amplification has become a major research priority. Often the approach is 'event-based', focusing on the structure and frequency of 'cold-air outbreaks' (CAOs) and 'warm-air intrusions' (WAIs) which potentially exchange heat between the Arctic and the mid-latitudes. This paper describes results from a carefully planned observational campaign using the HALO high-altitude research aircraft, in which flight paths were planned in conjunction with weather forecast information so that certain air masses were sampled more than once, giving information on transformation of the air mass properties. Complementary model studies are used to allow interpretation of the observations and assessment of the model capability. This paper, beyond its specific conclusions, is, through its comprehensive combination of observations and modelling, likely to set the standard for future research on this important topic.