An easterly tip jet event off Cape Farewell, Greenland, is described and analysed in considerable detail. In Part I of this study (this paper) comprehensive aircraft-based observations are described, while in Part II of this study numerical simulations and a dynamical analysis are presented. The easterly tip jet of 21 February 2007 took place during the Greenland Flow Distortion experiment. It resulted through the interaction of a barotropic synoptic-scale low pressure system in the central North Atlantic and the high topography Of Southern Greenland. hi situ observations reveal a jet core at the coast with peak winds of almost 50 m s(-1), about 100-800 m above the sea surface, and of 30 m s(-1) at 10 m. The depth of the jet increased with wind speed from similar to 1500 m to similar to 2500 m as the peak winds increased front 30 to 50 in s-1. The jet accelerated and curved anticyclonically as it reached Cape Farewell and the end of the barrier. The easterly tip jet was associated with a tongue of cold and dry air along the coast Of Southeast Greenland, general cloud cover to the east. and cloud streets to the south of Cape Farewell. Precipitation was observed during the low-level components of the flight. The very high wind speeds generated a highly turbulent atmospheric boundary layer and resulted in some of the highest surface wind stresses ever observed over the ocean.