The BOREAS HYD-04 work was focused on collecting data during the winter field campaign (FFC-W) to improve the understanding of winter processes within the boreal forest. Airborne remote sensing data (gamma, passive microwave) were acquired along a series of flight lines established in the vicinity of the BOREAS study areas. Ground snow surveys were conducted along selected sections of these aircraft flight lines. These calibration segments were typically 10-20 km in length, and ground data were collected at 1- to 2-km intervals. The development and validation of remote sensing algorithms will provide the means to extend the knowledge of these processes and states from the local to the regional scale.