From AMS Glossary
A board that holds graph paper on which is plotted information obtained from a pilot-balloon observation.
The vertical scale of the board, engraved on a metal plate, is an altitude scale indicating the assumed height of the balloon at the end of each minute of flight. The horizontal scale is engraved on a movable T-square and is calibrated in units of direction and speed. The result is a curve of wind speed and direction against height.