Martin Brenner's, Pilot Balloon Resources 

Introduction to Pibal Observation

Observations of pilot balloons are made for the purpose of determining wind direction and velocity at various altitudes. The procedures for setup, operation and data reduction of a single theodolite station are covered in the following sections. Observations are made by following the ascent of a small rubber helium or hydrogen filled balloon with a balloon theodolite. The balloon is inflated with an amount of helium or hydrogen so as to rise at a known rate. The light weight versus size of the balloons allows them to be transported by, and accurately reflect the speed and direction of the winds present at the altitudes that they climb through. The angles of azimuth and elevation from the theodolite station to the balloon are observed and recorded at specified intervals of time, usually each minute. 

The term elevation denotes the angular height of the balloon above the theodolite with 0 degrees being level and 90 degrees directly above. The azimuth is the horizontal relationship to true north with 0 or 360 degrees being true north, 90 being East, 180 South and 270 being West. The recorded angles along with the balloon’s altitude make possible the construction of a horizontal projection of the balloon’s movement and hence the wind direction and velocity at various altitudes.

Pilot-balloon observations can be made with by single-theodolite and double-theodolite methods. In single theodolite methods either a theoretical ascent rate is used calculate the height of the balloon at the observation intervals or using the "tail" method where a tail of a known length is attached to the balloon. With this method a theoretical ascent rate is not used in the calculation. For even greater accuracy set of observations can be made utilizing two theodolite stations at the ends of carefully surveyed baseline.