From AMS Glossary
A method of estimating horizontal divergence of air, using a triangular network of rawinsonde stations, devised by J. C. Bellamy in the late 1940s.
At any one altitude, the wind measurements at the vertices of the rawinsonde triangle are linearly interpolated along the sides of the triangle to find the average wind component perpendicular to the side. When these normal wind components are multiplied by the length of the respective side, summed over all sides, and divided by the area of the triangle, the result is the average horizontal divergence within the triangle. Note that this method is sensitive to wind measurement errors and to the spacing between rawinsondes.