Broad areas of relatively weak sink normally lie between evenly spaced thermals. The old Soaring Flight Manual proposed a sink rate approximately one fifth of the average lift. (For example, if thermals are averaging 1,000 fpm, the inter-thermal sink rate would be about 200 fpm.)
However, in an article for SOARING Magazine, Dick Johnson’s data from a number of test flights indicated ten percent, or 100 fpm sink between 1,000 fpm thermals.
This might seem a large disparity, but look closer. Johnson acquired his test data by flying a sailplane at one speed only, and the results describe the sink rate of the air plus the sink rate of the glider at best L/D speed. Of course we should always fly faster than best L/D speed in even weak sink (further increasing the total rate of descent), to maximize glide. So if we make the proper adjustment in speed-to-fly, our total sink rate will be significantly greater – something closer to what the Soaring Flight Manual suggests. Either way, inter-thermal speed should be at the very least a little faster than best L/D speed – and still more if we expect strong lift ahead.