For our purposes in soaring, the overall intensity of solar heating seems to affect the depth and power of shearlines.   When thermal conditions are weak or the convective layer does not reach above high terrain, mountains may impose outright barriers causing interaction between separate air masses to take place at lower levels, perhaps running straight across intervening valleys.   But when convection rises above the highest peaks, that is where the most powerful interaction will take place between stronger winds aloft.   In any case, once a shear exists increased solar heating can make it soarable even where it extends across level ground, far from any obvious geographical origin.

     Ponder the analogy of biscuits baking on a pan.   As they expand, their edges push together.   Growth from either side of a line between any two biscuits is directed toward that line, and because nothing can descend through the underlying surface, anything suspended between the two merging bodies can only go UP!   Yet even in perfect conditions the line will eventually end where there is no longer a material difference in the air or its movement on either side of that line.   This may occur as expansion ceases, or as a result of mixing… or at the edge of the pan (mountains).