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Two-dimensional (z,t) streak model. The streak is the product of the in-plane trajectory and the in-plane residence time, σt = σxy / |vy|. If σt is small (top row), the in-plane residence is small and thus the out-of-plane speed is fast. If σt is large (bottom row), the speed is slow. Sign ambiguity in vy arises because σt is insensitive to the sign of vy.
