Mean (top) and anomalous (bottom) 200-hPa stream function for the last 30 days.  Contour interval in top panel is 10 x 10⁶ m²s^-1.  The non-divergent component of the flow is directed along the contours with speed proportional to the gradient.  Thus, high (low) stream function corresponds to high (low) geopotential height in the Northern Hemisphere and to low (high) geopotential height in the Southern Hemisphere.  Anomalies are departures from the 1971-2000 daily base period means.