Figure 7. Atmospheric plane slices from NARR at 500 hPa at 18:00 UTC (a) 21 August 2013 (b) 30 August 2013. Vertical Cross sections at 90˚ W, 20-60˚ N 18:00 UT (c) 21 August 2013 (d) 30 August 2013. Blue contour lines show Potential Vorticity (PV) 106 PVU, Relative Humidity (RH) below 30% is shaded from light to dark, with darker shades representing the dryer air. The black lines on (a) and (b) are pressure height contours in m. While, the black lines on (c) and (d) represent potential temperature (isentropic surfaces) in K. Biomass burning sources tend to have low PV and can have moderate RH levels for pyroCbs, while stratospheric air masses have high PV and low RH. The blue dashed line PV contour (1.5 PV) shows the approximate boundary between stratospheric and tropospheric air masses. Arrows (green) show air-mass transport patterns from Stratospheric-Tropospheric Transport (STT) and from Biomass Burning (BB).