Figure 6 - Sounding. Plotted sounding from Oklahoma City, OK at 7 AM CDT, 8 June 1974. Horizontal lines represent height in pressure coordinates (millibars, or mb); diagonal lines represent temperature. Heavy solid lines show the vertical profile of observed temperature (right) and dew point (left). The blue line shows the temperature that a parcel of surface air would have if it were heated to about 38 degrees C (100 F, the forecast high that day), and then lifted.
This is a typical loaded gun sounding. A temperature inversion exists near 850 mb; the cap is represented by the warm layer above it, wherein the parcel would be cooler than the surrounding air (red area). Above the cap, the parcel would be warmer than the surrounding air and thus would accelerate upward (i.e., instability). At these levels, (above about 690 mb), positive area is seen as the green area. This area is related directly to the convective available potential energy or CAPE. The Lifted Index (LI) is shown by the temperature difference at 500 mb; in this case, it would be about minus 6. The convective temperature is found using the dotted lines; surface air would have to heat of about 43 C (109 F) to rise above the cap. This value assumes no subsequent changes in the sounding - a bad assumption on this day since a tornado touched down at the location of the sounding later that afternoon, despite surface temperatures in the 90s.