Both Standard Atmospheres (1964 and 1976) define the tropopause as having a constant temperature of -57.5 °C and a thickness of either 14 km (1964) or 9 km (1976). As the graph shows, in summer the tropopause goes away over north Florida and the minimum temperature decreases to about -72 °C (quite a bit less than -57.5 °C - note that CO2 solidifies at -78.5 °C at 1 atm) If you zoom in and look at the surface temperatures, you can see the Greenhouse Effect in action. (Aug 13, 2000 shows this best.) Notice that the surface temperature is less than the atmosphere slightly above. These temperature inversions are common in the morning and typically go away by mid-morning. They indicate that heat is leaving the atmosphere and returning to the surface. To be clear A negative slope indicates that more heat is leaving the atmosphere toward space. A positive slope indicates that more heat is leaving the atmosphere toward the surface. A vertical slope indicate no gain or loss of heat.