Drag the circles on the chart, or use the − / + buttons below it. The chart shows the day at its peak, in the afternoon. At your take-off time the ground is not yet fully heated, so thermals start cooler: that is the orange dotted line, “Thermal at 12:45” in the legend.
• Axes: pressure in hPa on the left, height in metres above sea level on the right. Slanted lines are isotherms (0 °C brighter). Brown dashes are dry adiabats, which a rising thermal follows below cloud base. Blue dots are moist adiabats, which it follows inside the cloud. Violet dots are lines of constant mixing ratio in g/kg.
• Cloud base is where the thermal’s dry adiabat meets the mixing-ratio line from the ground dew point. Cloud top is where the thermal is no longer warmer than the surrounding air. If the thermal stops before it reaches the condensation level, the day is blue: thermals without cumulus.
• Moisture: below cloud base (or below the inversion, if that is lower) the air is well mixed, so the dew point (green) follows a line of constant mixing ratio up from the ground dew point. Higher up it eases toward the value you set for the air above the inversion: drag the green handle left or right there to set how dry the air aloft is.
• What to look for: a small gap between red and green means moist air and more cloud. Moist air trapped under a strong inversion spreads the cumulus out into a layer. Moist air above a weak or no cap lets the cumulus grow into towers and showers. A large gap means dry air and small, separate cumulus. A strong wind with a clear inversion lines the cumulus up into streets.