CAPE - Convective Available Potential Energy
Today we are in Governador Valadares, Brasil. From the sounding we can expect thunderstorms today, so we decide not to fly. It also indicates cirrus, so even if it is very unstable, the thermals may be very weak because of the cirrus. The cirrus may, however, prevent the over development from happen. But we don't think it will. So better stay on the ground and write this article. But remember this is only a forecast, it is no guarantee that this is how it really is going to be. The most uncertain thing about this, is how much humidity there will be in the airmass coming in to the area today. Variety in humidity may give big change in instability and clouds formation. But usually the sounding is correct, and make the best basic for our guess.
We will make many articles abut the sounding so you can learn how to read it. But it is a big story to tell. We will try to make it step by step.
We start with the simple explanation for todays decision. The lower end of the green line (the dew point temperature curve) is quite far to the right. This means there is lots of humidity in the airmass and the condensation will happen at a relative high temperature. The dew point is the temperature at which the water vapor present in the air would condense out. The next thing we see for today, is that the red line (the temperature curve) lays very much over to the left. These two things indicate that the airmass has a big Convective Available Potensial Energy (CAPE) or instability constant. The third thing, that tell us the possibility for cirrus clouds, is that the red and green curves are close to each other from 400 mb and up.
One thing to remember, is that it may be very good conditions for flying, despite of the forecast for thunderstorms. You just have to be extra aware of the clouds and be sure that you are able to land with a good margin before the conditions turn crazy. Many days it is also possible to make good flights after the storm, as well.