Reading the sounding
When looking at the sounding, there are many things we can predict according to the flying conditions for paragliding. We will try to highlight the most useful ones, the way we see it. Will there be clouds? Will there be stable or unstable conditions? Is it going to be cumulus clouds? What will the cloud base be and how high can we expect the clouds to be? Is there big danger of thunderstorms/ cumulonimbus? These are questions to ask you self when you are planning the goal for today's flying. Remember again, these forecasts are only forecasts and not how it definitely is going to be. But in general, it is the best tool for qualified guess, and it help you to know what you should be looking out for this particular day.
Will there be clouds?
The green line shows what is the dew point temperature at different heights. This temperature is dependent on the vapor content of the air. The green line show at which temperature the relative humidity is 100%, or which temperature the condensation starts. The red line show you the actual temperature at different heights at the same spot. Of this follows that when the two lines are close together, there will be 100 relative humidity, or clouds.
If the lines are close to each other high up, this means you can expect cirrus clouds. If the are close to each other over a bigger area, you can expect thicker cirrus and less radiation from the sun, and no thermals.
If the lines are close to each other in the lower part, it is likely that it will be overcast. The separation between the lines in the bottom should be at least 2 degrees.
Stable or unstable?
The temperature gradient from the ground has to be bigger than the adiabatic cooling to make the air unstable. This means that the air has to be more than 1 degree colder 100 meter above the ground. It it is not, the air is stable and there will be no thermals for us to use. This we can read from the very lower part of the sounding. If the red line, from the lower end, is laying more to the left than the curve for the dry adiabatic cooling, it means that the air will be unstable. You can expect thermals. The more over to the left the red line goes, the more unstable and stronger thermals you will get.
Inversion is when the red curve is leaning over to the right. In this sounding that is the case between 750 and 700 mbar. We have learned the definition of inversion to be that the temperature increases with the height. In this case the temperature is still decreasing with the height, but the curve is leaning more to the right than both the dry and wet adiabatic curve. Even if it is outside the definition, we still call it an inversion, because it matters to our use. To be a true inversion it had to lean more over to the right than the iso temperature lines.
To have cumulus formation on the sky we need to have rising air on an unstable day. The humidity in the air on the surface has to be high enough so the temperature for condensation or 100 % relative humidity can be reached.
1. When a parcel of hot air is rising the temperature decreases and follow the line for dry adiabatic cooling.
2. The humidity (g/kg) in the parcel is the same and follow the line for the mixing rate. When it crosses the line for adiabatic cooling, it reaches the pressure and temperature needed for condensation. This is where the cloud base is expected to be.
3. In this case the cloud base will be at 780 mbar or about 2200 masl. There would be cumuluses.
4. If the humidity this day was only 10 g/kg, the thermal would stop where the line for dry adiabatic cooling crosses the red temperature curve. The thermal would then have the same temperature as the surrounding air and loose it potential to rise further. The thermal would stop and never reach to cross the mixing rate of 10 g/kg. There would be no cumuluses.
How high will the clouds be?
When the parcel of air from the ground reach the crossing point between the dry adiabatic cooling (1) and the mixing ratio line (2), the further cooling will follow the line for wet adiabatic cooling (5). As long as this line is to the right of the temperature curve, it means that the thermal has more potential to rise further. In this case it is big possibility for very high clouds, because there is nothing to stop the building of the cloud. The possibility for cumulunimbus is big and you can expect thunderstorms.
If the inversion was stronger (more over to the right with the temperature curve) or the humidity in the air on the surface was a little less, the line for wet adiabatic cooling (5) would be crossing the temperature curve. At this height the rising air would have the same temperature as the surrounding air, and the thermal would stop. The top of the cloud would be a little higher than this, because of the mass movement.
Is there big risk for thunderstorms?
Together with strong wind, thunderstorm is the highest risk to fly in on a day that looks great else. So it is nice to know what the possibility for thunderstorms are by looking at the sounding.
6. The hatched area is the space between the temperature curve and both the dry and we wet adiabatic cooling lines that the parcel of air from the ground follows. If the area is to the right of the temperature curve it gives the air a higher potential for thunderstorms. If the hatched area is to the left of the temperature curve, the air has potential to prevent thunderstorms. Sometimes the hatched area is on both sides, like when you have an inversion, the hatched area will be to the left over the inversion. Then it depends on which is the biggest area, left or right.
This potential can be calculated and is know as Convective Available Potential Energy, or just CAPE. It can be looked up in a meteogram and you can see the value of this potential.
If the CAPE is 100 J/kg or more you should be aware. In Drama the possibility of thunderstorm are high if the CAPE is 700 J/kg or more. But for other locations the value may be different. For this particular sounding the CAPE was 1600 J/kg, but it turned out to be a nice flying day. Anyway it may be nice to fly before the clouds too high. The air is unstable and the XC potential is big. But we need to be more aware of the clouds and be prepared to land when it is time.