What does adiabatic cooling mean?
A common misconseption is that the thermal will cool because it is surrounded by colder air on its way up from the ground. That is not the case. Actually, adiabatic means the direct opposite. The temperature in the thermal is not affected by the temperature of the surroundings. Adiabatic means that there is no heat exchange between the thermal and the air around it. This is almost the truth. There is some heat exchange, but since the thermal has a big mass, and the air is a bad conductor for heat, the heat exchange is negligible.
Adiabatic cooling must be caused by some other process. Let's look at some physical laws first.
The energy conservation law say that the amount of energy is constant, you can not make energy or consume energy, but the energy can change between different kinds of energy. Heat is one kind of energy that is important for thermals. We know that, since we get the heat from the sun.
Pressure decreases from the ground and up in the atmosphere. If you are at 5500 meter above sea level, the pressure is 50 % of the pressure at the sea level. This is because, at 5500 mASL you got half of the atmosphere below you and the other half above you.
Imagine now that you take a big package of air from 5500 mASL and force it down to ground without any heat exchange form the surroundings. The air around the package will compress the air in the package to a smaller volume. The air around has made a work on the air package by making the volume decrease. The energy from the work, made from the surroundings, will not be consumed (from the energy conservation law), but it is added to the package as heat energy that may be registered as an increase in temperature. If you now take the same package of air up to 5500 mASL, the package will expand, or do the same work on the surrounding as the surroundings did on the way down. But now it has to use the available energy from it self, and that is the heat energy. When using the heat energy without heat exhcange with the surroundings, the internal temperature in the airpackage will decrease. This is what causes adiabatic cooling.
A derivation of the ideal gas law (PV=nRT) will confirm this. It says that for a certain amount of gaseous substance the ratio Pressure* Volume/ Temperature is constant.
Dry adiabatic cooling happens as long as the air has less than 100% relative humidity. That means, as long as the temperature is higher than the dew point temperature. For dry adiabatic cooling the cooling ratio is close to 1 degreeC/ 100 m altitude. And this is actually without heat exchange from the surroundings, just caused by the expansion of the air package.
Wet adiabatic cooling is when the air has reached the temperature at which the water vapor starts to condence out, it reaches the dew point temperature. Then we will have clouds formation. Clouds are liquid water. When condence starts, energy will be released from the vapor causing the cooling ratio being less than for dry adiabatic cooling. All this together make the cooling ratio for wet adiabatic cooling to be close to 0,5 degree C/ 100 m altitude.
In the skew-T diagram the dry adiabatic cooling rate is shown as solid lines going from the right of the bottom to the left of the top. These are helping lines, and the thermal always go parallel to these lines before it reaches the dew point.
The dashed lines shows the wet adiabatic cooling rate. After the dew point is met, the thermal will go parallel to these dashed lines.