why does adiabatic rate of cooling change when condensation begins?How does stable air differ from unstable air?(

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The displacement of an air parcel to an environment of low pressure without exchange with surrounding air causes an increase in its volume & a consequent lowering of temperature .Since, volumetric increase involves work & consumption of energy so, both heat available per unit volume & temperature decreases. Such a temperature change involving no subtraction & addition of heat is called adiabatic temperature change.
Adiabatic lapse ate is the rate of temperature decrease in rising expanding air parcel. When this rising air parcel does not produce condensation it's called Dry adiabatic lapse rate. However, prolonged temperature decrease will result in super cooled water with latent heat energy stored  in the form of potential energy since the air parcel is moving against gravity & contains energy to the amount of 2260Joules/gram of this moisture laden air. Condensation will take place with the condensation taking place on suspended condensation nuclei in air , which also receives the 2260 Joules of energy from the water droplet by condensation. The total mass of air parcel increases due to density increase due to contraction & release of latent heat.
                                                       This in turn decreases the rate of cooling caused due to dry adiabatic lapse rate. This decrease in the rate of cooling is also caused as loss of latent heat causes the contraction of particles (atoms & molecules) due to cooling increasing vapor pressure but the air being saturated with condensation nuclei which raises the freezing point of water vapor ( as we know presence of solid increases the boiling point & freezing point. Eg.- presence of salt in water increases the boiling point & freezing point.)This is the reason why adiabatic rate of cooling changes when condensation begins.
                                    A stable air when forced up or down has a tendency to return to it's former position once the motivating force ceases.
An unstable air mass has a tendency to continue moving away from it's original level when set in motion.
                                     A parcel of air may be stable at lower layers. If the air (stable ) is forced to rise due to passage over a mountain range or due to local surface heating etc. conditional instability may arise, as instability is dependent on the air mass becoming saturated,since,the environmental lapse rate is frequently between dry & saturated adiabatic rates.   

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