To get air to rise freely, there must really be some atmospheric force upward that keeps the air parcel from sinking to the ground by gravity. This is the buoyancy force.

If an air parcel has the same temperature (and hence the same density as the surrounding air), there will be just enough buoyancy force upward to balance the gravitational force pulling down on the mass of air (hydrostatic equilibrium). We call this neutral buoyancy.

This is not very interesting for cloud development, because we need the air to rise.

When the object density is less than the surrounding fluid's density, there will be a net upward buoyancy force on the object. We call this positive buoyancy. A positively buoyant air parcel will have a higher temperature than the surrounding air, since warm air is less dense than cooler air at the same pressure.

When the object has a higher density than the surrounding fluid, there will be a net downward force on the object. This is called negative buoyancy. Air parcels that are colder than the surrounding air will exhibit this behavior.

So, as long as the air temperature inside the parcel is higher than the temperature outside, the air parcel rises due to a positive buoyancy force. In the illustration on the slide, we start with an air parcel that is warmer than the surrounding air, so it will rise by itself. The temperature inside the parcel decreases by the (dry) adiabatic lapse rate, 10C/km. But what if the environmental lapse rate is smaller than the dry adiabatic lapse rate, say 5C/km, as illustrated here? Then the air temperature outside and inside the parcel change differently (adiabatic process, so no heat can pass across the parcel boundary to equalize the temperatures), and at some point above the ground, both the temperature inside and outside will be the same (the two temperature lines intersect at this point). The parcel will stop rising because it stops being positively buoyant, since the parcel is not warmer than the surrounding air. This has a significant effect on cloud development, because what if the lifting condensation level (LCL) is higher than this altitude? Then no cloud forms. Subsequent screens demonstrate how different environmental lapse rates affect the air parcel motion and subsequently how clouds form under conditions of free convection.