Under what conditions does the net force acting on an object equal the sum of the individual forces?

Under any and all conditions.  That's the definition of "net force". The question is a lot like asking "Under what conditions is a block of ice composed of solid frozen water ?" .

explanation:

earth's surface is mainly heated by the absorption of infrared radiation from the sun.

evaporation occurs because molecules in liquid water are warmed ( usually by the sun) and vibrate quicker, becoming less dense and rise.

condensation occurs because molecules in water vapor are cooled and therefore vibrate more slowly thus becoming more dense and eventually fall to earth.

lines of equal barometric pressure on a weather map are called: isobars : lines of equal temp on a weather map are called: isotherms.​