Change occurs in nature when a system is not in equilibrium. Natural systems are frequently thrust out of balance, into a disequilibrium state that cannot persist in the long run. Sunlight shining on a puddle, for instance, heats the ground and forces the water to evaporate. Radiative transfer constitutes one way change and transformations are brought about. Atmospheric systems (e.g., clouds) routinely interact with their environments through exchanges of matter and energy, and they respond to external forces (e.g., pressure gradients). Environmental interactions shift the mechanical, thermal, and chemical balances that allowed a system to be at equilibrium in the first place, forcing the system to change, be transformed to another state. The atmosphere is never at equilibrium, so it is continually changing in one way or another. Indeed, clouds ultimately owe their existence to the disequilibrium forced by solar heating of the surface.

Deviations from equilibrium

Deviations from equilibrium drive changes in the observable properties of a system. A ladder that is kicked may suddenly topple, an example of mechanical forces that become imbalanced. A turkey placed in an oven gradually gets hot, an example of energy transfer because of a temperature difference. A mixture of air and natural gas (methane, CH4) exposed to a spark explodes, an example of one set of compounds (oxygen and methane) transforming to another (carbon dioxide and water) because of differences in the chemical potentials of the compounds involved.