A third natural source of energy is the energy contained in the oceans and rivers. These are the forms of energy that are most significant to natural processes on Earth's surface. Some of the energy is longer-wavelength thermal energy that is felt as heat. This form of energy is produced by the random motion of atoms and molecules of substances. The more vigorous the movement of its atoms and molecules, the hotter a substance becomes.
Temperature is a measure of average molecular motion.
Heat energy is the amount of energy in a substance. This is the energy of motion. The higher the speed at which something is moving, the more kinetic energy it has. For a given speed, the amount of kinetic energy is proportional to the mass of an object. If equal volumes of air and water are moving at the same speed, the water will have much more kinetic energy because of its much greater mass.