1. A form of energy released by atoms and molecules moving around randomly.
2. The transfer of energy from one substance to another one.

The energy flow will always be from the warmer substance (with a higher temperature) to the cooler substance (at lower temperature).

Amounts of heat are expressed in energy units; such as, the calorie, the joule, and the BTU or British Thermal Unit which is about 252 calories and about 4.2 calories is a joule (the basic unit of energy in the meter-kilogram-second system).

A heat pump in which the refrigerant exchanges heat (in a heat exchanger) with a fluid circulating through an earth connection medium (ground or ground water).

The fluid is contained in a variety of loop (pipe) configurations depending on the temperature of the ground and the ground area available.

Loops may be installed horizontally or vertically in the ground or submersed in a body of water.

Cloud-to-cloud lightning, common in the summer, which occurs behind a cloud or below the horizon but lights up the surrounding clouds.

Usually no thunder is heard because of the distance from the observer.