Air cooling is the most frequently used cooling technique for resistors because air constitutes an excellent insulator and can be easily supplied and removed. A disadvantage of air cooling is the low heat conductivity and the low heat storage capacity that is why transport of the excess heat is only economical when done via cooling air transport not via heat conduction. Cooling air transport can be achieved using the upward movement of heated air (natural air cooling, natural cooling) or via forced air cooling using a fan or blower, one important factor being the heat transfer.