Jet fuel is a mixture of a variety of hydrocarbons. Because the exact composition of jet fuel varies widely based on petroleum source, it is impossible to define jet fuel as a ratio of specific hydrocarbons. Jet fuel is therefore defined as a performance specification rather than a chemical compound. Furthermore, the range of molecular mass between hydrocarbons (or different carbon numbers) is defined by the requirements for the product, such as the freezing point or smoke point. Kerosene-type jet fuels (including Jet A and Jet A-1, JP-5, and JP-8) have the majority of their carbon number distribution between 8 and 16 (carbon atoms per molecule), while more-refined, more-expensive wide-cut or naphtha-type jet fuels (including Jet B and JP-4) have most of their a carbon chains 10 to 14 atoms long for enhanced performance in polar regions