Freezing Point : Because it is a mixture of many hundreds of individual hydrocarbons, each with its own freezing point, jet fuel does not become solid at one temperature the way water does. As the fuel is cooled, the hydrocarbon components with the highest freezing points solidify first, forming wax crystals. Further cooling causes hydrocarbons with lower freezing points to solidify. Thus, the fuel changes from a homogenous liquid, to a liquid containing a few hydrocarbon (wax) crystals, to a slush of fuel and hydrocarbon crystals, and, finally, to a near-solid block of hydro- carbons. The freezing point of jet fuel is defined as the temperature at which the last wax crystal melts, when warming a fuel that has previously been cooled until wax crystals form . Thus the freezing point of fuel is well above the temperature at which it completely solidifies.
The freezing point of jet A1 is -47C
The freezing point of jet A. Is -40C
So if you have a mixture of both, if you have refuelled is the USA, it is wise to use the jet A figures.