Fuel cools dramatically during a long flight in the high cold, but it cannot be allowed to cool too much. Typical freezing points of fuel are around minus 40 (a temperature that requires no C or F to follow it; it is the intersection of the Celsius and Fahrenheit scales) or colder. The static temperature of the air outside—the ambient temperature, shown perhaps on the moving map screen—is often colder than this. But the TAT, the experienced, wind-warmed temperature, is much higher. Indeed, nothing suggests the speed of airliners and the physicality of air quite like the fact that if the fuel
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Static air temp outside airplane at cruising is usually well below fuel freezing point, but ram rise (heat generated at leading edge of plane/wings due to air compressing at leading edges) helps warm fuel up. In fact, if fuel getting too cool, speeding up counteracts.
