What is most likely causing ice to form on the outside of a fuel tank in use?

The formation of ice on the outside of a fuel tank can be primarily attributed to the phenomenon known as evaporative cooling, which occurs when vaporized fuel is drawn into a burner. When fuel vaporizes, it absorbs heat from its surroundings to transition from a liquid to a gaseous state, leading to a drop in temperature. As the temperature of the fuel tank exterior decreases, moisture in the air can condense on the surface and freeze, resulting in ice formation.

This process is particularly relevant when the fuel is not in a heated state and is vaporized due to a pressure drop or a fast flow in the system, causing the tank to cool rapidly. Thus, the behavior of the fuel during operation directly affects the surrounding temperature, leading to this ice formation on the tank.

The other options, while they may relate to fuel systems in various ways, do not directly connect to the specific mechanism that leads to ice forming on the tank's exterior. For instance, while water vapor in the air can contribute to moisture, it is the cooling effect caused by the vaporization of the fuel that leads to ice. Additionally, excessive altitude might influence fuel properties but does not directly result in ice formation like the evaporative cooling does. Similarly, external temperature variation affects