I see now that I was not sufficiently clear what causes heating and cooling. It is what enemies shoot and the floor itself - standing on most tiles causes the tank to slowly or not so slowly reach its temperature, depending on the tile type. This is the sense in which I used the theme "The Floor Is...", in which it can be the "usual" lava, but it can also be ice. Player activities (driving, shooting, jumping and flying) have no effect on the temperature. Also, the current tank temperature is indicated by the thermometer on the top bar, it ranges from "-!!" and -9 to +9 and "+!!". Colors also indicate the temperature, with green being the midpoint between too hot and too cold.
Sorry for the confusion and thank you for playing this mess I made! :D