There are only so many letters to use! "T" is traditionally for Thrust Force, and "t" is forever reserved by time. Theta starts with "T", so it seemed like the next best symbol (in retrospect, tau is probably better here). Anyway, I started out designing the notes planning for more cases where aerodynamic forces and temperature pop up together, so I tried to use theta in case I ever couldn't use T. It didn't happen as much as I had anticipated...