I've noticed that with the bright explosions it's possible you simply don't see the heat bar.
Even with that not being the case, I've noticed that the faint black outline for the bar isn't quiet enough, maybe if it's green instead of white you could see it better, with a thicker black outline. There may be lots of grass but I haven't seen that many green explosions :P
Another option in my opinion is have the background UI thing darker, so during those explosions you can still see the bar, but not so dark that you can't see what's going on? I'm not entirely sure if I mentioned this before now :P Sorry if I have!
