Here in the southwest we have a hot air balloon season in which we will see a half dozen or more dot the sky. So pretty, so colorful, cheerful and fun. I even took my wife up in a balloon once. When we don't see these beautiful air conveyances is in the hotter months when the ambient temps are so high that no matter the effort at the torch, not enough pressure difference can be created to reliably generate lift. At least that is what I have heard is the reason.

Pressure differences. They're everywhere. I recall that when I trained mechanics I used to say, "nearly all the technology you will immerse yourself in in this industry is connected in some way to pressure differences." "PD" became a mantra of sorts with the students. When we reviewed the quizzes. When we summarized previous lessons. A fun time. And it's true, it's all over. The pressure difference created in the cylinder makes the engine intake air. The fuel rises in a carburetor due to the difference in pressure between the atmosphere and the fuel jet. Even electricity can't be left out--voltage is merely the difference in electrical pressure between one point and another in a circuit. Of course this shouldn't surprise. Weather works this way, too, as everyone knows. A lot of things. It's fun to think about.