Here in the southwest we have a hot air balloon season in which we will see a half dozen to a dozen at a time dot the air from the highway, sometimes very close. 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.

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. A fun time. And it's true. 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 simply the difference in electrical pressure (why engineers say, "potential") between one point and another in a circuit. Of course this shouldn't surprise. Weather works this way, a canal's water locks too. A lot of things. It's fun to think about.