Now that fall has officially begun, it looks like we will be on a slow boat to some warming temperatures. A stable ridge of high pressure is building back into the west coast, and we should have a steady rise in temperatures (low 60s Saturday, 70s by Monday, and the 80s by mid-week. While the days are warming, Saturday morning will be cold with widespread frosts and freezes, and a Freeze Warning is in effect until 9 am. Sunday morning could also have some freezes in the region as well.
So why does hot air rise? First, it’s better to describe hot air as being less dense instead of lighter. It’s a subtle distinction, but an important one. When one substance is less dense than its liquid or gaseous surroundings, that substance will “float”, just like Styrofoam (less dense) floats on water, while iron (more dense) sinks. So what makes hot air less dense?
Think of it this way. The density of the air is determined by the number of molecules in any unit volume. Temperature is a measure of the average kinetic energy of the air. In simple terms, that means that hot air molecules are moving faster than cold ones. The faster the air molecules are moving, the more often they are bouncing off each other, and the further apart they become on average. If all the molecules are further apart, there are less of them in any given volume, ergo it is less dense. And your balloon floats. Another way to put it is to say the balloon is “buoyant”.