Air Pressure

When I taught sixth grade science I taught about air pressure. I taught that cold air is high pressure (we visualized people who were cold crowding together for warmth) and warm air is low pressure (when you’re hot, you want people to get far away from you).

I posed the challenge question—“Why can’t you fly a hot air balloon in Phoenix in the middle of the day in the summer?” The answer is that the outside temperature has to be cold in order for the air blown into the balloon to be lighter/lower pressure than the regular air and to float.

Tonight I swung by Fred Meyer to get John a “congrats” balloon for his big debut. I carefully chose this particular balloon because it was fully inflated. By the time I got to the car, there was a full inch of depression and sag. What the heck?!

I realized that the cold air was compressing the balloon and the low pressure helium. How wild!

I left the balloon in the car and, by the time we got back to the car from the show, it looked like a 4 day old balloon. By this time, though, I’d figured out what was going on and told John. As soon as we got in the house and the balloon hit the warmer air, it filled right back out. You could hear the foil crackling as it expanded. I wish my 6th graders (who are now in high school!!) could see this :)