PhySP18SS6.1.3 From the Buoyancy Presentation Hot air balloons rise because the hot air inside them is less dense than the surrounding “colder” air. The colder air is more dense, but that wouldn’t make the balloon rise. It rises because the air pressure gets higher the closer the air parcel is to the ground because there is more OVERBURDEN on that lower parcel of air than there is on a higher parcel of air. All these air molecules are in Brownian Motion so they slam into any object that is near them. This slamming causes a force. When you take that combined force of trillions of these air molecules over a particular area (like a square inch) you get air pressure. Air pressure in Oklahoma averages about 14.7 psi (lbs per square inch). So more air molecules hit the bottom of the balloon per unit area (causing higher pressure). THere is an increased number of air molecules that are pushing up on the BOTTOM of balloon. So buoyancy is the measure of the extra bit of upward force from these lower outside air molecules.