In strong supercell thunderstorms, wind moves upwards at over 90mph. Fast-moving air creates an area of low pressure, or a partial vacuum. Nature abhors a vacuum, so nearby air rushes in to join the updraft. This skyward flow can be powerful enough to suspend grapefruit-sized hail.
The tendency of a speeding air current to suck in surrounding air molecules stems from the same idea that gets airplanes off the ground: Bernoulli’s principle. The faster a fluid moves, the lower its pressure. Airplane wings have more pronounced curves on the top side than they do on the bottom, so air follows a longer path, and moves faster, above the wing than below. Higher pressure from underneath the wing translates to an upward force– that’s lift.
Aircraft and thunderstorms provide some of the most dramatic examples of this concept, but it’s also surprisingly easy to explore at home or in class. All you need is moving air and something to reveal its motion! A long, narrow bag and a working set of lungs are the perfect tools. Try to blow up eight feet of plastic bag like you would a balloon and you’ll find yourself struggling for air. Instead, hold the bag’s opening ten inches from your face, and blow into it across the gap. What’s the difference?
When your breath travels through a volume of air before reaching the bag, Bernoulli’s principle does most of the work for you! Your exhalation is a fast flow, so it creates a sink of low pressure moving into the plastic bag. Nearby air rushes in to fill the partial vacuum, and before you know it, the bag is fully inflated.
Firefighters clearing smoke from buildings take advantage of Bernoulli’s principle in almost exactly the same way. A fan placed directly in a door or window will move air through the structure. Set the same fan a short distance back from the opening, and it pulls in more air, getting the job done faster.