Weatherz School: Air pressure

Air pressure. It matters when you’re wanting to toss the pig skin or take your bike out for the first time of the year, but why do we care about air pressure in meteorology?

Think of the atmosphere as a fluid in constant motion. As the atmosphere ebbs and flows, it drives all of the weather we experience.

Just as the surface of water rises and falls, the amount of air over us is constantly changing. We can track those changes by measuring the weight, or pressure, of the air. It’s important to note that air pressure decreases with height.

Elevation changes make it very difficult to compare the pressure at the ground from one area to another. To fix this, we convert air pressure readings to sea-level elevation. From New Orleans to the Rocky Mountains, air pressure readings reflect what would be observed if that location were at sea level.

We have two ways to measure sea level pressure: Inches of Mercury, with standard being 29.92 inches, and millibars, with a standard of 1013.25 mb. The instrument that measures this is called a barometer.
We can even make our own with a few simple items around the house.

First, cut a balloon and stretch it over a jar. Then glue a straw to the top of the balloon. You can add a toothpick or sewing needle for the pointer. We need an index card to mark the range of pressure, and we need a stand to hold it up. A pencil in a spool works nicely.

The container of air will expand or contract in response to atmospheric pressure. When the air pressure is lower than the pressure in the jar, the balloon is pushed up. If air pressure is higher than pressure in the jar, the balloon is pushed down.

Once you have your barometer, you can keep track of what the weather is like when pressure rises and falls.