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Sea Level: What is Atmospheric Pressure?

Written by: John Maguire

Feeling Pressured?

Air pressure is the force exerted on you by the weight of tiny particles of air (air molecules). Although air molecules are invisible, they still have weight and take up space. Since there's a lot of "empty" space between air molecules, air can be compressed to fit in a smaller volume.

When it's compressed, air is said to be "under high pressure". Air at sea level is what we're used to, in fact, we're so used to it that we forget we're actually feeling air pressure all the time!

Weather forecasters measure air pressure with a barometer. Barometers are used to measure the current air pressure at a particular location in "inches of mercury" or in "millibars" (mb). A measurement of 29.92 inches of mercury is equivalent to 1013.25 millibars.

How much pressure are you under? Earth's atmosphere is pressing against each square inch of you with a force of 1 kilogram per square centimeter (14.7 pounds per square inch). The force on 1,000 square centimeters (a little larger than a square foot) is about a ton!

Why doesn't all that pressure squash me? Remember that you have air inside your body too, that air balances out the pressure outside so you stay nice and firm and not squishy.

How Air Pressure Signals Changes in the Weather

Before Hurricanes could be spotted by satellites from space, people would keep a wary eye on their barometers during hurricane season. If the air pressure dropped, that was usually a good time to board up windows and head further inland!

As hurricanes pass over coastal areas, air pressure can drop significantly. At sea level air pressure is normally around 1013.25mb (29.92 inches of mercury). Extremely strong hurricanes are accompanied by air pressure drops of between 30 and 70mb. The greater the pressure difference between a low pressure area and a high pressure area, the stronger the winds! Wind is the natural result of having a low pressure area next to a higher pressure area since the air molecules in the higher pressure zone will migrate to the "more spacious" surroundings of the low pressure area.

Tornadoes, also known as Twisters, can be as destructive as hurricanes on a smaller scale. A falling barometer can indicate bad weather approaching and many people in the Midwest and central plains states will head into the cellar when the air pressure drops dramatically.

Air pressure can tell us about what kind of weather to expect as well. If a high pressure system is on its way, often you can expect cooler temperatures and clear skies. If a low pressure system is coming, then look for warmer weather, storms and rain.

What Happens if Air Pressure Changes?

Why do my ears pop? If you've ever been to the top of a tall mountain, you may have noticed that your ears pop and you need to breathe more often than when you're at sea level. As the number of molecules of air around you decreases, the air pressure decreases. This causes your ears to pop in order to balance the pressure between the outside and inside of your ear. Since you are breathing fewer molecules of oxygen, you need to breathe faster to bring the few molecules there are into your lungs to make up for the deficit.

As you climb higher, air temperature decreases. Typically, air temperatures decrease about 3.6° F per 1,000 feet of elevation.

Do you think a decrease in temperature could be explained in terms of air pressure? How?

Air Pressure Experiments

1. While holding your hand on your ribs, take a deep breath and observe what happens to your chest. Did you feel it expand? Did you see it expand? How would you explain what happened?

ANSWER: Your chest expands because, like blowing up a balloon, you are increasing the number of air molecules inside your lungs. This causes your lungs to expand in order to provide space for the increased number of air molecules.

2. Blow up a balloon and observe what happens. Does it expand? Why does it make a noise when it's popped?

ANSWER: When a balloon is blown up, the air pressure inside the balloon slowly becomes greater than the air pressure outside the balloon. Since the balloon is made of rubber and is expandable, it grows larger and larger. When the balloon is popped, the air escapes instantly. The sound you hear is from the molecules of air inside the balloon coming into sudden contact with the molecules of air outside the balloon.

3. Ask your parents for an empty plastic gallon milk jug with a screw top. Fill it about a quarter of the way full with very hot water. Cap it tightly and let it stand for about an hour. What did you expect to happen? What did happen?

ANSWER: The milk jug will crumple in on itself. When you added the hot water, it caused the air temperature inside the jug to rise. While the container was sealed no air could get into or out of the jug. When the water inside the jug cooled, the air cooled and caused the pressure inside the jug to decrease.

As the pressure on the inside walls of the jug decreased, the walls of the jug collapsed. Since there wasn't enough air pressure inside the jug to offset the air pressure on the outside of the jug!

The Mercurial Barometer: Measuring Pressure

Based on a principle developed by Evangelista Torricelli in 1643, the Mercurial Barometer is an instrument used for measuring the change in atmospheric pressure. It uses a long glass tube, open at one end and closed at the other. Air pressure is measured by observing the height of the column of mercury in the tube. At sea level, air pressure will push on the mercury at the open end and support a column of mercury about 30 inches high. If you used water instead of mercury, you would need a glass tube over 30 feet in length.

As atmospheric pressure increases, the mercury is forced from the reservoir by the increasing air pressure and the column of mercury rises; when the atmospheric pressure decreases, the mercury flows back into the reservoir and the column of mercury is lowered.

Make Your Own Barometer

Materials Needed:

  • Drinking straw (clear plastic).
  • Narrow-neck glass bottle.
  • A rubber or cork stopper which fits in the neck of the bottle


  1. Insert a drinking straw into the bottle.
  2. Fill the bottle about half-way full of water.
  3. Seal the neck of the bottle around the straw either with the rubber stopper or a cork.
  4. Make sure the end of the straw is immersed in the water and that the water level in the straw is above the top of the bottle.

As the air pressure outside the bottle decreases, the trapped air inside the bottle will push the water up the straw. As the air pressure outside the bottle increases, it will push the water farther down the straw.