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National Aeronautics and Space Administration, Science Mission Directorate. (2009). It's A Breeze. Retrieved , from Mission:Science website:

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Science Mission Directorate. "It's A Breeze" Mission:Science. 2009. National Aeronautics and Space Administration.

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!

A woman is standing on the surface of the Earth with arrows representing air pointing toward her with the caption, Air pressure is all around us

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.

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.

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.

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.

Swirling clouds over the ocean indicating the formation of a tropical storm.

A strong-low pressure system, referred to as a "depression", is the first sign a tropical storm may form. Generally, high- and low-pressure systems form when air mass and temperature differences between the surface of the Earth and the upper atmosphere create vertical currents. In a low-pressure system, these vertical winds travel upwards and suck air away from the surface of the Earth like a giant vacuum cleaner, decreasing the air pressure above the ground or sea. This decrease in surface air pressure in turn causes atmospheric currents moving parallel to the surface of the Earth near the base of the low to spin counter clockwise (clockwise in the Southern Hemisphere). Lows function like giant slow-moving hurricanes. The lower in pressure a low-pressure system gets, the more robust and larger this spinning circulation pattern becomes.

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. Tornadoes account for millions of dollars of damage and significant human suffering in the U.S. each year. Because of this, many scientists are studying the way in which tornadoes form and how they behave.

Map of the United Stated with small arrows showing the direction of the wind at given locations.

For example, this graph shows wind vectors across the U.S. and was created by the Global Tropospheric Experiment (GTE) at NASA. GTE is one example of how scientists are working to discover how our activities are affecting the weather.

How Does Weather Affect Air Pressure?

By NASA Scientist Dr. Louis Walter

The most fundamental thing you have to understand is that heavier gases weigh more than lighter gases. Now that's pretty straightforward - but what does it mean? Well, different chemical elements, as you know, have different atomic weights. Those which form gases (like nitrogen, oxygen, etc.) often combine two atoms at a time to form a gaseous molecule - like N2 (two nitrogens) or O2 (two oxygens).

Now the ATOMIC weight of nitrogen (N) is 14 and of oxygen (O) it's 16. The molecules N2 and O2 have MOLECULAR weights of 28 and 32, respectively. Doesn't take a rocket scientist to see that a gallon of oxygen weighs more than a gallon of nitrogen. Matter of fact, it turns out that - at room temperature and normal (sea level) atmospheric pressure, 28 grams of nitrogen occupies a volume of 22.4 liters and 32 grams of oxygen occupies the same volume!

In other words, under STP (standard temperature and pressure) the weight of 22.4 liters of a gas in grams equals the molecular weight of the gas. Now for some fun with numbers. Air is ABOUT 80% nitrogen and 20% oxygen. How much does a liter of air weigh? Well... If 22.4 liters of nitrogen weighs 28 grams; 0.8 liters weighs (0.8/22.4) X 28 grams = 1 gram (almost exactly) and 0.2 liters of oxygen weighs (0.2/22.4) X 32 = 0.286 grams, so... A liter of air weighs about 1.286 grams.

Now, think about this. How much does gaseous water weigh? Not LIQUID water - I mean steam or vapor. Let's figure it out. The chemical formula for water is H2O. One oxygen atom (atomic weight 16) and two hydrogen atoms (atomic weight 1). The total weight of the molecule is 18. Now how much does 22.4 liters (of the gas) weigh? 18 grams. One liter weighs 18/22.4 grams or 0.8 grams.

A series of images showing the correlation between pressure and weather forecast - low pressure equals stormy weather, high pressure equals clear sunny skies.

The barometer reading for air pressure helps forecast stormy weather.

Now we're getting close. You can see that air normally weighs 1.286 grams per liter but, if we substitute water for some of the air, the mixture becomes lighter. So, if there's water (otherwise known as humidity) in the air, the air mixure becomes LIGHTER - and it doesn't push down so hard on the mercury and the barometer's lower.

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.

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Citations

APA

National Aeronautics and Space Administration, Science Mission Directorate. (2009). It's A Breeze. Retrieved , from Mission:Science website:

MLA

Science Mission Directorate. "It's A Breeze" Mission:Science. 2009. National Aeronautics and Space Administration.