1. Air Molecules and Pressure
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2. Air is a Gas
Gasses are able to change their volumes. This means same amount of air can fit into different size containers. Here is a container of air with 48 air molecules. Since gasses take the shape of their container, they become evenly spaced:
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3. Compressed Air
If the volume of the container is made smaller, the same 48 air molecules will have to bunch up closer together: They are squeezed together tighter, pushing harder against each other and the sides of the container. This makes the air pressure inside the container higher.
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4. Expanded Air
If the volume of the container is made larger, the same 48 air molecules have more room to move around: They have more room and do not push so hard against each other and the sides of the container. This makes the air pressure inside the container lower.

5. Air Density
How loosely or tightly objects are packed into a space is known as “Density”. Since air is a gas, its molecules can be tightly or loosely packed into the same space. Note: the volume of the spaces below are the same: Higher Density Lower Density

6. Pressure in Our Atmosphere
Image credit: ILRS Image source link
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7. 48 air molecules weigh more than 22 air molecules.
Air Has Weight
Every single air molecule in Earth’s
atmosphere has a tiny amount of weight:
Air which is more dense weighs more because there are more air molecules taking up the same amount of space:
48 air molecules 22 air molecules
Heavier Lighter
48 air molecules weigh more than 22 air molecules.
The heavier air exerts more pressure on things around it.
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8. Pressure in Our Atmosphere
On Earth, our atmosphere extends up to around 50,00 feet.
Image credit: NASA ESPO/INTEX-NA Educational Outreach
Air molecules stack up and push down on each other.
The lower you go in the Earth’s atmosphere, there are more air molecules above you pushing down and all around you.
Air becomes more dense and the pressure increases at lower elevations

9. Air Pressure Is Around Us
Air molecules exert pressure in all directions on all things on Earth. Air pressure at ground level is determined by the weight of the all of the air molecules in the atmosphere above. This means that air pressure is influenced by gravity. At ground level, the average air pressure is 14.7 pounds per square inch (psi) or millibars (mb).
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10. The Mercury Barometer
The barometer measures the force of the air pressure around us. An old fashioned mercury barometer measures how far the air pressure will push mercury up a glass tube:
Image: Wikimedia Commons
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11. The Electronic Barometer
Mercury is a dangerous substance. It is much safer to use an electric barometer: The electronic parts inside measure the force of the air pushing against them. They are very sensitive and can detect the tiniest differences in air pressure.
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12. Disclaimer:
All images and photos – unless cited – are created by the author. They are free to use and retain no copyright.