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# Atmospheric Pressure To inflate a balloon, you must push air into it. When inflated, the air inside pushes outwards. This shows that air and other gases can cause pressure. ## The Pressure of the Atmosphere As you climb a high mountain, the air gets thinner (less dense). Its density decreases....

# Atmospheric Pressure To inflate a balloon, you must push air into it. When inflated, the air inside pushes outwards. This shows that air and other gases can cause pressure. ## The Pressure of the Atmosphere As you climb a high mountain, the air gets thinner (less dense). Its density decreases. Because the air is less dense at higher altitudes, its pressure is less. Atmospheric pressure is greatest at sea level, due to the weight of the entire atmosphere pressing down. Even though we can’t see air, it is real and exerts pressure. Atmospheric pressure changes with altitude, higher at sea level and decreasing as you go higher. Some weather systems have slightly higher pressure than others (high- and low-pressure systems). ## Measuring Atmospheric Pressure For a long time, atmospheric pressure has been measured using a mercury barometer. The first was invented in 1643 by one of Galileo's assistants. A mercurial barometer has a section of mercury exposed to the atmosphere. If atmospheric pressure increases, it forces the mercury to rise further in the glass tube, showing a higher reading. If pressure decreases, the mercury level lowers, showing a lower reading. These instruments are used in labs and weather stations, but are not easily moved. Measurements are often recorded in various units. Examples include inches of mercury (in Hg), pounds per square inch (psi), and pascals (Pa). The pascal is the standard unit in the metric (SI) system. Atmospheric pressure is sometimes expressed as "atmospheres" (atm). Weather forecasters often use millibars (mb) to describe low or high pressure weather systems. ## Summary At sea level, atmospheric pressure is 101,325 Pa = 1,013.25 mb = 1.013 bar.