Pressure

As humans living on the surface of the earth, we dwell at the bottom of an ocean of air. Each one of us supports on his or her shoulders the pressure caused by the weight of an air column that extends out to interstellar space.

Hold out the palm of your hand. Its area is approximately 20 in² (129 cm²) and the weight of the air resting upon it is nearly 300 lb (136 kg). Yet with all this weight, your hand does not crush. This is because our bodies are used to living under such pressures. The liquids and solids inside your body grow to exert an equal pressure from the inside.

Air particles are constantly hitting every part of our bodies and the pressure they cause is known as atmospheric pressure. At high altitudes, such as you would find in places like Mexico City or Aspen, there is less air above you and therefore less atmospheric pressure. Breathing becomes more difficult, but throwing a baseball for distance is easier because there is less air resistance experienced by the moving baseball.

The barometer, invented by Evangelista Torricelli in 1643, was the first instrument built to measure the pressure of the gases in our atmosphere. It consisted of a long glass tube closed at one end, filled with liquid mercury, and inverted into a dish of more mercury.

With this instrument, it has been observed that at sea level, atmospheric pressure can support the weight of about 760 mm of Hg (mercury). The exact figure depends on such things as weather conditions.

One standard atmosphere (1 atm) of pressure is the pressure exerted by a column of mercury that is 760 mm high at a temperature of 32°F (0°C). In the Universe, pressure varies from about 1 atmosphere on the Earth's surface to approximately zero in the vacuum of outer space. Much higher pressures are found at the center of stars and other massive bodies.

The pascal is the SI unit of pressure. One pascal is equal to the force of one newton applied to a surface whose area is equal to one squared meter, 1.0 Pa = 1.0 N / m². One atmosphere of pressure is equal to approximately 101.3 KPa.