AIR AND AIR PRESSURE

What is air?

Sometimes when people think of "air" the gas that comes to mind is oxygen. Actually, oxygen molecules (O2) are in the minority in our atmosphere. Only 21 percent of Earth's atmosphere is oxygen, with the majority of it being nitrogen gas (78 percent). The rest is a mixture of argon gas (0.9 percent), 0.035 percent carbon dioxide (CO2), water vapor, and traces of helium, xenon, methane (CH4), nitrous oxide (N2O), neon, and krypton (not to be confused with kryptonite, the stuff that hurts Superman), and a scattering of dust, pollen, and other particulates.

Is there actual salt in sea air?

Yes. The air above the sea and along the coast does contain salt. On average, the concentration of salt in sea air is about 3.5 million parts per cubic foot (100,000

Pollen for a wide variety of plants fills our atmosphere, sometimes causing allergic reactions in people.

Pollen for a wide variety of plants fills our atmosphere, sometimes causing allergic reactions in people.

parts per cubic meter). Salt air can penetrate inland up to thousands of miles, depending on wind and air pressure conditions, and can aid in the formation of clouds because the salt particles serve as nuclei around which drops of precipitation can form. Haze along seashores is also the result of salt particles, with haze droplets typically forming when the humidity level reaches 75 percent or more.

How much pollen is in our air?

Pollen from plants just within American borders produce about two billion pounds of particulates every year, or about seven pounds per person.

Are the gases in the atmosphere evenly distributed?

You are not likely to stroll down a street and encounter a suddenly high concentration of oxygen or a pocket of unmixed argon gas. The constant motion of the weather due to fronts, pressure changes, varying temperatures, storms, and so on, is like putting the atmosphere in a food processor and hitting the "blend" button, and never turning it off. The percentage of each gas, therefore, will be constant anywhere below an elevation of 50 or 60 miles (80 to 95 kilometers).

How much pressure does the atmosphere exert upon us?

Average air pressure is 14.7 pounds per square inch (1.03 kilograms per square centimeter) at sea level. Measured in inches of mercury, this comes to 29.92 inches, or

1,013 millibars. To put it another way, a cubic yard of sea-level air weighs a bit over two pounds (about 0.7 kilograms per cubic meter).

Who introduced use of the millibar to measure air pressure?

English meteorologist William Napier Shaw (1854-1945) was one of the leading scientists in his field and former director of Britain's Meteorological Office from 1905 to 1920. He suggested that air pressure be measured in millibars in 1909, but it was not adopted as an international standard until 1929.

Does air pressure change as elevation rises above sea level?

Yes, it does. The higher you go, the less air (or atmospheric) pressure there is. Air pressure is also involved in weather systems. Closer to the ground, air pressure decreases at a rate of about 0.01 inches of mercury for every 10 feet (3 meters). By the time you are at an elevation of 18,000 feet (5,500 meters), air pressure is about half what it is at sea level. A low-pressure system is more likely to bring rain and bad weather versus a high-pressure system, which is usually drier.

What is Gay-Lussac's Law and why is it important in meteorology?

Joseph Louis Gay-Lussac (1778-1850) was a French physicist and chemist best known for two laws of physics about gases. One of these laws states that, in chemical reactions, gases combine in simple ratios according to volume. For example, it takes two carbon monoxide molecules (CO) to combine with one oxygen molecule (O2) to create carbon dioxide (CO2). This became known as Gay-Lussac's Law and is important for understanding chemical reactions of gases within our atmosphere. Gay-Lussac also published a law about gas expansion by volume occurring linearly as a function of temperature. Sometimes credited as another of Gay-Lussac's laws, it is more correctly called Charles' Law (and Gay-Lussac was one of those who gave credit where it was due). It was discovered by another French physicist, Jacques Alexandre César Charles (1746-1823), who was also a mathematician. Charles found that gases such as oxygen and nitrogen increased their volume by 1/273 for every 1.8°F (1°C) increase in temperature. From this, he extrapolated the possibility that, at absolute zero (-273°C, or -459.4°F) the volume of a gas would also be zero. Both scientists were also balloonists, which was handy, given their interest in the atmosphere.

 
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