This column is a continuation of a previous one on the atmosphere. The term “column” is appropriate.
Think of one square inch on the Earth’s surface at sea level, and imagine an inch square column of air above it extending to the top of the atmosphere.
The air in that column would weigh 14.7 pounds due to the Earth’s gravity.
Thus, our atmosphere exerts a pressure at the earth surface of 14.7 pounds per square inch.
This is what we call atmospheric pressure — also known as barometric pressure.
Most would say they do not feel the almost 15 psi.
We have evolved to live quite comfortably at sea level, so we actually think it is completely normal to be somewhat pressurized.
However, the pressure we are under is far from trivial.
Atmospheric pressure is the equivalent of about 34 feet of water.
Yes, if you were to sit at the bottom of a 34-foot deep swimming pool, you would feel two times normal atmospheric pressure, one from our atmosphere and one from the 34 feet of water.
I don’t know about you, but my ears start to hurt and pop when I dive just 10 feet or so.
In weather reports, we hear other measures of atmospheric pressure.
All of the below are equal to one atmosphere of pressure (14.7 psi).
• 760 mm (millimeters) of mercury (Hg);
• 29.92 inches of mercury;
• 1013.25 millibars; and
• 33.8 feet of water.
The highest atmospheric pressure ever recorded was in Mongolia in 2001 at 32.06 inches of mercury.
The lowest (sea level) pressure, not including tornadoes, was 25.69 inches of mercury during a typhoon in 1979.
As you can see, atmospheric pressure at sea level, even in the extreme, varies less than 15 percent from normal.
Relatively small changes in atmospheric pressure are the primary cause of wind.
Air moves from areas of higher pressure to areas of lower pressure.
I will discuss the wind in another column.
I will leave you with a question.
If changes in atmospheric pressure cause wind, and if atmospheric pressure gets less and less as we go up through the atmosphere, why doesn’t the wind blow straight up?