Air pressure a vital measure

One of the weather variables you will almost always see reported during a weathercast is the barometric pressure. Barometric pressure is just the weight of the atmosphere (the air around us), measured with a barometer. Most people recognize “inches of mercury” as the unit used to report air pressure, but meteorologists and scientists use millibars of hectopascals, which are part of the metric system. The unit “inches of mercury” comes from the design of the first mercury barometer invented by Evangelista Torricelli in 1643. Recognizing that the atmosphere had weight, he placed a glass tube, in which the air had been removed, into a dish of mercury. The weight of the air on the mercury in the dish forced some of it up the glass tube. On the Earth’s surface on average, the mercury would be pushed about 30 inches up the tube (equivalent to 1,013 millibars). Changes in air pressure resulted in changes of the height of the mercury in the tube, with a normal range of a couple of inches. Water was also tried in place of mercury, but because water is much less dense, the glass tube needed to be 60 feet long to accommodate the same pressure measurements.

Though average air pressure at the surface is 30 inches of mercury, extreme values of pressure can be found in the center of storms such as hurricanes and tornadoes. The lowest air pressure ever measured was in the center of Typhoon Tip in the western Pacific Ocean in 1979. A reconnaissance aircraft recorded a reading of 25.69 inches of mercury. The highest air pressure ever measure occurred on a frigid day in Agata, Soviet Union, in 1968. The air temperature was as cold as 58 degrees below zero, and the pressure was 32.01 inches of mercury.

Why do we care about the air pressure measurements? For one, the horizontal differences in air pressure are responsible for the wind that you feel as air moves from higher to lower pressure. Secondly, pressure trends can clue us in to what type of weather to expect. Falling pressure usually indicates stormy weather, because as air converges on an area of low pressure, it is forced to rise. The rising motion can lead to clouds, which can lead to precipitation. Conversely, increasing air pressure usually means sinking air, which often results in fair weather. A single value of barometric pressure does not yield much additional information. It is the pressure change over time (tendency) at a given location, or the differences in pressure across a horizontal area, that really have any meaning. It makes me wonder why TV weathercasters (myself included) even show the current single value of barometric pressure on the air. In the event you have ever tried to use the barometric pressure we report on TV to compare with your backyard barometer, take note that the air pressure reported is of “mean sea level pressure.” All pressure readings at official weather stations are corrected to mean sea level for consistency. A backyard barometer will give you the air pressure at your particular elevation, which will be different.

I’ve often read that some people’s joints are very sensitive to changes in air pressure, kind of like a human barometer. You might have known a person whose swelling elbow or aching knee could predict an approaching storm. Our bodies also respond to lower pressures because of lower oxygen density. It is harder to breathe at high elevations because there is less atmosphere above you, meaning lower air pressure. At approximately 19,000 feet, the air pressure is roughly half of the value at sea level (500 millibars or 14.77 inches of mercury).