Weather Whys; Barometers

This is weather-wise. There are two main types of barometers in use today. They both measure air pressure, but they work in different ways. The mercury barometer is a descendant of the one invented by Evangelistatora Cheli in 1644. The instrument is a 1 meter long glass tube, which is sealed at one end and is filled with mercury. The tube is turned open-end down into another pool of mercury. The quicksilver in the tube then rises or falls in response to rising or falling air pressure. The other type of barometer is the aneroid barometer. It's the kind you'd probably find in your home. It has a small metal bellows in it, which expands with decreasing pressure and contracts as pressure increases. The expansion or contraction moves a lever, which is calibrated to point to the correct air pressure. Some versions called barographs have an ink pen connected to the lever and a chart wrapped around a rotating cylinder.

The pen leaves a continuous trace of ink on the chart following the changes in barometric pressure over time. When Tori Cheli created the first mercury barometer, he saw that the column of mercury in the tube on that day was about 30 inches high. That's where the expression inches of mercury comes from. But scientists generally use two other units of measure, Pascals and millibars. A Pascal is defined as a force of one Newton acting on an area of one square meter. That's about a third of an ounce per square foot. A millibar is the equivalent of 100 Pascals or a little over 2 pounds per square foot. So a sea level pressure of 1000 millibars means there's more than 2,000 pounds of pressure on every square foot of your body. We'd like to know what you think about weather-wise. Please write to us at KGOU Radio, Norman Oklahoma, 73019, for Weather-wise, I'm Drew Barlow.