Barry Emig, age 11, of Williamsport, Pennsylvania, for his question:
How do they weigh the air?
We weigh such things as stones and sugar by how hard they press down on a solid surface. If, for example, the amount of sugar is equal to a man made pound weight, we call it a pound of sugar. This method of measuring is possible because everything is pulled to the surface of the globe by the earth's gravity. The weight of an object is the amount of gravity exerted upon it. The same force also gives weight or pressure to the air above the earth's surface.
It seems to be weightless as well as invisible, yet the air in an average living room weighs two or more pounds. It belongs to the great ocean of atmosphere that enfolds our globe and extends hundreds of miles above the surface. Its total weight is estimated to be 5,600 million million tons. This weight equals the pressure it exerts on the entire globe, as its mass is hugged to the surface by the earth's gravity.
Meteorologists would be lost without a way to measure small local samples of atmospheric pressure. This is because the density of the the air is in constant turmoil and changing pressures indicate what sort of weather is on the way. The basic method for measuring local atmospheric pressure was invented in 1643. The original gadget for the job has been improved and refined, but its basic principle is still the same.
The instrument is the barometer, meaning the weight measurer. The original model used a column of mercury to measure the weight of a column of air extending from the ground to the top of the atmosphere. It was a glass tube 30 inches tall. The tube was almost filled with liquid mercury and its open end was turned upside down and set into a small glass dish. Some of the mercury in the rube escaped down into the dish but most of it remained in the tube to indicate the weight of the air.
It worked because the atmospheric column above the dish exerted pressure on the surface of the mercury. Its weight pushed hard enough to keep most of the mercury inside the glass tube. A scale of inches beside the tube indicated pressure changes in the air sample. Later the scale was translated into more convenient centimeters. And still later, the "bar" was invented as the standard unit for measuring the weight of atmospheric pressure.
Naturally, the weight of the sample air column grows lighter as we ascend higher into the atmosphere. Standard air pressure is the weight of the column of air at sea level. Its weight, under normal conditions, exerts enough pressure to push the mercury in a barometer tube up to 29.92 inches or 760 millimeters. The "bar" of atmospheric pressure equals a million dynes per square centimeter, or 1,000 millibars.