Like every matter, air has weight. Weight of air is called air pressure.
What is air pressure? Air pressure is the weight of air molecules pressing down on the Earth. The pressure of the air molecules changes as you move upward from sea level into the atmosphere. The highest pressure is at sea level where the density of the air molecules is the greatest.
Measuring air pressure on island of Hawaii. Left picture taken at 14,000 ft at Mauna Kea observatory when the bottle was sealed. Middle picture of the sealed bottle at 9000 ft. Last picture taken at 1000 feet where the bottle was crushed.
Barometers measure air pressure
Meteorologists use barometers to measure the pressure of the air when making weather forecasts and studying weather patterns. Barometric pressure given in weather reports is giving the barometric reading in either inches of centimeters. Pressure on the barometer moves the column of mercury in a cylinder upward or downward. Readings of barometric pressure are taken by looking at the markings on the outside of the glass cylinder. Barometers are calibrated with inches or centimeters.
Mercury used for measuring pressure
The United States National Weather Service uses inches of mercury when they issue weather reports about the Earth’s surface. Measurements aloft are given in millibars. Millibars and barometers using inches and centimeters are slowly being phased out.
Hectopascal measurements
Today hectopascal measurements are worldwide by meteorologists because they are a direct measurement of pressure. A hectopascal measurement is similar to a measurement in pounds per square inch. Measurements in hectopascal are preferred because they can be used by meteorologists to study weather patterns. They can also be used with computer programs to predict the weather.