Rocket unrolls a chart up the wall, ground at the bottom, sky at the top. "The atmosphere, drawn to scale."
Nova shades the bottom slice. "The National Weather Service says half of all the air molecules are in this part."
Raven measures it. "That is only about 18,000 feet. Airliners fly higher than that."
"So above that line the air is thin," Rocket says. "Fewer molecules, less weight, less pressure."
"What do you notice about the numbers on the side?" Raven asks. "Millibars, and inches of something."
"Inches of mercury," Nova says. "An old way and a new way to say the same thing."
Rocket writes 1013.25 at the very bottom. "Sea level. Our starting line."
The National Weather Service explains that as elevation increases, the number of air molecules decreases.
Fewer molecules means the air is less dense, and so there is less air pressure.
One half of the air molecules in the atmosphere are contained within the first 18,000 feet (5.6 kilometers).
This decrease with height is why the pressure at a mountain station cannot be compared directly with one at sea level.
| Unit | Standard pressure at sea level | Who uses it |
|---|---|---|
| Millibars (mb) | 1013.25 | Meteorologists today |
| Hectopascals (hPa) | 1013.25 (1 hectopascal equals 1 millibar) | The same size as a millibar |
| Inches of mercury | 29.92 | Historically, with mercury barometers |
| Pounds per square inch | 14.7 | The weight of air above each square inch |
Millibar values used in meteorology range from about 100 to 1050.
Historically, pressure was described in inches of mercury: how high the air could push a column of mercury in a glass tube.
Today meteorologists use millibars, and weather stations use electronic sensors that measure pressure continuously.
The National Weather Service glossary describes a mercury barometer as a sealed glass tube in which mercury rises or falls.
Mercury barometers are heavy and fragile, and mercury is never used at home. Your station will not use one.
An aneroid barometer uses no liquid. A needle attached to a sealed box moves as the box top bends in or out with pressure.
The idea of a dry barometer came from Gottfried Wilhelm Leibniz around 1700. The first working aneroid barometer was built in 1843 by Lucien Vidie.
The aneroid barometer is the most common type for home use. Tomorrow you build a simple one.
| Statement | True or false? |
|---|---|
| Air pressure increases as you go higher. | ? |
| Millibar values in meteorology range from about 100 to 1050. | ? |
| Mercury barometers are a good choice for a home station. | ? |
| Today, weather stations use electronic sensors to measure pressure. | ? |
Units mastered. Tomorrow's Station Lab builds your own barometer from a jar.