Air pressure, also known as atmospheric pressure, is the force exerted by the weight of air molecules above a given point. Imagine the air around you as a vast ocean, and just like water pressure increases as you go deeper, air pressure is greatest at sea level and decreases as you ascend in altitude. This is because the higher you go, the less air is above you pressing down.
Air Pressure and Altitude: Why Does it Decrease?
The Earth’s atmosphere is composed of gas molecules, and these molecules are most densely packed closer to the Earth’s surface due to gravity. As altitude increases, the number of air molecules present in a given volume decreases, leading to a reduction in air density. Consequently, with fewer air molecules overhead, the weight of the air column pressing down is less, resulting in lower air pressure. To put it in perspective, about half of all the air molecules in our atmosphere are located within the first 18,000 feet (5.6 kilometers) above sea level. This rapid decrease in air density at higher altitudes is the primary reason why air pressure drops as you go up.
Standardizing Air Pressure: Sea Level Pressure
Because air pressure is significantly affected by altitude, comparing pressure readings from different locations becomes challenging, especially if those locations are at varying elevations. To make pressure measurements meaningful and comparable across different weather stations worldwide, meteorologists convert all pressure readings to a standard reference point: sea level.
This conversion process essentially calculates what the air pressure would be if the weather instrument were located at sea level. By using sea level as a common baseline, meteorologists can accurately analyze pressure variations and patterns across geographical areas, regardless of the actual elevation of the weather station. This standardized measurement is known as sea-level pressure.
Units of Air Pressure: Inches of Mercury and Millibars
In the United States, two common units are used to measure air pressure: inches of mercury and millibars.
Inches of Mercury: This unit originates from early barometers that measured air pressure by the height of a column of mercury in a glass tube. “Inches of mercury” refers to this height, measured in hundredths of inches. You’ll often hear this unit used in weather reports from NOAA Weather Radio and news sources. Standard air pressure at sea level is approximately 29.92 inches of mercury.
Millibars (hPa): The term “bar” comes from the Greek word “báros,” meaning weight, and is the original unit of pressure. A millibar is one-thousandth of a bar. In modern meteorology, the term hectopascal (hPa) is equivalent to millibar and is the standard unit of atmospheric pressure used globally. Sea-level standard air pressure is 1013.2 millibars (or hPa). Weather maps that depict surface pressure are typically drawn using millibar or hectopascal values, allowing for easy visualization of pressure systems and patterns.
Understanding air pressure is fundamental to comprehending weather patterns and atmospheric science. By grasping what air pressure is and how it is measured, we gain valuable insights into the dynamics of our planet’s atmosphere.
