The key difference between barometric pressure and atmospheric pressure is that barometric pressure is the pressure we measure using a barometer whereas atmospheric pressure is the pressure exerted by the atmosphere.
Atmosphere pressure and barometric pressure are two important concepts in pressure and thermodynamics. It is vital to have a clear understanding of these concepts in order to excel in such fields.
CONTENTS
1. Overview and Key Difference
2. What is Barometric Pressure
3. What is Atmospheric Pressure
4. Side by Side Comparison – Barometric Pressure vs Atmospheric Pressure in Tabular Form
5. Summary
What is Barometric Pressure?
A barometer is a device that consists of a glass tube that is closed at one end and filled with a high-density liquid. There is a vacuum between the top of the liquid and the tube, and the other end of the tube is submerged in an open container containing the same liquid. When we use mercury as the liquid, we name this apparatus as mercury barometer.
Figure 01: A Barometer
Since the pressure of the vacuum is zero and the pressure at the liquid surface is P, the pressure difference is also P. Thus, this pressure difference is responsible for holding the liquid column. Therefore, the force from the pressure difference is equal to the weight of the column. Cancelling out the area on both sides, we get P = hdg, where h is the height we measure using the barometer is the barometric pressure. Here, P is equal to the atmospheric pressure if the open end is in the atmosphere.
What is Atmospheric Pressure?
It is important to understand the concept of pressure in order to understand atmospheric pressure. We can define the pressure as the force per unit area that applies perpendicularly on a surface. The pressure of a static fluid is equal to the weight of the fluid column above the point we measure the pressure. Therefore, the pressure of a static (non-flowing) fluid depends only on the density of the fluid, the gravitational acceleration, the atmospheric pressure and the height of the liquid above the point the pressure is measured.
Moreover, we can define pressure as the force exerted by the collisions of particles. In this sense, we can calculate the pressure using the kinetic molecular theory of gasses and the gas equation. Atmospheric pressure is the force per unit area exerted against a surface by the weight of air above that surface in the Earth’s atmosphere.
Figure 02: A Mercury Barometer
When going to high altitudes, the air mass above the point decreases, thereby reducing the atmospheric pressure. Usually, we take the atmospheric pressure at the sea level as the standard atmospheric pressure.
Furthermore, we measure the pressure in Pascal (unit Pa). The Pascal unit is also equivalent to Newton per square meter (N/m2). Other than that, we use units such as Hgmm or Hgcm to measure the pressure. The atmospheric pressure at the sea level is 101.325 kPa or sometimes we take it as 100 kPa.
What is the Difference Between Barometric Pressure and Atmospheric Pressure?
The key difference between barometric pressure and atmospheric pressure is that barometric pressure is the pressure we measure using a barometer, whereas atmospheric pressure is the pressure the atmosphere exerts. Usually, we measure atmospheric pressure in the unit Pascal, but the barometer usually gives the reading in “atmospheres” or “bar”. So, the unit of measurement contributes to another difference between barometric pressure and atmospheric pressure.
Furthermore, barometric pressure is the pressure we specifically measure from a barometer. However, we can measure atmospheric pressure using either a barometer or based on the depth of water; it is because one atmosphere equals the pressure caused by the weight of a column of fresh water of approximately 10.3 m.
The following infographic summarizes the difference between barometric pressure and atmospheric pressure.
Summary – Barometric Pressure vs Atmospheric Pressure
Sometimes, we call atmospheric pressure as barometric pressure as well. It is because we usually measure the atmospheric pressure using a barometer. The key difference between barometric pressure and atmospheric pressure is that barometric pressure is the pressure we measure using a barometer, whereas atmospheric pressure is the pressure the atmosphere exerts.
Reference:
1. Worboys, Jenny. “How a Barometer Works and Helps Forecast Weather.” ThoughtCo, Jun. 22, 2018, Available here.
Image Courtesy:
1. “Aneroid barometer” By David R. Ingham (CC BY-SA 3.0) via Commons Wikimedia
2. “MercuryBarometer” By Danomagnum – Own work (Public Domain) via Commons Wikimedia