Atmospheric Pressure: What do you need to know?

The below article talks about atmospheric pressure, the factors that affect the atmospheric pressure, reading and interpreting barometric pressure readings, about isobars on weather maps, along with some frequently asked questions about atmospheric pressure and interpreting them. 

What is atmospheric pressure?

Atmospheric pressure, a.k.a. Barometric pressures refer to the amount of force exerted per unit area in a specified atmospheric column. It is measured using a mercury barometer, a.k.a. Barometric pressure, or an aneroid barometer. There are several factors that determine the changes to atmospheric pressure including the earth rotation, gravitational force, temperature, altitude, humidity, etc. The below article talks in detail about atmospheric pressure. 

Atmospheric Pressure 

When we look up into the sky, there is one thing we never see- air, lots of it. We live at the very bottom of the atmospheric layers and the weight of the air above us can be referred to as the atmospheric pressure. It is estimated that there are 14.7 pounds of air per square inch of the Earth; which means that the air exerts 14.7 pounds per sq. inch (psi) of pressure on the surface of the Earth (UCAR, n.d.). As we move higher up into the atmospheric layers, the atmospheric pressure decreases; and this can be explained by the fact that as we move higher there are lesser air molecules present to exert the same kind of force we see in the lower atmospheric levels. 

Since the surface of the Earth receives unequal amounts of heating, air pressure over the surface of the Earth is prone to constant change. When air is warmed by the Sun, the pressure decreases as warm air rises, and these areas are called low-pressure systems, and areas with higher air pressure are referred to as high-pressure systems 

Source: UCAR, n.d.

A high-pressure system is characterized by higher pressure at the center in comparison to other areas around it. Therefore, winds tend to move away from the center into lower pressure areas in the surroundings, and thus rotate clockwise in the northern hemisphere and counterclockwise in the southern hemisphere. This is called an anti-cyclonic flow, and due to this phenomenon, air from the higher atmospheric levels tends to move downwards as air from the center of the system moves outwards into lower pressure areas. On maps, a high-pressure system is labeled with an “H”. A low-pressure system is characterized by lower pressure at the center in comparison to other areas around it. Wind tends to move towards areas of lower pressure, and air rises. As the air rises, water vapor condenses to form clouds and eventually causes rainfall. Due to the Earth’s spin and the Coriolis effect, a low-pressure wind system that moves counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. This phenomenon is also called the cyclonic flow and on maps are labeled with an “L”. 

Factors that influence Atmospheric Pressure

There are several factors that contribute towards determining a high or low pressure system. Some of these factors have been listed below and how it affects a pressure system has been described. 

Temperature

Air expands on heating, which causes a decrease in density and thus lower pressure. Similarly, the density of the air increases as temperatures decrease, thus creating high-pressure systems. Areas towards the equator, therefore, are low-pressure systems, and areas towards the polar regions are high-pressure systems. Due to which it is said that these two elements are inversely proportional, and their relationship is captured in the below sentence:

“When the mercury of the thermometer rises, the mercury of the barometer falls.”

Altitude Changes 

Atmospheric pressure depends on the number of air molecules that are present in the air, right above the area of focus. Therefore, atmospheric pressure is the highest at sea level, and lower as we move upwards into the different atmospheric levels. Moving upwards into the different atmospheric levels means we leave behind the heavy gases and pollutants within the lower atmosphere layers, therefore with the lack of air molecules at higher areas and the non-availability of these gases or pollutants, density decreases and is therefore, a low-pressure area. 

The decrease in pressure is said to be approximately 1 cm of mercury for every 110 m of ascending into the higher atmospheric levels (Shaalaa, n.d.). 

Humidity 

Conversion of water from liquid to vapour form due to evaporation is known as the humidity (the amount of moisture in air). Since water vapor is light-weight, they are able to rise, and the pressure of the air decreases. Since the water content in the atmosphere keeps changing over time and is different across the Earth’s surface at different times, the atmospheric pressure also keeps varying as a result. 

Earth’s Gravity 

Atmosphere levels warp around the Earth due to the planet’s gravitational strengths. The strength of this force decreases, the further you move from the Earth’s core. Therefore, the air molecules are held more concentrated to the Earth’s surface and is lesser as we move upwards. This is another reason why polar regions have higher atmospheric pressures in comparison to equatorial regions, as polar regions are comparatively closer to the Earth’s core. 

Rotation of the Earth

Earth’s rotation produces centrifugal force, which has the tendency to push things away from the core of the planet. This is another reason why air pressure is lower at the equators, as the centrifugal force tends to push away the air molecules and gaseous particles from the equatorial regions. 

Reading a Barometer 

In general, a digital or an analog barometer would likely show atmospheric pressure in the unit of “inches of mercury” (inHg). However, the standard unit used is Pascal (Pa), which is approximately 3386.389 inHg. Other units used are millibars (mb) and is equal to 100,000 Pa (Oblack R., 2020). 

High Pressure 

A reading above 30.20 inHg / 102268.9 Pa / 1022.689 mb is considered as high pressure and the area is referred to a high-pressure system. A rising or steady pressure could mean fair weather, a slowly falling pressure could mean continued fair weather, whereas a rapid descend of pressure would mean cloudy and warmer conditions. 

Normal Pressure

A reading between 29.8 inHg and 30.20 inHg / 100914.4 Pa and 102268.9 Pa / 1022.689 mb and 1009.144 mb is considered as normal pressure readings and is accompanied by a steady weather. Rising or a steady maintain of the pressure shows continued conditions of existing weather, slowly falling pressure levels could indicate changes in the weather at a mainimal level, and rapidly falling pressure would indicate likely rainfall, or snow if the temperatures are cold enough. 

Low Pressure 

A reading below 29.8 inHg / 100914.4 Pa / 1009.144 mb is generally considered as low pressure and the area is considered to be a low pressure system. A rising or steady pressure could indicate clear and cooler weather, a slowing falling pressure would indicate possible rainfall, and rapidly falling pressure would indicate an imminent storm moving towards the area. 

Understanding Isobars 

Metereologists use lines called as isobars on maps to connect areas of equal atmospheric pressure. A weather map could feature lines that connect all points with a similar pressure reading, and the gradation of these lines on the map shows that points above a certain line would show lower pressure systems and below a line would show higher pressure systems. Isobars are helpful for weather researchers and meterologists to identify possible weather patterns and cast projections. 

Below is an isobar map of the United States on December 20, 2021. It clearly represents through isobar lines that areas of similar pressures and which points are high-pressure systems or low-pressure systems with corresponding pressure readings. 

Source: NOAA, 2021

Frequently Asked Questions (FAQs): What do I need to know about atmospheric pressure?

Which part of the world has the highest atmospheric pressure?

An extremely cold dome of high atmospheric pressure across eastern Asia has produced the highest ever barometric pressure reading on Earth. At 7 AM, on December 29, 2020, the pressure at Tsetsen-Uul in Mongolia had shown a record-breaking reading of 1,094.3 millibars/ 32.31 inches. This was the highest pressure reading ever reported with the record temperatures at -45.5 ℃. Tsetsen-Uul is at an altitude of 6,325 feet or 1,928 meters. The previous record was also found in Mongolia in Tosontsengel in 2004. 

(Henson B., 2020)

How to interpret the weather using barometer readings?

Mercury barometers can let us know of the immediate weather changes, just on the basis of the changes made to atmospheric pressure. Here are some ways you could interpret a barometric reading:

  • The barometer reading will rise when air is dry, cool, and pleasant 
  • A rising level in a barometer usually means that the weather is improving
  • A falling barometric reading usually means that the weather is getting worse 
  • If the pressure readings drop suddenly, it could indicate a storm 
  • If the pressure is steady over time, there is likely no changes in the weather to be expected. 

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References

Henson B. (2020, December 29). A frigid air mass in Mongolia may have just crushed a world record for surface air pressure. Washington Post. Viewed on 12-20-2021. https://www.washingtonpost.com/weather/2020/12/29/mongolia-air-pressure-record/ 

National Oceanic and Atmospheric Administration (NOAA). (2021, December 20). Maps showing present situation. National Weather Service. Viewed on 12-20-2021. https://www.weather.gov/ffc/mapslast  

Oblack R. (2020, March 04). How to read a barometer: Use rising and falling air pressure to predict the Weather. ThoughtCo. Viewed on 12-20-2021. https://www.thoughtco.com/how-to-read-a-barometer-3444043 

Raha P. (n.d.). Air Pressure: Factors & Distribution | Atmosphere | Earth | Geography. Geography Notes. Viewed on 12-20-2021. https://www.geographynotes.com/atmosphere/air-pressure/air-pressure-factors-distribution-atmosphere-earth-geography/5438 

Shaalaa.com. (n.d.). Explain briefly the factors that affect atmospheric pressure. Geography. Viewed on 12-20-2021. https://www.shaalaa.com/question-bank-solutions/explain-briefly-the-factors-that-affect-atmospheric-pressure-atmospheric-pressure_133851  

UCAR Center for Science Education (UCAR). (n.d.). The highs and lows of air pressure. How Weather Works. Viewed on 12-20-2021. https://scied.ucar.edu/learning-zone/how-weather-works/highs-and-lows-air-pressure 

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