Are there changes to air mass with pressure changes?

 

The below article talks about changes caused to an air mass due to changes in pressure, why it is so, along with some frequently asked questions about air mass. 

Are there changes to air mass with pressure changes?

Air masses are large volumes of air that usually have the same temperature and pressure. Changes in temperatures and pressures can create unstable air masses, and unstable air masses can cause meteorological events like storms. 

What is an air mass?

An air mass is a large volume of air present in the atmosphere, and generally has a uniform level of temperature and pressure. Air masses extend all over the earth’s surface for over thousands of kilometers, in any direction, and can stay across various levels of the atmosphere. Changes in temperature and pressure of an air mass can create events such as storms over the area that the air mass covers. As mir masses usually form over regions with constant temperature and pressure conditions, these regions are also known as source regions. Low wind speeds allows for air masses to stay stationary in its source region, and attain the features of the source region, be it hot or cold weather conditions. 

As winds start picking up, the air mass turns unstable and moves over to new regions causing for changes in the weather of the new regions. This can be explained by the fact that this travelling air mass meets an already existent air mass over the new region, which may have different temperature, humidity, and pressure conditions, and a collision between these two masses cause what we know as storms. Experts classify air masses into 4 types depending on where they are formed: 

  • Arctic air masses are formed in the arctic region and are very cold 
  • Tropical air masses occur in low-latitude areas and display moderately warm characteristics 
  • Polar air masses are formed over high-latitude regions and are cold 
  • Equatorial air masses form near the equatorial zone and is warm

Air masses can also be categorised depending on whether they form over land or water. Maritime air masses would form over water, and is humid, whereas, continental air masses are dry and form over lands. 

What causes high pressure in air masses?

High-pressure air masses are usually formed by sudden cooling of the air masses, either from the regions below the air mass such as being cooled by the ocean waters or from above the air mass which happens via infrared cooling of masses over land where cooling of the air mass would precede any condition that may cause its warming. High-pressure air masses at a ground level are caused by air that is moving downwards, and in a high-pressure area, i.e., where the atmospheric pressure at the surface is higher than that of the environment leads to an anticyclone. This is when winds from the high-pressure area move outwards from the center into low-pressure areas. 

What causes a low-pressure air mass? 

Low pressure refers to pressure put downwards onto the surface of the earth. A low-pressure system is when the center of the system has lower pressure than its surrounding environments. Winds would blow inwards from high-pressure areas, and the air in the center of the mass would rise up, causing a cyclone. As air rises, water vapor condenses, forms clouds, and gives precipitation. Usually, tropical and warm air masses have lower pressure.

Below is a diagram that shows the changes to air masses when the pressure changes in their environment. Low-pressure systems lead to cyclones and high-pressure systems lead to anticyclones. 

Source: Jensen D., 2017

Frequently Asked Questions (FAQs): Are air masses affected by changes in pressure?

How are air masses depicted in maps?

Air masses are classified and denoted on weather maps using the following symbology (Rutledge K. et.al., 2011):

  • A lowercase letter describes the amount of moisture in the air mass: 
    • m for maritime (moist) 
    • c for continental (dry)
  • An uppercase letter denotes the heat of the air mass: 
    • E for equatorial
    • T for tropical
    • M for monsoon
    • P for polar
    • A for Arctic or Antarctic
    • S for superior (refers to when dry air is formed as a result of powerful downward movement of the atmospheric levels) 
  • A lowercase letter to show the relationship between the air mass and the surface of the earth: 
    • k signifies that the air mass is colder than the ground below it
    • w describes an air mass that is warmer than the ground below it

What factors affect the air pressure? 

There are 3 factors that can affect air pressure: temperature, altitude or elevation, and moisture or water vapor (Lisbdnet, 2021). 

References 

Jensen D. (2017, April 03). What causes an air mass to have low pressure. Viewed on Quora on 02-02-2022. https://www.quora.com/What-causes-an-air-mass-to-have-low-pressure 

Lisbdnet. (2021, December 31). Response to: What causes an air mass to have high pressure? Viewed on 02-02-2022. https://lisbdnet.com/what-causes-an-air-mass-to-have-a-high-pressure/ 

Rutledge K., Ramroop T., Boudreau D., McDaniel M., Teng S., Sprout E., Costa H., Hall H., & Hunt J. (2011, March 30). Air Mass. National Geographic Society. Viewed on 02-02-2022. https://www.nationalgeographic.org/encyclopedia/air-mass/ 

UCAR. (n.d.). The highs and lows of air pressure. Center for Science Education. Viewed on 02-02-2022. https://scied.ucar.edu/learning-zone/how-weather-works/highs-and-lows-air-pressure#:~:text=A%20low%20pressure%20system%20has,forming%20clouds%20and%20often%20precipitation.  

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