Why is the Bay of Bengal Monsoon Called 'Purvaiya' in Bihar? The Himalayan Geography Behind India's Monsoon

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Indian Subcontinent Monsoon: How the Thar Desert Acts as the Thermal Engine of South Asia's Rainfall System

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The Thar Desert as an Atmospheric Pressure Cooker: A Simple Analogy

  • Most of us have seen a pressure cooker placed on a kitchen burner. When the flame is first lit, the pressure cooker does not react immediately, but takes time because the cooker continuously absorbs heat from the burner. As heat accumulates, the temperature inside gradually rises. After reaching a certain threshold, the water inside begins to transform into steam. Since hot steam is lighter than the surrounding air, it naturally rises upward in a vertical direction.
  • As long as the burner continues to supply heat and water remains available inside the cooker, the process continues. More water is converted into steam, and the upward movement of steam persists. However, if the heat source is removed or the water is exhausted, steam generation gradually ceases, and the process slows.
  • A remarkably similar phenomenon can be observed in the atmosphere, particularly over hot, desert regions such as the Thar Desert. 


Inter-Relationship between High-pressure -& Desert formation 

  • During the summer season, the Sun acts as a giant burner, continuously supplying solar energy to the desert surface. How, at a particular time or period of the year, the  SUN acts as a "giant burner" this we have discussed in detail in one of our previous monsoon series blogs. For a detailed study, click here. 
  • Look at the following infographics-

  • The concentration of the world's major hot deserts around the subtropical high-pressure belts (approximately 20°–30° latitude in both the Northern and Southern Hemispheres) is not a coincidence. Rather, it is a direct consequence of global atmospheric circulation and the descending branch of the Hadley Cell.
  • As the air descends, it undergoes "adiabatic compression and warming," reducing its relative humidity, which creates a persistent high-pressure zone characterized by atmospheric stability and the suppression of vertical air movement. As a result, these regions remain dry throughout most of the year with less than 25 cm of rainfall, based on which categorized as "Desert".
  • Simultaneously, the subtropical regions receive intense solar radiation due to their geographical location, where temperatures enhance the rate of evaporation, causing moisture to be removed from the soil and surface water bodies more rapidly than it can be replenished through precipitation.
  • Over long periods, the combined effect of minimal rainfall and high evaporation leads the land to gradually transform into arid and semi-arid landscapes characterized by sand dunes, rocky surfaces, and desert soils, and the combination od all above mentioned factor leads to the formation of air currents, as explained in the following infographics.


Formation of Ait Currents -& Wind blowing movement


  1. We need to study these air currents formation with an understanding of the interrelationship of temperature & pressure, which we have discussed in one of our previous blogs. For detail study, click here. 
  2. The land absorbs this intense heat throughout the day, causing the air above it to become increasingly warm. As the temperature rises, the air expands, becomes lighter, and begins to rise in vertical air currents.
  • Just as steam rises inside a pressure cooker, warm air rises over the Thar Desert. 

  • This upward movement, as shown in the above image, transfers heat from the lower layers of the atmosphere to the upper layers. As air continues to rise, a region of relatively lower pressure develops near the surface. In simple terms, a low-pressure "vacuum-like" zone forms because air is continuously lifted away from the ground.
  • Nature constantly seeks to balance such pressure differences. Consequently, the surrounding air begins to move toward this low-pressure area in the form of wind. We know that -
           "movement of air in which it blows from a high air pressure area to a low pressure are hirizontally known as wind."


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Location of "Thar Desert" as the thermal engine of the monsoon
  • Now, look at the following infographics-


  • As has been shown in the above,  in the Indian monsoon, moisture-laden winds from the Indian Ocean are drawn toward the low-pressure zone that develops over northwestern India and the adjoining Thar Desert region. 
  1. The stronger the heating over the desert, 
  2. The stronger the upward air currents, and 
  3. The greater the attraction of incoming winds.
  • So, 
  1. As long as intense solar heating continues during the summer months, 
  2. warm air continues to rise, and 
  3. The low-pressure system remains active. 
  4. Resulting in the continuous inflow of moisture-bearing winds helps sustain the monsoon circulation.
  • Therefore, the Thar Desert can be compared to a giant atmospheric pressure cooker/, in which-
  1. Continuous solar heating generates rising air currents, 
  2. creates a powerful low-pressure system, and 
  3. helps pull moisture-laden monsoon winds deep into the Indian subcontinent in form of "the Arabian Sea branch" & "Bay of Bengal branch." 
  • This is why the Thar Desert is often described as "the thermal engine" of the Indian monsoon system.


Thanks.

Critical COMMENTS are welcome.


👉NCERT-GEOGRAPHY-CLASS-6-NOTES👀


What Comes Next?

  • In the next blog of this series, we will examine the “Thermal Axis of the Indian Monsoon”—how it forms and influences the intensity of monsoon rainfall across the Indian subcontinent.
  • Following that, we will move into the upper and lower atmospheric circulation systems that strengthen the monsoon. We will discuss the formation and significance of the 
  1. Anti-Trade Winds, 
  2. the Somali Jet (Findlater Jet), and 
  3. The Tropical Easterly Jet (TEJ).
  • Understanding these interconnected processes will help us uncover how the Indian monsoon evolves from a simple "thermal contrast" into one of the most complex and powerful atmospheric systems on Earth.



 


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