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

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Evolution of Monsoon Science: From Halley's Thermal Theory to the Thermal Axis of the Indian Monsoon

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  • The nature of science is continuous evolution. Every generation builds upon the knowledge of the previous one. While "Edmond Halley's Thermal Theory" provided the first scientific explanation of the monsoon through differential heating of land and sea, later research revealed that the Indian Monsoon is far more complex than a simple land-sea breeze on a continental scale.
  • Following the severe droughts experienced in India during the 1960s and 1970s, Indian meteorologists, climatologists, and geographers intensified efforts to understand the causes of monsoon variability in collaboration with India's evergreen friend country's geographers, and, yes, I mean here Russia. 
  • Through- 
  1. upper-air observations, 
  2. atmospheric surveys, and 
  3. international scientific collaboration, researchers discovered that the monsoon system is influenced not only by the heating of the Indian landmass but also by several large-scale thermal centres across South Asia.
  • Three major thermal components emerged as particularly significant:

  1. The Thar Desert
  2. The Tibetan Plateau
  3. The Himalayan Snow-Albedo System



The Thar Desert: Surface Thermal Engine

      


  • During the pre-monsoon months of May and June, the Thar Desert experiences extreme surface heating, with temperatures frequently exceeding 45°C. Why does this happen? Click here to study. 
  • The dry surface, sparse vegetation, low soil moisture, and intense solar radiation allow rapid warming of the land and the overlying air.
  • As warm air rises, a strong thermal low-pressure zone develops over northwestern India and adjoining Pakistan. This thermal low strengthens the pressure gradient between the Indian Ocean and the Asian continent, helping attract moisture-laden monsoon winds toward the Indian subcontinent.



The Tibetan Plateau: Elevated Heat Source

     
  • The Tibetan Plateau plays a unique role because of its extraordinary elevation. Often referred to as the "Roof of the World," the plateau stands at an average elevation of about 4,500 metres above sea level.
  • During summer, intense solar radiation heats the plateau and the air above it. Because this heating occurs at a great altitude, the Tibetan Plateau acts as an elevated heat source, warming the middle and upper troposphere and influencing large-scale atmospheric circulation across Asia.
  • This elevated heating helps strengthen the summer monsoon circulation and contributes to the development of important "upper-air wind systems."


The Himalayan Snow-Albedo Effect

    



  • The Himalayan mountain system influences the monsoon not only as a physical barrier but also through its snow cover.
  • Snow possesses a very high albedo, meaning it reflects a large proportion of incoming solar radiation back into space. When snow cover is extensive, less solar energy is absorbed by the surface, resulting in weaker heating. Conversely, reduced snow cover lowers albedo, allowing greater absorption of solar energy and stronger regional heating.
  • Therefore, variations in Himalayan snow cover can significantly influence the thermal conditions over South Asia and affect the strength of the monsoon circulation.

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Formation of a Thermal Axis

     


  • Together, the Thar Desert, the Tibetan Plateau, and the Himalayan thermal system create a broad thermal axis extending across South Asia. This thermal belt traps and concentrates heat over the region lying south of the Himalayas, stretching broadly from Pakistan in the west to Bangladesh in the east.
  1. The stronger the heating over these thermal centres, 
  2. The stronger the pressure gradients and atmospheric circulation that support the Indian Summer Monsoon. 
  • When these thermal drivers are collectively strong, monsoon circulation tends to be more vigorous. Conversely, when one or more of these thermal components weaken, the monsoon circulation may also weaken.
  • The intense heating over South Asia contributes to the establishment of important upper-air circulation systems, including "the Tropical Easterly Jet," which plays a significant role in monsoon dynamics.
  • The detailed mechanism behind the formation of-
  1. anti-trade winds, 
  2. the Tropical Easterly Jet, and 
  3. their influence on monsoon rainfall will be examined in the next lecture.
  • For the present discussion, the Thar Desert, the Tibetan Plateau, and the Himalayan thermal system together constitute the principal thermal axis of the Indian Monsoon, creating the large-scale heat engine that drives atmospheric circulation over the Indian subcontinent.



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