Classical Theory or The Thermal Concept of Monsoon

Classical Theory or The Thermal Concept of Monsoon
Posted on 18-08-2023

Classical Theory or The Thermal Concept The tenth-century Arab scholar Al Masudi made notable contributions by intricately detailing the characteristics of monsoon winds and their distinct pattern of seasonal reversal.

In 1686, Sir Edmund Halley undertook the task of elucidating the origin of monsoons through what came to be known as the 'classical theory.' This theory, often referred to as the thermal concept, posits that monsoons find their genesis in the realm of immense land and sea breezes, magnified to an extraordinary scale. The driving force behind these colossal air movements is the dissimilar and fluctuating seasonal heating exhibited by continental and oceanic expanses.

This classical theory delves into the heart of the matter by underscoring the fundamental principle of differential rates of temperature transformation between landmasses and oceanic expanses. The distinctive nature of monsoons, as envisioned by this concept, arises from the interplay of heat absorption and dissipation across these diverse terrain types.

Explanation of Monsoons by Thermal Origin

The concept of the thermal origin of monsoons was elucidated by Sir Edmund Halley in 1686, subsequently recognized as the 'classical theory' of the Asiatic monsoon's genesis. This theory posits that monsoons are essentially amplified versions of land and sea breezes, occurring on a grand scale due to the varying heating and cooling patterns of continental and oceanic regions.

In the summer season of the northern hemisphere, when the sun is directly overhead the Tropic of Cancer, the Indian landmass experiences heightened heating compared to the adjacent seas. This leads to the formation of a low-pressure zone over the Indian subcontinent in contrast to the Indian Ocean. Consequently, a thermal pressure gradient emerges, directing winds from the ocean towards the Indian subcontinent, culminating in the initiation of south-westerly winds originating from the Indian Ocean and moving towards India. These winds, known as the southwest monsoon, transport substantial moisture and give rise to abundant rainfall over the land.

Conversely, land cools at a faster pace, causing the Indian Ocean to remain warmer than the Indian subcontinent during the winter season. This alteration in temperature results in a reversed pressure gradient that prompts winds to blow from the northeast to the southwest. These winds, termed the northeast monsoon, traverse from the Indian subcontinent to the Indian Ocean. Given the land-to-sea direction of these winds, they carry limited moisture, resulting in a predominantly dry winter period over the Indian landmass.

The thermal theory of monsoons interprets this phenomenon as a manifestation of large-scale land and sea breezes occurring at a sub-continental level. Unlike the typical local diurnal oscillation of land and sea breezes, the direction of monsoons reverses seasonally rather than daily.

Critique of the Thermal Theory or Classical Theory of Monsoons

  1. This theory confines monsoons within the scope of regional surface winds.

  2. It struggles to account for the erratic and unpredictable nature of dynamic monsoons.

  3. Contemporary climatologists raise skepticism regarding the thermal basis of the low (summer) and high (winter) pressure systems over the land (Indian subcontinent). According to their perspective, the positions of these pressure areas undergo abrupt shifts, driven not only by thermal conditions but also by dynamic factors. The winter high-pressure zone, they argue, emerges due to anticyclonic conditions prevailing over the Indian subcontinent, influenced by southerly westerly jet streams. Similarly, the low-pressure areas during the summer are linked to cyclonic lows.

  4. Monsoon precipitation is not solely influenced by orographic factors; rather, it encompasses a blend of orographic, cyclonic, and convectional influences.

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