Climate of India

India stands as the quintessential representation of a monsoonal climate on a global scale. The Indian monsoon system is characterized by distinct wet and dry seasons, accompanied by significant annual temperature variations, which culminate in three primary climatic phases across much of the country. These are: (1) a hot and wet season commencing around mid-June and extending until the close of September, (2) a cool and dry season beginning in early October and lasting through February, and (3) a hot and dry season—typically marked by elevated atmospheric humidity—from approximately March to mid-June. It is important to note that the exact length of these phases can fluctuate by several weeks, and this variability is observed not only across different regions of India but also from one year to the next. Furthermore, regional climatic disparities, which can be quite significant, arise due to a variety of local factors, such as altitude, topography, and the proximity to water bodies.

The monsoons

A monsoonal system is typified by the cyclical inversion of dominant wind currents, which results in distinct periods of precipitation and aridity. In the context of India, the monsoon season, often referred to as the southwest monsoon, spans from mid-June to the onset of October. During this interval, the Indian Ocean acts as the source of humid air that traverses the subcontinent, precipitating extensive rainfall that can lead to significant inundation. Approximately 75% of India’s annual rainfall is concentrated within this timeframe. Conversely, the nation experiences its most parched months, known as the retreating monsoon phase, from November to February. In this phase, arid air from the continental interior of Asia advances over India towards the ocean. The months of October and March through May are generally marked by erratic winds that lack a definitive directional trend.

The southwest monsoon

The Southwestern Monsoon, despite its name, encompasses two primary trajectories: one that moves eastward from the Arabian Sea and another that advances northward from the Bay of Bengal. The Arabian Sea branch commences its journey by impacting the western coast of the Indian peninsula and then ascending the adjacent Western Ghats. As this branch traverses the Ghats, the air undergoes cooling, thereby reducing its capacity to hold moisture, which results in substantial rainfall on the windward side of this elevated terrain. The annual precipitation in certain areas of this region can surpass 100 inches (2,540 mm), reaching up to 245 inches (6,250 mm) in Mahabaleshwar at the summit of the Western Ghats. On the contrary, the descending winds on the leeward side of the Ghats experience an increase in moisture retention, leading to the formation of a semi-arid zone with annual precipitation falling below 25 inches (635 mm) in some areas.

The Bay of Bengal monsoon branch similarly traverses eastern India and Bangladesh, inducing rainfall through a comparable mechanism to that seen along the Western Ghats. This is especially evident on the Shillong Plateau, where Cherrapunji records an average annual rainfall of 450 inches (11,430 mm), ranking among the highest globally. The Brahmaputra valley to the north also undergoes a rain-shadow effect. However, the proximity of the Himalayas causes the winds to ascend once more, creating a zone of significant rainfall. The Himalayas act as a barrier, redirecting the Bay of Bengal monsoon branch westward across the Gangetic Plain, arriving in Punjab by the first week of July.

Upon reaching the Gangetic Plain, both branches converge. By the time they arrive in Punjab, their moisture content has substantially diminished. This decrease in rainfall from east to west is marked by a reduction from 64 inches (1,625 mm) in Kolkata to 26 inches (660 mm) in Delhi, and further west, it transitions into arid conditions. In the northeastern part of the Indian peninsula, the two monsoon branches occasionally merge, resulting in weak weather fronts that still manage to deliver significant rainfall, with the Chota Nagpur plateau receiving more than 60 inches (1,520 mm) in some areas.

Rainfall during the retreating monsoon

In India, the monsoon season typically withdraws with limited and rather mild rainfall across most regions. However, this pattern is notably different along the southeastern coastline and extends to some areas inland. During this period, as the monsoon winds reverse direction and flow from the northeast across the Bay of Bengal, they gather considerable moisture. This moisture is then deposited over the Indian peninsula as the winds move inland. Consequently, from October to December, the coastal areas of Tamil Nadu receive a substantial portion of their annual rainfall, amounting to approximately 40 inches (1,000 mm), which represents at least half of their yearly precipitation. This phenomenon, which brings rainfall during the otherwise dry retreat of the monsoon, is referred to as the northeast or winter monsoon.

Additionally, the northern regions of India experience a distinct form of winter precipitation. This is attributed to weak cyclonic disturbances that originate in the Mediterranean basin. Upon reaching the Himalayan region, these disturbances result in prolonged periods of light rain and overcast conditions, followed by bouts of cold weather and snowfall. The state of Jammu and Kashmir, in particular, receives a significant amount of its annual precipitation from these weather systems.

Tropical cyclones

Intense tropical cyclones frequently develop in India throughout various seasonal phases, including the pre-monsoon, the onset of the monsoon, and the post-monsoon periods. These cyclones, which emerge from the Bay of Bengal as well as the Arabian Sea, are capable of reaching speeds exceeding 100 miles per hour (approximately 160 kilometers per hour). They are particularly infamous for generating heavy rainfall and significant storm surges upon making landfall along the Indian coastline. Coastal regions of Andhra Pradesh, Odisha, and West Bengal are particularly vulnerable to the impacts of these powerful storms.

Importance to agriculture

Monsoons are a critical factor in the agricultural sector of India, significantly influencing crop production due to the considerable fluctuations in rainfall patterns. The inconsistency in the timing and volume of monsoon rains introduces a notable degree of uncertainty in the nation’s agricultural output. Favorable monsoon seasons can result in substantial crop yields, whereas inadequate rainfall has the potential to lead to widespread crop failure, particularly in regions where irrigation systems are not well-established. Additionally, instances of extensive flooding can inflict damage on agricultural lands. It is generally observed that regions with higher average annual rainfall tend to have more reliable precipitation patterns. However, very few regions in India receive a sufficiently high average annual rainfall to completely mitigate the risk of sporadic droughts and the associated risk of crop failure.

Temperatures

In India, peak temperatures are typically observed in May or June, immediately before the onset of the southwest monsoon’s cooling rains. There is often a secondary peak in temperature during September or October as rainfall diminishes. Coastal regions experience a narrower temperature range compared to inland areas, and this range tends to expand with increasing latitude. For instance, the seasonal temperature variation near the southern tip of India is minimal, with Thiruvananthapuram in Kerala showing an average temperature deviation of only 4.3 °F (2.4 °C) from its annual mean of 81 °F (27 °C). In contrast, the temperature range in the northwest is considerably wider. Ambala in Haryana, for example, experiences temperature swings from 56 °F (13 °C) in January to 92 °F (33 °C) in June. Elevated regions, including many Himalayan hill stations—remnants from the British colonial era—provide a respite from the intense heat that can afflict India. Extreme heat waves, like the one that affected much of the subcontinent in mid-2015, pose a significant threat to life.

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