SEASONALITY

SEASONALITY

A season is a period of the year distinguished by specific climatic conditions. The four primary seasons—spring, summer, fall, and winter—occur in a regular cycle, each defined by unique light, temperature, and weather patterns.

Primary Reasons for Seasons:

1. Tilted Axis of Earth:
   • Seasons occur due to the tilt of the Earth on its axis relative to its orbital plane. The hemisphere tilted towards the Sun experiences summer, while the hemisphere tilted away experiences winter.
   • In June: The Northern Hemisphere is tilted toward the Sun, resulting in longer daylight hours and the summer season.
   • In December: The Northern Hemisphere is tilted away from the Sun, leading to shorter daylight hours and the winter season.
2. Rotation and Revolution of the Earth:
   • The Earth’s movement on its own axis and around the Sun has a direct impact on the climatic conditions experienced throughout the year.

SEASONS IN INDIA

The climate of India is not uniform but highly varied due to its location in the tropical and subtropical zones, the influence of oceans, and the presence of diverse topographical features, such as mountain ranges.

Climatic Regions of India:

India’s climatic regions are determined by the varying rainfall patterns and geography. The country can be divided into the following four broad climatic zones based on temperature and precipitation:

• Wet zone
• Intermediate zone
• Dry zone
• Arid zone

Climatic Zones of India:

India is divided into four major climatic zones based on rainfall, location, vegetation, and main agricultural crops:

1. Wet Zone:
   • Average annual rainfall: More than 200 cm.
   • Location: Western Ghats including the western coastal parts of Mumbai, Karnataka, Kerala, Assam, Eastern half of Bengal, Terai region of UP, Orissa, Chhattisgarh, and North Bihar.
   • Natural Vegetation: Evergreen and semi-evergreen tropical forests in the west coast and Assam; dry deciduous forests in other parts.
   • Main Agricultural Crop: Paddy.
2. Intermediate Zone:
   • Average annual rainfall: Above 100 cm but less than 200 cm.
   • Location: South-west Bengal, Bihar, Orissa, Madhya Pradesh, East UP, North-eastern Andhra Pradesh, and the Eastern slopes of the Western Ghats and East Tamil Nadu.
   • Natural Vegetation: Dry deciduous forests and scrub forests.
   • Main Agricultural Crop: Rice.
3. Dry Zone:
   • Average annual rainfall: Between 50 cm and 100 cm.
   • Location: Western UP, North-eastern Punjab, Delhi, South-eastern Mumbai, Eastern Rajasthan, and parts of Gujarat.
   • Natural Vegetation: Dry deciduous forests, shrubs, and thorn scrubs.
   • Main Agricultural Crop: Wheat and millets.
4. Arid Zone:
   • Average annual rainfall: Less than 50 cm.
   • Location: South-western parts of Rajasthan and Punjab, North-eastern Mumbai, and North-western Gujarat.
   • Natural Vegetation: Thorny scrubs and short-lived herbs.
   • Main Agricultural Crop: None mentioned.

Indian Monsoons:

Monsoons are seasonal winds characterized by their periodic reversal, bringing rains during summer and flowing from land to sea during winter. There are two primary monsoon seasons in India:

1. Southwest Monsoon Season
2. Retreating Monsoon Season

Mechanism of the South-West Monsoon:

1. Differential heating and cooling of land and water: The land heats up quickly, creating low pressure, while the surrounding seas experience higher pressure.
2. Shift of ITCZ (Intertropical Convergence Zone): In June, the Sun is directly overhead at the Tropic of Cancer, shifting the ITCZ northwards to the Indo-Gangetic plains, leading to southwest monsoon winds blowing from the Arabian Sea and Bay of Bengal.
3. Coriolis Force: The southeast trade winds from the southern hemisphere cross the equator and change direction to southwest, moving toward the landmass under the influence of the Coriolis effect.
4. Withdrawal of the Westerly Jet Stream: The westerly jet stream, which had been sitting over northern India, retreats as the ITCZ moves, marking the onset of the southwest monsoon.
5. Onset of Easterly Jet Stream: The easterly (Somali) jet stream forms at around 15°N latitude after the westerly jet stream withdraws, playing a crucial role in the burst of the monsoon in India.
6. Bifurcation of Monsoon Winds: As the monsoon winds approach the land, they bifurcate into two branches due to the relief and thermal low pressure:

• Arabian Sea branch: This branch originates over the Arabian Sea.
• Bay of Bengal branch: Deflected by the Arakan Hills along the coast of Myanmar, a portion of this branch is directed towards West Bengal and Bangladesh from the southeast instead of directly from the southwest.

1. Breaks in Rainfall: The monsoon rains do not occur continuously but take place in intervals. These breaks are often connected to the movement of the monsoon trough.

Mechanism of the Retreating Monsoon:

• October heat: After the southwest monsoon ends, the land still experiences high temperatures and humidity, resulting in oppressive conditions known as the “October heat.”
• Temperature drop: By the second half of October, temperatures start falling, particularly in northern India.
• Regional variation: The retreating monsoon is generally dry, except for the rain that occurs in the eastern part of the peninsula. The months of October and November tend to experience rainfall in this region.
• Cyclonic depressions: Widespread rainfall in this season often results from cyclonic depressions originating from the Andaman Sea and affecting the eastern coast of southern India. These cyclones are typically very destructive.

Monsoon Prediction in India:

• Statistical weather model: Gilbert Walker, a British physicist who led the Indian Meteorological Department (IMD), developed an empirical model for weather prediction based on the relationship between two weather phenomena.
• Numerical models: From 2014, the IMD began utilizing numerical models to supplement its long-range weather forecasting.

Issues of Monsoon Prediction in India:

1. District-wise weather data: The IMD provides district-wise data, but this can be inadequate as predictions indicate scattered rainfall over large areas. This means only 25-50% of the district may receive rain when rainfall is forecasted for the entire district.
2. Insufficient data collection: The IMD gathers weather data through 679 automatic stations, 550 surface observatories, 43 radiosonde or weather balloons, 24 radars, and 3 satellites, but the data still falls short of providing the accuracy required for precise forecasting.
3. Lack of data on current weather conditions: Prediction models currently require significant data for proper functioning, and until more comprehensive data is available, highly advanced prediction models (like those using supercomputers) may not provide accurate forecasts.

1. Substandard automatic weather stations: These stations need regular calibration and cleaning, which is often neglected, resulting in inaccurate data collection.
2. Data gaps: Significant gaps exist, especially regarding dust, aerosols, soil moisture, and maritime conditions, which are critical for accurate weather prediction.
3. Lack of indigenous advancement: Many weather models used in India are based on Western data and conditions. Little effort has been made to adapt or fine-tune these models for Indian climatic conditions.

Indian Initiatives:

1. Monsoon Mission of India: An initiative by the Ministry of Earth Sciences to enhance the forecasting ability of the Indian Meteorological Department (IMD) using high-resolution data, super-parameterization, and data assimilation techniques.
2. Collaboration with the Indian Council of Agricultural Research (ICAR): ICAR provides district-level agro-meteorological advisories to farmers through 130 agro-met field units in local languages.
3. Indo-US expedition: In 2018, the Indian Ocean Research Vessel ‘Sagar Nidhi’ departed from Chennai as part of an Indo-US expedition to better understand the southwest monsoon over the Bay of Bengal and gather data to improve monsoon prediction models.
4. National Supercomputing Mission: This mission aims to address gaps in computational power required for better monsoon forecasting by developing advanced supercomputing capabilities in India.