Weather and Climate

WEATHER AND CLIMATE

**Weather** is the momentary or day-to-day state of the atmosphere. It includes temporary changes in atmospheric elements like temperature, pressure, wind direction and velocity, humidity, and precipitation. Weather can change within a day or week.

**Climate** refers to the average of weather conditions over a longer period of time, typically 30-50 years or more. Climate changes imperceptibly and is noted only after studying data over many decades. India's climate is fundamentally **monsoonal** in rhythm and character, meaning it is dominated by seasonal reversal in wind direction.

**Key Differences:**

Weather: Changes rapidly, temporary variations

Climate: Long-term average, relatively stable patterns

Weather affects daily life (clothes, water consumption, activities)

Climate determines regional agriculture, vegetation types, and settlement patterns

UNITY AND DIVERSITY IN THE MONSOON CLIMATE

Despite being classified under a single monsoonal climate type, India displays **significant regional variations** in weather and climate across different regions. This apparent contradiction represents the dual nature of Indian climate.

**Regional Variations in Temperature:**

Western Rajasthan (summer): Mercury touches 55°C

Leh (winter): Drops to -45°C (temperature range of 100°C across India)

Churu, Rajasthan (June): 50°C or more

Tawang, Arunachal Pradesh (June): Barely 19°C on the same day

Drass, Ladakh (December night): -45°C

Chennai/Thiruvananthapuram (December night): 20-22°C

**Daily Temperature Variations:**

Coastal areas (Kerala, Andaman Islands): Day-night difference only 7-8°C (moderated by ocean)

Thar Desert: Day temperature 50°C, night temperature 15-20°C (difference of 30-35°C due to dry air and lack of water moderating influence)

**Regional Variations in Precipitation:**

Cherrapunji and Mawsynram (Khasi Hills, Meghalaya): Over 1,080 cm annual rainfall (wettest places in India)

Jaisalmer, Rajasthan: Less than 9 cm annual rainfall (driest regions)

Tura, Garo Hills: Single day rainfall equals 10 years of Jaisalmer's rainfall

North-west Himalayas and western deserts: Less than 10 cm annually

Meghalaya: Exceeds 400 cm annually

**Seasonal and Spatial Variations in Rainfall:**

Ganga delta and Odisha coastal plains: Strong rain-bearing storms almost every 3-5 days in July-August (Southwest monsoon region)

Coromandal Coast (Tamil Nadu): Generally dry during July-August months

Most of India: Rainfall during June-September (Southwest monsoon period)

Tamil Nadu coastal areas: Rainfall in early winter season (October-November), contrary to national pattern

**Key Examination Point:** These variations represent **sub-types of monsoon climate**, not different climate systems. The unifying factor is the seasonal reversal of winds (monsoon mechanism).

FACTORS DETERMINING THE CLIMATE OF INDIA

LATITUDE

Tropic of Cancer passes through central India in east-west direction

**Northern India (north of Tropic of Cancer)**: Located in subtropical and temperate zones

Far from equator

Experiences extreme climate with high daily and annual temperature range

Cold winters, hot summers

Example: Delhi shows 25°C+ annual temperature range

**Southern India (south of Tropic of Cancer)**: Located in tropical zone

Closer to equator

High temperatures throughout the year

Small daily and annual temperature range

Example: Chennai/Thiruvananthapuram remain warm year-round

THE HIMALAYAN MOUNTAINS

The Himalayas act as a **critical climatic divide** for India:

**Shield Effect**: The lofty mountain chain protects the subcontinent from cold northern winds originating near the Arctic Circle. These winds blow across central and eastern Asia but cannot penetrate India due to the Himalayan barrier.

**Moisture Trapping**: The Himalayas force monsoon winds to shed their moisture within the subcontinent. This causes high rainfall on windward (southern) slopes and creates dry conditions on leeward (northern) slopes.

**Structural Function**: Acts as a southern extension of cold air masses from Central Asia, preventing their southward penetration.

DISTRIBUTION OF LAND AND WATER

India's unique geographical position creates distinct climatic advantages:

**Geographical Setting**: Flanked by Indian Ocean on three sides (south), girdled by Himalayan mountain wall (north)

**Differential Heating**: Water heats up and cools down slowly compared to land. This differential heating creates different air pressure zones in different seasons.

**Pressure Reversal**: Difference in air pressure between land and ocean causes **reversal in wind direction**, which is the fundamental mechanism of monsoon.

**Example**: During summer, intense heating of northern plains creates low pressure, attracting moisture-laden winds from the ocean. In winter, cooling of land creates high pressure, reversing wind direction.

DISTANCE FROM THE SEA

**Coastal Areas**: Have **equable climate** with small temperature variations

Example: Mumbai and Konkan coast have minimal temperature extremes

Ocean moderates temperature throughout the year

Minimal seasonal contrasts

**Interior Areas**: Far from moderating influence of sea

Experience extremes of climate

Large daily and annual temperature ranges

Example: Delhi, Kanpur, Amritsar show dramatic seasonal contrasts affecting all aspects of life (agriculture, clothing, water availability)

**Practical Impact**: Coastal regions require different adaptation strategies than interior regions

ALTITUDE

Temperature decreases with increasing height. The lapse rate (typical decrease) is approximately 6.5°C per 1,000 meters.

**Example**: Agra and Darjeeling are on the same latitude (26-27°N)

Agra (plain, 216 m elevation): January temperature = 16°C

Darjeeling (hill station, 2,134 m elevation): January temperature = 4°C

Difference of 12°C despite identical latitude due to altitude difference of ~1,900 m

**Mountain Effect**: Thin air in mountains absorbs less solar radiation, causing cooler temperatures

RELIEF AND PHYSIOGRAPHY

The terrain and landscape features significantly influence climate:

**Windward Slopes**: Receive high rainfall as moisture-laden winds are forced to rise

Example: Western Ghats (windward side): High rainfall during June-September

Assam: Receives abundant rainfall

**Leeward Slopes**: Remain dry as descending air causes warming and evaporation

Example: Southern Plateau (leeward side of Western Ghats): Dry during monsoon season

Creates rain shadow effect

**Wind Direction and Speed**: Relief features channel and accelerate winds

Mountain passes funnel winds with increased velocity

Valleys may provide shelter, reducing wind speed

**Rainfall Distribution**: Physiography determines where moisture-laden winds encounter resistance and precipitate

THE NATURE OF INDIAN MONSOON

The monsoon remains a complex climatic phenomenon that has puzzled scientists despite centuries of observations. **No single theory fully explains the monsoon**. Recent breakthroughs came from studying the monsoon at the **global scale** rather than regional level.

**Definition**: Monsoon connotes the climate associated with **seasonal reversal in the direction of winds**. India experiences a hot monsoonal climate prevalent across south and southeast Asia.

ONSET OF THE MONSOON

MECHANISM OF MONSOON ONSET

**Traditional Understanding (Differential Heating Theory):**

The classical explanation centers on differential heating of land and sea:

**Timing**: April-May, when sun shines vertically over Tropic of Cancer

**Northern Landmass Effect**: Large landmass in north of Indian Ocean gets intensely heated

**Low Pressure Formation**: Intense heating causes formation of low pressure cell in northwestern part of subcontinent

**Ocean Conditions**: Indian Ocean south of landmass remains high pressure (water heats slowly)

**Wind Movement**: Low pressure cell attracts southeast trade winds across the Equator

**ITCZ Shift**: This attracts Intertropical Convergence Zone (ITCZ) to move northward

INTERTROPICAL CONVERGENCE ZONE (ITCZ)

The ITCZ is critical to understanding monsoon mechanism:

**Definition**: **ITCZ is a low-pressure zone located at the equator where trade winds converge**, creating an area where air tends to ascend and condense.

**Characteristics:**

Permanent feature of equatorial atmospheric circulation

Marked by convergence of northeast and southeast trade winds

Area of rising air, cloud formation, and rainfall

Follows the sun's direct rays (vertical heating)

**Position Changes:**

**January (Winter)**: Located near equator (0-10°S latitude)

**July (Summer)**: Shifts northward to 20-25°N latitude over Gangetic Plain

**Called Monsoon Trough in July**: When positioned over India

**Role in Southwest Monsoon:**

Northeast trades of northern hemisphere cross equator between 40-60°E longitude

Deflected by Coriolis force into southwest direction

Become southwest monsoon winds

ITCZ movement encourages thermal low development over north and northwest India

JET STREAMS AND MONSOON ONSET

**Westerly Jet Stream Withdrawal:**

Located south of Himalayas during winter months

Steers temperate cyclones and weather systems

Must withdraw from north Indian plain for monsoon onset

Withdrawal creates space for easterly jet stream to develop

**Easterly Jet Stream:**

Sets in along 15°N latitude only after westerly jet withdraws

Held responsible for **burst of monsoon** in India (sudden onset)

Associated with strengthening of southwesterly flow

ENTRY OF MONSOON INTO INDIA

The southwest monsoon enters India in a progressive manner:

**Kerala Coast**: Onset around **1st June** (earliest arrival point)

**Western Coast**: Reaches Mumbai by 10th June

**Eastern Coast**: Reaches Kolkata by 13th June

**Entire Subcontinent**: Engulfed by mid-July

**Progress**: The monsoon moves swiftly northward and inland, covering the entire country within 2-2.5 months.

**Figure 4.2 Reference**: Map shows normal dates of onset of southwest monsoon across India, with progressive dates from June 1 (Kerala) to July 15 (northwest).

BREAK IN THE MONSOON

**Definition**: **Break in monsoon occurs when rain fails for one or more weeks during the southwest monsoon period after having rains for a few days**. Dry spells are quite common during the rainy season.

**Regional Causes:**

**Northern India:**

Rains fail when rain-bearing storms are not frequent along the monsoon trough

ITCZ (monsoon trough) over this region becomes weak

Results in temporary cessation of rainfall

**Western Coast (Konkan, Western Ghats):**

Dry spells associated with days when winds blow **parallel to the coast**

When monsoon winds blow parallel instead of perpendicular to coast, they cannot generate significant rainfall on western slopes

Example: Some days in July-August see no rainfall despite being monsoon season

**Examination Point**: Breaks are natural phenomenon, not monsoon failure. They represent temporary interruption, not complete system breakdown.

EL-NIÑO AND THE INDIAN MONSOON

**Definition**: **El-Niño is a complex weather system appearing once every 3-7 years, bringing drought, floods, and extreme weather to different parts of the world**.

**Mechanism:**

Oceanic and atmospheric phenomenon

Characterized by appearance of warm currents off Peru coast in Eastern Pacific

Warm equatorial current temporarily replaces cold Peruvian (Humboldt) current

Water temperature increases by 10°C on Peruvian coast

Affects weather patterns globally, including in India

**Consequences:**

Distortion of equatorial atmospheric circulation

Irregularities in evaporation of sea water

Reduction in plankton population

Significant decrease in fish population in affected regions

**Etymology**: Name means "Child Christ" because current appears around Christmas in December (summer month in Southern Hemisphere).

**Impact on Indian Monsoon:**

**Forecasting Tool**: Indian Meteorological Department uses El-Niño data for long-range monsoon rainfall prediction

**1990-91 Event**: Wild El-Niño event delayed southwest monsoon onset by 5-12 days across most of India

**Monsoon Variability**: One of key factors affecting monsoon behavior and rainfall distribution

**Practical Significance**: Understanding El-Niño helps in seasonal rainfall forecasting and agricultural planning in India.

THE RHYTHM OF SEASONS

Indian climate is best understood through an **annual cycle of four distinct seasons**. Meteorologists recognize:

1. COLD WEATHER SEASON

**Duration**: Mid-November to February

**Temperature Characteristics:**

Sets in by mid-November in northern India

December and January: Coldest months in northern plains

Mean daily temperature: Below 21°C over most of northern India

Night temperature: Can go below freezing point in Punjab and Rajasthan

**Peninsular exception**: No well-defined cold weather season in south due to proximity to equator and moderating ocean influence

Example: Thiruvananthapuram mean maximum temperature in January = 21°C; June = 29.5°C (small difference)

**Reasons for Excessive Cold in North India:**

1. **Continental Location**: States like Punjab, Haryana, and Rajasthan far from moderating sea influence, experience continental climate with temperature extremes

2. **Himalayan Snowfall**: Nearby snowfall in Himalayan ranges creates cold wave situations, bringing freezing air southward

3. **Western Disturbances**: Around February, cold winds from Caspian Sea and Turkmenistan bring cold waves with frost and fog over northwestern India

**Pressure and Winds:**

**High Pressure Zone**: Develops over northern plains

**Lower Pressure**: Over Indian Ocean (south)

**Wind Direction**: Blow from northwestern high pressure toward southern low pressure over ocean

**Isobar Position**: 1019 mb isobar in northwest, 1013 mb isobar in far south

**Rainfall Pattern:**

Winter monsoons do NOT cause significant rainfall as they move from land to sea

Two reasons: (i) Low humidity content, (ii) Anticyclonic circulation on land reduces rainfall possibility

**Most of India**: No winter rainfall

**Exceptions to Winter Rainfall:**

1. **Northwestern India**: Weak temperate cyclones from Mediterranean Sea cause rainfall

Areas affected: Punjab, Haryana, Delhi, western Uttar Pradesh

Amount: Meager but highly beneficial for rabi crops

Snowfall: In lower Himalayas

Significance: Snow sustains Himalayan river flow during summer months

Pattern: Decreases west to east in plains, north to south in mountains

Example: Delhi = 53 mm average winter rainfall, Punjab and Bihar = 25-18 mm

2. **Central India and Northern Peninsula**: Occasional winter rainfall

3. **Northeastern India**: Arunachal Pradesh and Assam receive 25-50 mm during winter

4. **Coromandal Coast (Tamil Nadu)**: Receives rainfall in early winter (October-November) from northeast monsoon while most of India is dry

**Western Jet Stream Role**: Question asked in exams — jet streams steer Mediterranean depressions into India during winter months

2. HOT WEATHER SEASON

**Duration**: March/April to May/June

**Temperature Characteristics:**

Period of excessive heat in northern half of country

Temperatures rise progressively from March to May

Maximum temperatures reach 45-50°C in most parts

Marked by falling air pressure in northern regions

**Pressure and Wind System:**

**ITCZ Northward Movement**: ITCZ moves northward due to intense heating, positioned at 25°N in July

**Monsoon Trough**: Elongated low-pressure zone extends from Thar Desert (northwest) to Patna and Chotanagpur Plateau (east-southeast)

**Surface Circulation**: Monsoon trough creates surface wind circulation

Southwesterly winds on west coast

Southwesterly winds along West Bengal and Bangladesh coasts

Easterly or southeasterly winds over north Bengal and Bihar

**Origin of Monsoon Winds**: Southwest monsoon winds are **displaced equatorial easterlies** of southern hemisphere, deflected by Coriolis force

**By Mid-June**: Influx of southwesterly monsoon winds brings change in weather toward rainy season

**Local Wind Phenomena:**

**Loo Wind:**

Dry, hot winds blow in afternoon from heart of ITCZ in northwest

Common in May in Punjab, Haryana, eastern Rajasthan, Uttar Pradesh

Continue into midnight

Associated with **dust storms in evening**

Bring temporary respite: Light rains, cool breeze

**Local Thunderstorms:**

Occur when moisture-laden monsoon winds contact dry air at ITCZ periphery

Sudden temperature and humidity contrast causes violent storms

Features: Violent winds, torrential rains, hailstorms

Significance: Provide relief from intense heat

**Rainfall Pattern:**

Most areas receive little rainfall during hot season

Rainfall increases from May as monsoon approaches

Coastal areas may receive significant rainfall in late May

3. SOUTHWEST MONSOON SEASON

**Duration**: June to September

**Characteristics**: Covered extensively in monsoon onset section above

**Rainfall Distribution:**

**Primary rainy season** for most of India

Rainfall varies dramatically by region:

Western coastal regions: 400+ cm

Windward slopes: High precipitation

Leeward plateaus: Low precipitation

Interior regions: Moderate to low

**Wind Pattern:**

Dominated by southwest monsoon winds

Low pressure gradient creates light winds (3-5 km/hour)

Topography influences wind direction

Westerly or northwesterly down Ganga Valley

Northerly in Ganga-Brahmaputra delta

Clearly northeasterly over Bay of Bengal

4. RETREATING/WITHDRAWAL MONSOON SEASON

**Duration**: October to November

**Characteristics**: Represents transition as monsoon weakens and withdraws

**Wind Reversal:**

Southwest monsoon weakens and retreats

ITCZ moves southward with sun's apparent movement

Trade winds reassert themselves

Wind direction reverses from northeast

Winds become northeast to south and southwest (northeast monsoons)

**Rainfall Pattern:**

**Northeast monsoon season begins** in October-November

Affects primarily east and south coasts

Coromandal Coast: Significant rainfall (exception to July-August dry period)

Bay of Bengal: Cyclonic activity increases

Most of interior India: Transition to drier conditions

**Environmental Effect:**

Temperatures begin cooling in north

Gradual transition toward cold weather season

UNDERSTANDING THE MONSOON (FORECASTING)

Modern monsoon forecasting involves sophisticated methods:

**Measurement Indices:**

**Southern Oscillation**: Measured by pressure difference between:

**Tahiti** (roughly 20°S, 140°W in French Polynesia, East Pacific)

**Port Darwin** (12°30'S, 131°E in northern Australia)

**Indian Meteorological Department (IMD):**

Uses **16 indicators** to forecast monsoon behavior

These indicators include pressure indices, temperature patterns, jet stream positions, oceanic conditions, and El-Niño parameters

**Practical Application:**

Long-range monsoon forecasts help agricultural planning

Farmers prepare based on expected rainfall

Reservoir management based on monsoon predictions

Policy decisions on irrigation and water resources

**Examination Importance:** IMD's role and 16 indicators often tested as factual knowledge questions

REGIONAL CLIMATE TYPES OF INDIA

While entire India experiences monsoonal climate, regional variations create distinctive climate types:

**Western Himalayas:**

Temperate to cold climate

Winter rainfall from Mediterranean depressions

Heavy snowfall

Alpine meadows in high altitude

**Northwest India (Rajasthan, Punjab):**

Hot and dry (arid) monsoon type

Low annual rainfall (less than 50 cm)

Extreme temperature ranges

Desert and semi-desert vegetation

**Northern Plains and Ganga Valley:**

Subtropical monsoon climate

High summer temperatures (45-50°C)

Moderate monsoon rainfall (75-150 cm)

Distinct winter cold

**Western Coastal Region (Konkan, Malabar):**

High rainfall monsoon type (200-400+ cm annually)

Moderate temperatures year-round

High humidity

Tropical evergreen vegetation

**Peninsula and Deccan Plateau:**

Tropical monsoon climate with seasonal variation

Moderate rainfall (50-200 cm)

Hot summers, mild winters

Dry deciduous vegetation

**Eastern India (Ganga Delta, Assam):**

High rainfall monsoon type (150-250+ cm)

Cyclonic activity in pre-monsoon and post-monsoon seasons

Hot and humid

Tropical vegetation

**Southern Coasts:**

High rainfall area (100-200+ cm)

Bimodal rainfall: Southwest monsoon and northeast monsoon

High humidity and moderate temperatures

Tropical vegetation

**Examination Strategy:** Maps showing these regional climate types are crucial. Practice identifying regions on rainfall maps, temperature maps, and physiographic maps.

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**Key Map References for Board Exams:**

Figure 4.1: Onset of Monsoon (showing ITCZ position and wind directions)

Figure 4.2: Normal Dates of Onset of Southwest Monsoon (showing June 1 in Kerala to July 15 in northwest)

Seasonal rainfall distribution maps

Temperature isotherm maps showing regional variations

Pressure and wind direction maps for different seasons

**High-Value Examination Topics:**

ITCZ concept and its role

Monsoon mechanism and factors

Differential heating and pressure reversal

Regional climate variations

Monsoon breaks and their causes

Seasonal progression and rhythm

El-Niño impacts

Jet stream withdrawal mechanism