Description

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Context

India Meteorological Department (IMD) has issued a yellow alert for a heatwave in Delhi on April 25–26 forecasting maximum temperatures between 40–42°C with clear skies, dry westerly winds & sustained poor air quality despite slight improvements across Delhi-NCR.

IMD Heatwave Classification (Plains Region including Delhi)

Criteria	Threshold Value	Remarks
Maximum Temperature ≥ 40°C	With departure of ≥ 4.5°C to 6.4°C	Heatwave
Maximum Temperature ≥ 40°C	With departure of > 6.4°C	Severe Heatwave
Absolute Maximum Temperature ≥ 45°C	Irrespective of normal	Heatwave
Absolute Maximum Temperature ≥ 47°C	Irrespective of normal	Severe Heatwave
Minimum Temperature Influence	Not considered directly	But contributes to thermal stress during heatwave spells
Duration	Usually 2 or more consecutive days	But warning may be issued even for 1 day in exceptional cases

Air Quality Index (AQI) Color Categories – CPCB India

AQI Range	Color Code	Category	Associated Health Impacts
0 – 50	Green	Good	Minimal or no health risk. Air quality is considered satisfactory.
51 – 100	Light Green	Satisfactory	Acceptable; may cause minor breathing discomfort to sensitive individuals.
101 – 200	Yellow	Moderate	Breathing discomfort to people with asthma, heart disease, elderly & children.
201 – 300	Orange	Poor	Increased likelihood of respiratory symptoms among sensitive groups.
301 – 400	Red-Purple	Very Poor	Respiratory illness on prolonged exposure; affects general public health.
401 – 500	Maroon	Severe	Serious health effects even on healthy people; emergency conditions for sensitive groups.

Air Quality Alerts Based on AQI Levels

Alert Type	AQI Range	Description	Health Advisory
No Alert	0 – 50	Air quality is considered satisfactory with little or no risk to general population.	No action required.
Moderate	51 – 100	Air quality is acceptable; however there may be some health effects for sensitive individuals (e.g. asthma or breathing issues).	People with respiratory conditions should limit prolonged exertion outdoors.
Unhealthy for Sensitive Groups	101 – 200	Health effects may be felt by sensitive individuals such as children, elderly people & those with respiratory or heart conditions.	Sensitive individuals should avoid prolonged outdoor activities.
Unhealthy	201 – 300	Health effects are likely to occur for general population not just sensitive groups. Outdoor activities should be limited.	Everyone may begin to experience health effects; sensitive individuals should avoid outdoor exertion.
Very Unhealthy	301 – 400	Health alert. Everyone may experience more serious health effects.	Everyone should avoid outdoor activities; stay indoors with air conditioning.
Hazardous	401 – 500	Emergency conditions. Serious health effects for entire population.	All individuals should remain indoors & avoid physical activities.

Western Disturbance (WD)

A Western Disturbance (WD) refers to an extratropical storm that originates in Mediterranean region or North Africa & moves eastward across Middle East & northern Indian subcontinent primarily affecting India, Pakistan & Nepal.

Aspect	Description
Origin	Mediterranean & North African regions where warm & moist air meets cold air from polar regions.
Nature	These disturbances are low-pressure systems & are responsible for bringing rainfall & colder conditions.
Movement	They generally travel eastward from Mediterranean crossing Iran, Afghanistan & Pakistan into northern India.
Impact on Weather	WD brings rain to northwestern & northern India (Punjab, Haryana, Delhi, Himachal Pradesh, Jammu & Kashmir) & can cause fluctuations in winter temperatures.
Duration	Typically these disturbances last 3-5 days & affect western Himalayan region with varying intensities.
Precipitation Type	The precipitation associated with Western Disturbances is mostly light to moderate rainfall or snowfall in hilly areas although heavy rainfall may occur in some cases.
Seasonality	Predominantly observed during winter season (from October to March) with some weakening or absence during summer monsoon months.
Influence on Indian Climate	WDs are crucial for winter precipitation in northwestern & northern India & play a significant role in maintaining agricultural productivity of these regions.
Thunderstorms	WDs also bring thunderstorms particularly in plains of northern India often associated with sudden temperature drops & shifting wind patterns.

Impact of Western Disturbance (WD) on India

Snowfall in Himalayas WDs cause snowfall in higher altitudes of western Himalayas, which affects water resources in region.

Rainfall in Plains Regions like Delhi, Punjab, Haryana & Uttar Pradesh receive rainfall often leading to a sudden drop in temperatures.

Temperature Variations WDs can cause sharp fluctuations in temperatures with warm spells followed by cold spells. They can also lead to fog & low visibility during winter months.

Cyclonic Storms: In certain instances if disturbance intensifies it can lead to cyclonic storms though this is rarer.

East India Thunderstorms

Thunderstorms in East India including states like Odisha, West Bengal, Bihar, Jharkhand & parts of Uttar Pradesh are typically linked to warm, moist air & significant atmospheric instability. These storms are particularly active during pre-monsoon & monsoon months.

Key Characteristics of Thunderstorms in East India

Aspect	Description
Formation	Thunderstorms in East India generally form due to convergence of moist air from Bay of Bengal & hot, dry air from north. The region experiences frequent occurrences of low-pressure areas.
Seasons	Thunderstorms are most common during pre-monsoon season (March to June) & monsoon season (June to September). However they can occur in post-monsoon period as well.
Conditions Favoring Formation	High surface temperatures, moisture availability from Bay of Bengal & atmospheric instability are key ingredients for thunderstorm development.
Wind Patterns	Strong easterly winds coupled with influx of moisture from Bay of Bengal help in creating conditions for thunderstorms.
Intensity	These thunderstorms can be severe often producing heavy rainfall, strong winds (30-60 km/h), lightning & occasionally hail.
Tornadoes	Tornadoes are rare but can occur especially with intense thunderstorms in this region.
Duration & Frequency	Thunderstorms typically last for 1-2 hours but can be frequent in summer months especially in region’s hilly terrain & coastal areas.
Impact on Weather	The thunderstorms bring much-needed rain to agricultural regions of East India but can also result in localized flooding, property damage & loss of life.
Rainfall	Thunderstorms often lead to heavy & localized rainfall which can cause flash floods particularly in hilly areas.
Lightning	Lightning strikes are frequent & can cause fires, power outages & injuries especially in rural & agricultural areas.

Thunderstorm Dynamics in East India

Development of Low-Pressure Areas Low-pressure areas or cyclonic circulations over Bay of Bengal can significantly enhance thunderstorm activity by strengthening convergence of air masses.

Influence of Sea Breeze The sea breeze from Bay of Bengal plays a critical role in triggering convection particularly in coastal regions like Odisha & West Bengal where afternoon thunderstorms are common.

Convection & Instability High temperature gradients between land & sea cause vertical convection resulting in upward movement of warm, moist air. As air rises, it cools, condenses & forms clouds leading to thunderstorm activity.

Thunderstorms & Crop Damage These storms often lead to damage of standing crops due to strong winds, hail & flooding. The region heavy reliance on agriculture makes these storms both a boon (for rainfall) & a bane (for crop destruction).

Key Meteorological Factors Affecting East India Thunderstorms

Sea Surface Temperatures (SSTs) Warm SSTs in Bay of Bengal increase moisture content in air enhancing thunderstorm development.

Atmospheric Instability A sharp contrast between warm surface air & cooler air aloft fosters conditions for deep convection & thunderstorm formation.

Low-Level Jet Streams The presence of low-level jet streams which transport moisture into region is crucial for intensity of these thunderstorms.

Impact of Western Disturbance on Delhi Weather

Aspect	Impact on Delhi Weather	Details
Temperature Variations	Sudden Drop in Temperature	WDs cause cold northerly winds leading to a sharp decrease in temperature particularly during winter months.
Precipitation	Light to Moderate Rain	Light rainfall often intermittent caused by WDs passing over Delhi. Rainfall can last over a few days increasing humidity levels.
Fog Formation	Dense Fog	WDs lead to foggy conditions especially in mornings due to drop in temperature & high humidity.
Hailstorms	Occasional Hailstorms	WDs may occasionally cause hailstorms in Delhi especially during winter though this is rare.
Wind Patterns	Cold Westerlies, Gusty Winds	Strong westerly winds reduce temperatures & may bring brief gusts of rain followed by clear skies.
Visibility	Reduced Visibility (Fog)	The fog reduces visibility especially in early mornings which can affect air & road transport.

Impact of East India Thunderstorms on Delhi Weather

Aspect	Impact on Delhi Weather	Details
Temperature Variations	Hot & Humid Conditions	Thunderstorms from East India bring warm, moist air to Delhi raising temperatures & increasing humidity particularly before or during thunderstorms.
Precipitation	Unpredictable Rainfall	Brief, sudden rainfall occurs in Delhi especially in pre-monsoon months but is usually lighter compared to eastern states.
Humidity	Increased Humidity	The moist air from East India thunderstorms causes a rise in humidity levels leading to uncomfortable conditions especially during pre-monsoon months.
Air Quality	Deterioration in Air Quality	Humidity & rising temperatures can exacerbate pollution levels, causing smog & haze & occasionally dust storms.
Heat Index	Higher Apparent Temperature	Due to increase in humidity, heat index (feels-like temperature) is higher than actual temperatures making it feel hotter.
Dust Storms	Higher Dust Concentration	Strong winds from thunderstorms may carry dust & particulate matter into Delhi worsening air quality.

For more such articles, please refer to IAS GYAN

Sources:

BUSINESS STANDARD

PRACTICE QUESTION

Q. Discuss how Western Disturbances & East India-origin thunderstorms differently impact weather patterns of Delhi particularly in terms of temperature, precipitation & air quality.