Week 7: Precipitation, Humidity

 

Precipitation, Humidity, Cloud Formation, and Types of Rainfall

Precipitation, humidity, and cloud formation are fundamental meteorological concepts that influence weather, climate, water distribution, and ecosystems. These processes are essential in agriculture, hydrology, climate studies, and disaster management.

Why This Topic Matters?

• Regulates Earth’s Water Cycle: Ensures water availability across continents and oceans.
• Influences Weather & Climate: Determines droughts, storms, and flooding patterns.
• Affects Agriculture & Economy: Crops depend on rainfall distribution.
• Impacts Human Health & Comfort: High humidity affects thermal comfort and disease spread.

Fig No. 1 - Water Cycle

In recent years, the India Meteorological Department (IMD) has played a vital role in weather forecasting, climate studies, and artificial rain experiments, particularly in Delhi, where air pollution is a severe issue. The concept of artificial rain (cloud seeding) has been explored to improve air quality by inducing rainfall to wash away pollutants.

The Water Cycle (Hydrological Cycle)

• The continuous exchange of water between the atmosphere, oceans, and continents occurs through:
• Evaporation: Transformation of water into vapor.
• Transpiration: Water release from plants.
• Condensation: Vapor cooling and turning into clouds.
• Precipitation: Water returning to Earth in various forms.
• The total amount of moisture remains constant, balancing evapotranspiration and precipitation.

Key Components of the Water Cycle

• Evaporation

• Definition: The process where water transforms from liquid to gas due to solar heating.
• Major Sources: Oceans (contribute ~84% of atmospheric moisture), lakes, rivers, and soil.
• Example: The Indian Ocean is a key source of moisture for the South Asian monsoon.

• Transpiration

• Definition: Plants release water vapor through tiny pores (stomata) in leaves.
• Example: The Amazon Rainforest releases 20 billion tons of water vapor daily, affecting global rainfall patterns.

• Condensation

• Definition: Water vapor cools and turns into liquid droplets, forming clouds.
• Example: Fog formation over San Francisco Bay, where warm air meets the cold Pacific Ocean.

• Precipitation

• Definition: When condensed droplets become heavy enough to fall as rain, snow, sleet, or hail.
• Example: Cherrapunji, India holds the record for highest annual rainfall (~11,777 mm).

• Runoff & Infiltration

• Runoff: Water flows into rivers, lakes, and oceans.
• Infiltration: Water seeps into the ground, replenishing groundwater.
• Example: The Ganges River Basin relies on monsoon runoff for agriculture.

Fig No. 2 - Hydrologic cycle

Humidity

    Definition & Importance

• Humidity is the amount of water vapor in the air.

• It influences:

• Heat absorption and radiation.
• Storm and cyclone formation.
• Human comfort (perceived temperature).

    Types of Humidity

1. Absolute Humidity: The total mass of water vapor present in a unit volume of air (grams per cubic meter). Example: Coastal cities like Mumbai experience high absolute humidity (~80%) due to oceanic influence.
2. Relative Humidity (RH): The percentage of moisture in the air compared to its maximum holding capacity at a given temperature. 

• RH formula:

           

• Higher RH reduces evaporation and makes air feel warmer.
• Example:
1. Dubai (Desert Climate) – RH drops to 20% in daytime.
2. Kolkata (Humid Climate) – RH often exceeds 85% during monsoon.

Fig No. 3 - A graph showing the relationship between temperature and relative humidity, dew point

3. Specific Humidity: Ratio of water vapor mass to total air mass (grams per kg).
• Unlike absolute and relative humidity, it remains constant despite temperature changes.
4. Dew Point: The temperature at which air reaches 100% RH and condensation begins.
• Example: Cold winter mornings in Delhi, where dew forms on grass due to night-time cooling.

    Factors Affecting Humidity

• Temperature: Warm air holds more moisture.
• Evaporation: Higher over oceans due to available water.
• Air Movement: Winds replace saturated air, increasing evaporation.

Fig No. 4 - A graph showing the relationship between temperature and relative humidity, dew point

Cloud Formation

    Process of Cloud Formation

Fig No. 5 - Diagram showing adiabatic cooling, where rising air expands, cools, and condenses into clouds.

• Condensation occurs when air cools below its dew point, forming tiny water droplets or ice crystals.

• Requires:

• Cooling Mechanisms:
• Adiabatic Cooling (rising air expands and cools).
• Radiational Cooling (night-time cooling).
• Contact Cooling (air touching a cold surface).
• Condensation Nuclei: Dust, smoke, or salt particles that facilitate droplet formation.

Fig No. 6 - Cloud formation

    Cloud Classification

        Clouds are categorized based on altitude and appearance:

Type	Description	Altitude
High Clouds	Cirrus, Cirrostratus, Cirrocumulus	Above 6000m
Middle Clouds	Altostratus, Altocumulus	2000-6000m
Low Clouds	Stratus, Stratocumulus, Nimbostratus	Below 2000m
Vertical Clouds	Cumulus, Cumulonimbus (thunderstorm clouds)	Low to high

• Cirrus: Wispy, high-altitude ice clouds.

• Cumulus: Puffy, cotton-like clouds, indicating fair weather.

• Stratus: Uniform grey layers covering the sky, leading to drizzle.

• Nimbus: Dark, dense clouds that bring rain.

Fig No. 7 - Different Types of Clouds

Precipitation

    Definition & Forms

• Precipitation is any form of water (liquid or solid) falling from the atmosphere to the Earth's surface.

• Forms:

• Rain (drops > 0.5 mm)
• Drizzle (drops < 0.5 mm)
• Snow (ice crystals)
• Sleet (frozen raindrops)
• Hail (layered ice pellets)

Fig No. 8 - Forms of precipitation

Factors Affecting Precipitation

1. Humidity levels (higher RH leads to more precipitation).
2. Temperature (determines rain vs. snow).
3. Wind patterns (move moist air).
4. Topography (mountains force air up, cooling it).

Types of Rainfall

    Rainfall is classified based on its origin and cause:

    Convectional Rainfall

• Occurs due to intense heating of the Earth's surface.
• Process:
• Warm air rises → cools → condenses → rains.
• Characteristics:
• Common in equatorial regions (Amazon, Congo).
• Short, intense afternoon showers.
• Forms cumulonimbus clouds.

    Orographic (Relief) Rainfall

• Occurs when moist air is forced to rise over mountains.
• Process:
• Air rises on the windward side → cools → condenses → rains.
• Leeward (rain-shadow) side remains dry.
• Examples:
• Western Ghats (India): Windward slopes get heavy rain (600 cm), while Pune (leeward) gets only ~70 cm.
• Andes Mountains, Rockies.

Fig No. 9 - Adiabatic Process

    Cyclonic (Frontal) Rainfall

• Occurs when warm and cold air masses meet.
• Process:
• Warm air rises over cold air → condenses → rains.
• Types:
• Tropical Cyclonic Rain (e.g., hurricanes, typhoons).
• Temperate Cyclonic Rain (frontal activity in mid-latitudes).
• Examples:
• North-West Europe (Atlantic depressions).
• US (hurricane-induced rainfall - Katrina).

    Monsoonal Rainfall

• Caused by seasonal wind reversals.
• Process:
• Summer: Warm land draws moist ocean winds → rains.
• Winter: Dry land winds dominate → little rain.
• Examples:
• South Asian Monsoons (India, Bangladesh, Myanmar).
• Heavy rainfall in Cherrapunji (~1000 cm/year).

Fig No. 10 - Summer Monsoon Dates

Comparative Analysis

Type	Cause	Region	Cloud Type	Characteristics
Convectional	Surface heating	Equatorial (Amazon)	Cumulonimbus	Short, intense showers
Orographic	Mountain barriers	Windward slopes	Stratus, Cumulus	Heavy rain on windward, dry leeward
Cyclonic	Air mass collision	Mid-latitudes	Nimbostratus	Steady, widespread rain
Monsoonal	Seasonal wind shift	South Asia	Varied	Wet summers, dry winters

Fig No. 11 - South Asian monsoon wind patterns

Role of IMD in Meteorology and Rainfall Studies

    What is the IMD?

• The India Meteorological Department (IMD) was established in 1875 and is responsible for weather forecasting, climate monitoring, and meteorological research in India.
• It operates Doppler radars, satellites, and weather observatories to track precipitation, cloud formation, and humidity levels.
• IMD plays a crucial role in issuing monsoon forecasts, cyclone warnings, and heatwave alerts.

    IMD’s Role in Rainfall Monitoring

• Monsoon Prediction: IMD uses climate models to forecast Southwest and Northeast monsoons, which are essential for India’s agriculture and water supply.
• Cloud Observations: IMD tracks cloud formation, rainfall patterns, and humidity levels using satellites and ground stations.
• Cyclone & Extreme Weather Alerts: Monitors and predicts tropical cyclones, thunderstorms, and heavy rainfall events.

Fig No. 12- (a). Network of Class I surface observatories of IMD. (b). Network of radiosonde observatories of IMD. (c). Current Doppler Radar network of India Meteorological Department. (d). Network of Lightning sensors of IITM

Artificial Rain in Delhi (Cloud Seeding)

    What is Artificial Rain?

• Artificial rain, or cloud seeding, is a technique where chemicals like silver iodide, potassium iodide, or sodium chloride are dispersed into clouds to stimulate condensation and precipitation.

    Why Was Artificial Rain Considered in Delhi?

• Delhi faces severe air pollution, especially in winter, due to factors like:
• Vehicular emissions
• Industrial pollution
• Construction dust
• Crop stubble burning
• Artificial rain was proposed as a way to reduce air pollution by washing away particulate matter (PM2.5 and PM10) from the atmosphere.

    Expert Opinion on Cloud Seeding in Delhi

• IMD and CPCB (Central Pollution Control Board) reviewed the feasibility of cloud seeding in Delhi but found several challenges:
• Winter clouds in Delhi are primarily formed by Western Disturbances, which already bring natural rain.
• High-altitude clouds (>5-6 km) cannot be seeded due to aircraft limitations.
• Dry air below cloud level can cause raindrops to evaporate before reaching the ground.
• Effectiveness of cloud seeding is uncertain and depends on existing moisture levels.

Global Case Studies on Artificial Rain

• China: Used cloud seeding during the 2008 Beijing Olympics to clear pollution before events.
• UAE: Regularly conducts cloud seeding to enhance rainfall in desert areas.
• USA: Used in California for drought mitigation.

Fig No. 13 - Cloud Seeding

Advanced Insights & Future Developments

• Artificial Rainfall (Cloud Seeding):
• Uses silver iodide or salt particles to stimulate precipitation.
• Used in China, UAE, India (proposed for Delhi smog reduction).
• Challenges: Requires specific cloud conditions, ethical concerns.
• Climate Change Impacts:
• Increasing extreme rainfall events.
• Shifting monsoons affecting agriculture.
• Desertification due to changing precipitation patterns.

Steps Taken for Abatement of Air Pollution in Delhi

    Air Pollution & Precipitation in Delhi

• Precipitation plays a key role in cleansing the atmosphere by removing air pollutants.
• Lack of rainfall during winter worsens Delhi’s air quality, trapping pollutants due to temperature inversions.
• IMD and CPCB monitor air quality and rainfall patterns to suggest mitigation strategies.

    Key Measures Taken to Reduce Air Pollution in Delhi

1. Air Quality Monitoring
• IMD & CPCB track real-time Air Quality Index (AQI) and meteorological data.
• GRAP (Graded Response Action Plan) is implemented based on AQI levels.
2. Vehicular Emission Control
• Strict emissions norms for vehicles (BS-VI fuel, electric vehicle promotion).
• Odd-Even traffic scheme introduced during high pollution periods.
3. Industrial & Construction Regulations
• Ban on pet coke and furnace oil in industries.
• Dust control measures at construction sites.
4. Stubble Burning Management
• Subsidies for crop residue management equipment to prevent farmers from burning stubble.
• Ex-situ utilization of paddy straw (e.g., converting it into biofuel).
5. Cloud Seeding Consideration
• The Delhi government requested artificial rain, but scientific limitations were identified.

Fig No. 14 - Daily Average Air Quality Index