MEV 012: Unit 06 - Weather Phenomenon

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 UNIT 6: WEATHER PHENOMENON

6.0 Introduction

Weather phenomena represent the dynamic atmospheric processes that occur on a daily basis and influence human life, agriculture, transport, and environment. This unit explores key atmospheric concepts such as air masses, cyclones, monsoons, jet streams, and climatic classifications.

6.1 Objectives

By the end of this unit, learners will be able to:

  • Define and distinguish between weather and climate.
  • Understand the origin and classification of air masses.
  • Explain the formation of fronts, temperate and tropical cyclones.
  • Understand jet streams and their impact on weather.
  • Describe the Indian monsoon system and its variations.
  • Understand global climatic events such as ENSO.
  • Interpret major climate classification systems.

6.2 Weather: An Introduction

Weather refers to the day-to-day atmospheric conditions (temperature, precipitation, wind, humidity) at a specific place and time. Unlike climate, it changes rapidly and is influenced by local and global factors like pressure systems, moisture, and solar radiation.

6.3 Introduction to Air Masses

An air mass is a large body of air with relatively uniform temperature and humidity characteristics throughout.

6.3.1 Classification of Air Masses

Air masses are classified based on their source region and thermal properties:

  • Continental Arctic (cA): Very cold, dry.
  • Continental Polar (cP): Cold, dry.
  • Maritime Polar (mP): Cool, moist.
  • Continental Tropical (cT): Hot, dry.
  • Maritime Tropical (mT): Warm, moist.
  • Equatorial (E): Very warm, humid.

When different air masses meet, they form fronts, leading to significant weather changes.

6.4 Fronts and Temperate Cyclones

A front is the boundary between two air masses of different densities and temperatures. Types of fronts:

  • Cold Front: Cold air displaces warm air—leads to thunderstorms.
  • Warm Front: Warm air overtakes cold air—leads to steady rainfall.
  • Occluded Front: Cold front overtakes a warm front—causes complex weather.
  • Stationary Front: No movement—leads to prolonged cloudiness.

Temperate Cyclones (or extratropical cyclones) form in mid-latitudes due to frontal activity. They rotate counterclockwise in the Northern Hemisphere and bring widespread precipitation.

6.5 Tropical Cyclones

Tropical cyclones are intense low-pressure systems forming over warm ocean waters near the equator. Known as:

  • Cyclones in the Indian Ocean,
  • Hurricanes in the Atlantic,
  • Typhoons in the Pacific.

Features:

  • Eye (calm center)
  • Eye wall (most intense weather)
  • Spiral rainbands

Conditions for formation:

  • Sea surface temperature > 26.5°C
  • High humidity
  • Low vertical wind shear
  • Coriolis force

6.6 Jet Streams

Jet streams are high-altitude, fast-moving air currents located near the tropopause (9–16 km altitude). They influence weather systems by steering storms and regulating air mass movement.

Main Jet Streams:

  • Polar Jet Stream
  • Subtropical Jet Stream

In India, the subtropical westerly jet affects winter weather, while the tropical easterly jet is crucial for monsoon development.

6.7 South-West and North-East Monsoons

Monsoons are seasonal winds that reverse direction between summer and winter, bringing heavy rainfall and climatic changes.

6.7.1 South-West Monsoon

  • Onset: Early June
  • Caused by low pressure over north India
  • Divides into Arabian Sea branch and Bay of Bengal branch
  • Major rainfall season in India

6.7.2 North-East Monsoon

  • Occurs from October to December
  • Winds blow from land to sea
  • Brings rainfall to southeast coast of India (Tamil Nadu)

6.8 El Niño Southern Oscillation (ENSO)

ENSO is a climate phenomenon affecting global weather patterns, including the Indian monsoon.

  • El Niño: Warming of the central and eastern Pacific Ocean—leads to weak Indian monsoon.
  • La Niña: Cooling of the Pacific Ocean—can lead to stronger monsoon.
  • Southern Oscillation: Refers to changes in air pressure patterns between Tahiti and Darwin.

ENSO events disrupt normal trade winds and ocean currents, causing droughts or floods worldwide.

6.9 Classification of Climate by Köppen and Thornthwaite

Köppen Climate Classification

  • Based on temperature and precipitation patterns.
  • Five major groups:
    • A (Tropical)
    • B (Dry)
    • C (Temperate)
    • D (Continental)
    • E (Polar)
  • Widely used due to simplicity and global applicability.

Thornthwaite Classification

  • Focuses on evapotranspiration and moisture index.
  • Useful in agricultural and water resource planning.
  • Categories: Arid, Semi-arid, Humid, Perhumid, etc.

6.10 Let Us Sum Up

In this unit, we explored key weather phenomena including air masses, fronts, cyclones (tropical and temperate), and jet streams. A detailed look at the Indian monsoon system and ENSO highlighted how global climatic interactions affect regional weather. Finally, two major climatic classification systems—Köppen and Thornthwaite—were introduced.

6.11 Keywords

·         Weather-Day-to-day atmospheric conditions of a specific place.

·         Climate-Long-term average of weather patterns over a region (30+ years).

·         Air Mass-Large body of air with uniform temperature and humidity.

·         Front-Boundary between two different air masses.

·         Temperate Cyclone-Low-pressure system forming in mid-latitudes, associated with fronts.

·         Tropical Cyclone-Severe storm system forming over warm tropical oceans.

·         Jet Stream-Fast-moving air currents at high altitudes influencing weather systems.

·         Monsoon-Seasonal reversal in wind direction, causing rainfall, especially in India.

·         El Niño-Warming of the Pacific Ocean that disrupts global weather patterns.

·         La Niña-Cooling phase of ENSO that can intensify monsoons.

·         ENSO-Combined ocean-atmosphere phenomenon involving El Niño and Southern Oscillation.

·         Köppen Classification-Climate classification based on temperature and precipitation.

·         Thornthwaite Classification-Climate system based on moisture availability and evapotranspiration.

·         Coriolis Effect-Apparent deflection of winds due to Earth's rotation.

·         Evapotranspiration-Combined process of evaporation and transpiration from land and vegetation.

 

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