Airmass

Air Mass

• An Air Mass is a large body of air that has similar temperature, humidity, and pressure conditions throughout its horizontal extent.
• It can extend over thousands of kilometers and remain relatively uniform.
• The uniformity develops because the air mass stays over a particular region for a long time and absorbs the characteristics of that region.
  

Source Region of Air Mass

• The source region is the place where an air mass forms and acquires its physical properties.

Conditions required for a good source region:

• • The surface should be extensive and uniform (like deserts, oceans, ice sheets).
  • The atmosphere should be stable, meaning there is less vertical mixing.
  • Winds should be light, so the air can remain over the region long enough.
  • Examples:
    • Deserts → produce hot and dry air masses
    • Oceans → produce moist air masses
    • Polar regions → produce cold air masses

Classification of Air Masses

Air masses are mainly classified based on moisture content and temperature.

A. Based on Moisture

1. Continental Air Mass (c)

• • Forms over land areas
  • Has low moisture content (dry air)
  • Leads to clear skies and low rainfall
    

2. Maritime Air Mass (m)

• • Forms over oceans or seas
  • Contains high moisture
  • Responsible for cloud formation and rainfall
    

B. Based on Temperature

1. Tropical (T)

• • Originates in low latitudes
  • Hot or warm in nature

2. Polar (P)

• • Originates in high latitudes
  • Cold air mass

3. Arctic/Antarctic (A)

• • Forms in extreme polar regions
  • Very cold and dense

4. Equatorial (E)

• • Forms near the equator
  • Very hot and highly humid
    
    

C. Combined Classification

• Air masses are usually described by combining moisture + temperature symbols:

Properties of Air Mass

An air mass is mainly characterized by:

• • Temperature – determines whether it is hot or cold
  • Humidity – determines moisture content
  • Pressure conditions
  • Stability

Stability of Air Mass:

• • Stable Air Mass:
    
    • Air does not rise easily
    • Leads to clear and calm weather
  • Unstable Air Mass:
    
    • Air rises easily due to convection
    • Leads to clouds, rainfall, storms
      

Modification of Air Mass

• When an air mass moves away from its source region, it undergoes modification.

Causes of modification:

• Heating or cooling of the surface
• Gain or loss of moisture
• Mixing with other air masses

Result:

• Changes in:
  
  • Temperature
  • Humidity
  • Stability

Example:

• A cold, dry air mass (cP) moving over warm ocean water becomes warmer and more humid

Air Masses Affecting India

Air masses play a crucial role in shaping India’s climate.

1. Maritime Tropical (mT)

• Comes from Indian Ocean
• Warm and moisture-laden
• Responsible for Monsoon rainfall

2. Continental Tropical (cT)

• Originates from Thar Desert
• Hot and dry
• Causes heat waves (Loo winds) in North India

3. Continental Polar (cP)

• Comes from Central Asia
• Brings cold waves during winter

4. Maritime Polar (mP)

• Associated with Mediterranean region
• Leads to Western Disturbances, causing winter rainfall in North India

Interaction of Air Masses – Fronts

• When two different air masses meet, they do not mix easily.
• The boundary between them is called a Front.

Types of Fronts:

1. Cold Front

• Cold air pushes under warm air
• Causes sudden weather changes
• Leads to heavy rainfall, thunderstorms

2. Warm Front

• Warm air rises over cold air
• Causes gradual weather change
• Leads to light, continuous rainfall

3. Occluded Front

• Cold front overtakes warm front
• Produces complex weather conditions

4. Stationary Front

• No movement of air masses
• Leads to prolonged weather conditions
  
  

Role of Air Masses in Weather Systems

• Air masses are the basic building blocks of weather.
• They are involved in:
  • Formation of cyclones and anticyclones
  • Development of monsoon systems
  • Occurrence of storms and rainfall
• Example:
  • Interaction of warm moist air (mT) and cold dry air (cP) leads to temperate cyclones

Link with Jet Streams

• Jet streams are fast-moving winds in the upper atmosphere.
• They influence:
  
  • Movement of air masses
  • Formation of fronts
• Example:
  • The Subtropical Jet Stream affects the onset and withdrawal of the Indian Monsoon.