Air Mass
An air mass is a large body of air that has relatively uniform temperature, humidity, and pressure characteristics throughout its horizontal extent. It forms when air remains over a particular region for a sufficient period, allowing it to acquire the climatic properties of that area. Air masses play a crucial role in determining the weather and climate patterns of different regions across the world.
Definition
In geography and meteorology, an air mass is defined as a vast, homogeneous parcel of air covering thousands of square kilometres, characterised by similar physical properties at any given altitude. When two air masses of different characteristics meet, they form a front, leading to changes in weather such as cloud formation, precipitation, or storms.
Formation of Air Masses
Air masses develop in regions known as source regions, where the atmosphere remains relatively stable for long periods, allowing air to take on the temperature and moisture conditions of the surface below.
Ideal conditions for air mass formation include:
- Extensive flat surfaces such as oceans, deserts, or plains.
- Light surface winds that minimise vertical mixing.
- Prolonged stability, allowing uniform heating or cooling of the air.
Common source regions include the subtropical oceans, polar ice caps, and continental interiors.
Classification of Air Masses
Air masses are classified according to their source region (latitude) and surface nature (continental or maritime). The combination of these two factors produces several major types.
Based on Latitude (Temperature):
- Equatorial (E): Very warm and moist; found near the equator.
- Tropical (T): Warm; originates between 15° and 35° latitudes.
- Polar (P): Cold; forms in subpolar regions between 50° and 70° latitudes.
- Arctic (A) or Antarctic (AA): Extremely cold; originates near the poles.
Based on Surface Nature (Moisture):
- Continental (c): Dry air mass formed over land.
- Maritime (m): Moist air mass formed over oceans or seas.
Combining both systems, meteorologists identify the principal types of air masses:
| Type | Source Region | Temperature and Humidity Characteristics | Typical Weather Effects |
|---|---|---|---|
| cT (Continental Tropical) | Hot deserts and dry subtropical regions | Hot and dry | Clear skies, hot weather |
| mT (Maritime Tropical) | Warm tropical oceans | Warm and moist | Humid, cloudy, and rainy |
| cP (Continental Polar) | Cold continental interiors | Cold and dry | Clear, cold weather |
| mP (Maritime Polar) | Cool oceanic regions at mid-latitudes | Cool and moist | Cloudy with drizzle or snow |
| cA (Continental Arctic) | Arctic or Antarctic regions | Very cold and dry | Extremely cold, stable air |
| mE (Maritime Equatorial) | Equatorial oceans | Hot and very moist | Heavy rainfall, thunderstorms |
Properties of Air Masses
Each air mass has distinctive meteorological characteristics depending on its source region:
- Temperature: Determined by latitude and surface heating or cooling.
- Moisture Content: Depends on whether the air forms over land (dry) or water (moist).
- Stability: Varies according to temperature gradient and movement over warmer or cooler surfaces.
Modification of Air Masses
As air masses move away from their source regions, they undergo modification, changing their original properties. Modification occurs through processes such as:
- Heating or cooling from below: When air moves over warmer or colder surfaces.
- Addition or loss of moisture: Through evaporation or precipitation.
- Mixing with local air: Reduces the uniformity of temperature and humidity.
For example, a continental polar (cP) air mass moving over a warm ocean becomes warmer and more humid, transforming gradually into a maritime polar (mP) air mass.
Fronts and Weather Changes
When contrasting air masses meet, they do not mix immediately due to their different densities. The boundary between them is called a front, which can be of several types:
- Cold Front: Where a cold air mass advances and lifts warmer air, often causing thunderstorms and showers.
- Warm Front: Where a warm air mass moves over a retreating cold air mass, leading to steady rainfall.
- Stationary Front: When neither air mass advances, producing prolonged cloudy weather.
- Occluded Front: When a cold front overtakes a warm front, resulting in complex weather patterns.
The interaction of air masses at these fronts is responsible for much of the world’s variable weather, especially in the mid-latitudes.
Major Air Masses Affecting the World
- North America: Influenced by mT (Gulf of Mexico), cT (Mexico deserts), mP (North Pacific), and cP (Canada).
- Europe: Dominated by mP (North Atlantic), cP (Eurasia), and mT (Mediterranean).
- Asia: Affected by cP (Siberia) in winter and mT (Indian Ocean) during summer monsoon.
- Australia: Influenced by mT (Coral Sea) and cT (central desert).
- South America and Africa: Experience equatorial and tropical maritime air masses, producing humid climates along coasts.
Role of Air Masses in Climate and Weather
- Weather Formation: Movement and interaction of air masses create varied weather phenomena such as rainfall, storms, fog, and temperature shifts.
- Climate Control: Long-term dominance of specific air masses shapes the climatic characteristics of regions (e.g., polar air masses create cold climates, tropical maritime air masses cause humid conditions).
- Cyclone and Anticyclone Formation: The meeting of contrasting air masses often leads to the development of low-pressure (cyclonic) or high-pressure (anticyclonic) systems.
Example: Air Mass Influence in India
India experiences the effects of tropical maritime (mT) and tropical continental (cT) air masses. The seasonal reversal of winds associated with these air masses forms the basis of the monsoon system. During summer, moist mT air from the Indian Ocean brings heavy rainfall, while in winter, dry cT air from the continental interior results in dry conditions.