Air Masses, Fronts & Cyclones

1. Air Masses

An air mass is a large body of air (thousands of km across) having little horizontal variation in temperature and moisture. It acquires these properties by remaining stationary over a uniform surface (source region) for a long time.

Classification of Air Masses

Based on source regions:

• Maritime Equatorial (mE): Warm and extremely wet.
• Maritime Tropical (mT): Warm and moist (influences trade winds).
• Continental Tropical (cT): Hot and dry (Sahara, deserts).
• Maritime Polar (mP): Cool and moist.
• Continental Polar (cP): Cold and dry (Siberia, Canada).
• Continental Arctic/Antarctic (cA): Extremely cold and dry.

2. Fronts

When two different, contrasting air masses meet, the boundary zone between them is called a front. The process of its formation is frontogenesis. Fronts are the primary cause of weather changes in mid-latitudes.

Types of Fronts

1. Cold Front: A cold, dense air mass aggressively moves into warmer air, forcing the warm air up rapidly. This creates steep slopes, towering cumulonimbus clouds, and heavy, intense, but short-duration storms.
2. Warm Front: A warm air mass gently glides up over a heavier, retreating cold air mass. The slope is gentle, leading to thick nimbostratus clouds causing steady, long-duration, gentle precipitation over a wide area.
3. Stationary Front: The boundary between the two air masses is not moving.
4. Occluded Front: The faster-moving cold front completely overtakes the warm front, lifting the warm air mass entirely off the ground.

3. Temperate Cyclones (Extra-Tropical Cyclones)

Also known as mid-latitude depressions, these develop between 35° and 65° latitudes in both hemispheres due to the interaction of warm (westerly) and cold (polar easterly) air masses along the Polar Front.

• Mechanism (Polar Front Theory by Bjerknes): A wave develops along the stationary polar front. It deepens into a low-pressure center with a distinct warm sector and cold sector, separated by warm and cold fronts. Finally, the cold front rapidly occludes the warm front, and the cyclone dissipates.
• Characteristics: They are extremely large (up to 2000 km in diameter), form over both land and sea, and travel West to East steered by the westerlies. They bring steady, extensive rainfall.
• Western Disturbances: Temperate cyclones forming over the Mediterranean Sea that travel east to bring crucial winter rainfall to Northwest India.

4. Tropical Cyclones

Violent, intense low-pressure storms originating over warm tropical oceans (between 8° and 20° latitudes, never on the equator due to zero Coriolis force). They move West under the influence of trade winds.

Favourable Conditions for Formation

• Large sea surface with temperature > 27°C (providing enormous latent heat from water vapor).
• Presence of Coriolis force to create rotation.
• Small variations in the vertical wind speed.
• A pre-existing weak low-pressure area.

Structure & Characteristics

• The Eye: The center of the storm characterised by remarkably calm weather, clear skies, low pressure, and slowly subsiding (descending) air.
• The Eyewall: A ring of towering cumulonimbus clouds surrounding the eye. It experiences the most violent, hurricane-force winds and heaviest torrential rainfall.
• Spiral Rainbands: Bands of clouds spiralling outward.
• Dissipation: They weaken rapidly ("landfall") when crossing over land or moving to cooler waters because their energy source (warm moisture) is cut off.

Nomenclature

Known by different names: Hurricanes (Atlantic / Eastern Pacific), Typhoons (Western Pacific, SE Asia), Cyclones (Indian Ocean), Willy-Willies (Western Australia).