Discover the Pressure — pressure belts; winds-planetary seasonal and local, air masses and fronts; tropical and extra tropical cyclones

Pressure & Factors Affecting Pressure

A column of air exerts weight, which is felt as pressure on the Earth's surface. The force of this air column at a specific time and place is termed air pressure or atmospheric pressure. Barometers are used to measure atmospheric pressure. Pressure is expressed as force per unit area, with millibars as the standard unit. One millibar equals approximately the force of one gram per square centimeter.

Factors Influencing Pressure Systems

Pressure differences causing high and low-pressure systems arise due to two main factors:

• Thermal Factors
  • When air is heated, it expands, becomes less dense, and creates low pressure. Cooling causes contraction, increasing density and forming high pressure.
  • Examples: Equatorial low-pressure zones and polar high-pressure zones.
• Dynamic Factors
  • Dynamic factors like the pressure gradient and the Earth's rotation (Coriolis force) influence pressure belts.

Understanding Pressure Gradient

The pressure gradient refers to the rate at which pressure changes between two points on Earth's surface. On weather maps, this is shown using isobars, lines that connect areas of equal pressure. Tightly spaced isobars indicate a steep gradient, while widely spaced isobars suggest a gentler gradient.

Vertical Distribution of Pressure

Pressure decreases with altitude due to the compressive weight of the air above. Lower atmospheric layers are denser and exert more pressure than upper layers. Factors like temperature, water vapor, and gravity influence pressure at specific locations. Variations in these factors cause fluctuations in the rate of pressure decrease with height. Rising pressure signals stable weather, while falling pressure may indicate instability and cloud formation.

Horizontal Distribution of Pressure

Horizontal pressure variation arises due to several factors:

• Air Temperature
  • Uneven heating of Earth's surface causes low pressure in warm equatorial areas and high pressure in cooler polar regions.
  • Warm air ascends at the equator, reducing surface pressure, while cold air sinks at the poles, increasing pressure.
• The Earth's Rotation
  • Rotational forces produce centrifugal effects, deflecting air and reducing pressure in some areas.
  • This process creates low-pressure subpolar zones and high-pressure subtropical zones.
• Water Vapor
  • Humid air exerts less pressure than dry air because it is less dense.

Role of Atmospheric Pressure & Wind

Atmospheric pressure and wind are interconnected systems that shape Earth's weather and climate. They result from the uneven heating of the Earth's surface and are influenced by various forces and global pressure belts.

Atmospheric Pressure

Definition and Characteristics: Atmospheric pressure is the force exerted by the weight of air above a given point. It decreases with altitude and is measured using tools like:

• Mercury Barometer
• Aneroid Barometer

At sea level, atmospheric pressure averages 1,013.2 mb. Variations in pressure due to temperature differences cause air movement.

Forces Affecting Wind Movement

Wind movement is influenced by several forces:

• Pressure Gradient Force:

  This force drives winds from high-pressure areas to low-pressure areas. The rate of change in pressure determines the gradient:

  

  • Strong gradients: Closely spaced isobars → Strong winds.
  • Weak gradients: Widely spaced isobars → Weak winds.
• Coriolis Force:

  This force, caused by the Earth’s rotation, alters wind direction:

  

  • Deflects winds to the right in the Northern Hemisphere.
  • Deflects winds to the left in the Southern Hemisphere.

  It is strongest at the poles and negligible at the Equator. It influences:

  • Geostrophic Winds: Winds flowing parallel to isobars due to a balance of forces.
  • Cyclonic Circulation: Converging winds in low-pressure areas.
  • Anticyclonic Circulation: Diverging winds in high-pressure areas.

  

• Frictional Force:

  This force reduces wind speed, most significant near the Earth’s surface (up to 1–3 km). Over oceans, friction is minimal, allowing freer wind flow.

• Gravitational Force:

  Acts vertically downward, maintaining atmospheric pressure and holding air close to Earth’s surface.

Global Pressure Belts

The Earth’s surface is divided into seven distinct pressure belts that influence global wind circulation:

• Equatorial Low Pressure Belt:

  Located between 0°–5° North and South of the Equator, this belt is characterized by intense solar heating, which causes warm air to rise and create low-pressure zones. Known as the doldrums, this region is calm and windless.

• Sub-tropical High Pressure Belts:

  Located around 30° North and South, these regions experience descending air currents that create high-pressure zones, often called the Horse Latitudes. Winds from this region flow:

  • Toward the Equator as Trade Winds.
  • Toward the Sub-polar Low Pressure areas as Westerlies.
• Sub-polar Low Pressure Belts:

  Located between 60°–70° in both hemispheres, these regions form where warm Sub-tropical air collides with cold Polar air. This creates low-pressure zones and results in frequent violent storms, especially during winter.

• Polar High Pressure Belts:

  Located between 70°–90° at the poles, these areas have cold, descending air that forms high-pressure zones. These regions are characterized by permanent ice caps.

Wind Circulation and Weather Systems

Wind circulation determines global weather patterns by redistributing heat and moisture:

• Low-pressure areas: Air converges and rises → Cloud formation and precipitation.
• High-pressure areas: Air subsides and diverges → Clear skies.

Upper-level winds, free from surface friction, are driven by pressure gradient and Coriolis forces, forming geostrophic winds and contributing to global weather systems.

The Coriolis Effect & Wind Circulation

Definition: The Coriolis effect arises due to Earth’s rotation, causing the deflection of moving air:

• Deflects winds to the right in the Northern Hemisphere.
• Deflects winds to the left in the Southern Hemisphere.

It is strongest at the poles and negligible at the Equator. The effect prevents tropical cyclones from forming near the Equator and plays a critical role in global wind circulation.

Introduction to Atmospheric Circulation

Atmospheric circulation refers to the large-scale movement of air that, along with ocean currents, redistributes heat across the Earth. This system is critical for shaping weather patterns and regulating the global climate.

General Circulation of the Atmosphere

The general circulation of the atmosphere is driven by several factors:

• Latitudinal variation in heating: Uneven heating of Earth's surface due to its curvature.
• Pressure belts: Development of pressure zones like the equatorial low and subtropical high.



• Seasonal migration of belts: Movement of pressure belts following the sun's apparent path.
• Distribution of land and oceans: Influences heat retention and wind patterns.
• Earth's rotation: Causes the Coriolis effect, which deflects wind directions.

This circulation regulates global wind patterns and influences ocean currents, playing a significant role in climate dynamics.

Atmospheric Cells and Wind Circulation

The atmosphere is divided into three primary circulation cells in each hemisphere, facilitating heat transfer across latitudes:

• Hadley Cell
  • Dominates tropical regions with intense solar heating at the Inter-Tropical Convergence Zone (ITCZ).
  • Warm air rises, moves poleward at the tropopause, cools, and sinks at around 30° N/S, forming subtropical highs.
  • Surface winds, known as Trade Winds, return to the equator, completing the cell.
  • Associated phenomena:
    • Horse Latitudes: Zones of descending air with minimal winds, often linked to desert formation.
• Ferrel Cell
  • Operates in mid-latitudes (30°-60°).
  • Warm air rises at subtropical highs, while cold polar air sinks, creating surface winds known as Westerlies.
  • Interaction with polar air masses results in dynamic weather and the formation of fronts.
• Polar Cell
  • Cold air sinks at the poles and flows equatorward as Polar Easterlies.
  • Air rises at 60° latitude, creating low-pressure zones and interacting with the Ferrel Cell.

Seasonal and Local Wind Systems

Wind systems undergo seasonal shifts and include notable local patterns:

• Seasonal Wind Changes
  • Monsoons: Seasonal reversal of winds, especially in Southeast Asia, caused by shifts in pressure and heating.
• Local Winds
  • Land and Sea Breezes:
    • Day: Land heats faster than the sea, creating a low-pressure zone. Cooler sea air flows to the land (sea breeze).
    • Night: Land cools faster than the sea, reversing the airflow (land breeze).
  • Mountain and Valley Winds:
    • Day: Warm air rises along slopes (valley breeze).
    • Night: Cool air descends into valleys (mountain breeze).
  • Katabatic Winds: Dense, cold air flows down slopes, particularly over snow-covered regions.

Air Masses and Fronts

Air masses are large volumes of air with uniform temperature and humidity characteristics:

• Types of Air Masses:
  • Maritime Tropical (mT)
  • Continental Tropical (cT)
  • Maritime Polar (mP)
  • Continental Polar (cP)
  • Continental Arctic (cA)

Fronts are boundaries between two air masses:

• Types of Fronts:
  • Cold Front: Cold air advances into warm air.
  • Warm Front: Warm air moves over cold air.
  • Stationary Front: Neither air mass advances.
  • Occluded Front: Warm air is lifted by converging cold air.

Cyclones and Severe Weather

• Cyclones
  • Tropical Cyclones: Intense storms fueled by warm ocean water, also known as hurricanes or typhoons depending on the region.
  • Extra-Tropical Cyclones: Form in mid to high latitudes, featuring frontal systems and impacting large areas.
• Thunderstorms and Tornadoes
  • Thunderstorms: Formed by convection, causing heavy rainfall, hail, or dust storms.
  • Tornadoes: Rotating columns of air causing localized destruction.

Walker Circulation and ENSO

The Walker circulation is an east-west air movement across the tropics:

• Upwelling: Nutrient-rich cold water rises along South America's coast.
• Rainfall: Warm air causes precipitation near Australia.

El Niño and ENSO significantly impact global weather patterns:

• El Niño: Weakened trade winds shift warm water eastward, causing:
  • Heavy rainfall in South America.
  • Droughts in Australia and India.

Conclusion

Atmospheric circulation plays a crucial role in redistributing heat, shaping weather patterns, and sustaining life. The interaction of circulation cells, winds, and ocean currents is central to understanding Earth's climate system.