Atmosphere: Structure, Air Pressure, and Wind System Study Notes

Introduction to the Atmosphere

• The atmosphere is defined as the gaseous uninterrupted envelope of air surrounding the earth from its surface up to some height. It is essentially an ocean of air that extends vertically up to an altitude of around $10,000\,km$ from the earth's surface.
• The atmosphere remains attached to the earth due to the earth's gravitational force.
• It is composed of colourless gases, water vapour, and dust particles.
• The atmosphere is a critical component of the earth's physical environment. Along with the lithosphere (land) and hydrosphere (water), it facilitates the creation of the Biosphere, or life-world, which allows for the origin, growth, and sustenance of living creatures.
• Essential functions includeproviding gases for humans, plants, and animals; distributing and circulating heat and moisture; and maintaining a congenial temperature for life. Movement of air helps certain areas avoid extremes of excessive heat or cold.
• The atmosphere acts as a shield, obstructing harmful ultraviolet (UV) radiation from reaching the earth. It is also the site for weather and climate formation, including rainfall, without which life could not exist. These factors make earth unique among the planets in the solar system.

Gaseous Composition and Proportions

• The composition and characteristics of the atmosphere vary with altitude. As altitude increases, the volume of gases and elements changes, leading to changes in air density, pressure, and temperature.
• Nitrogen and Oxygen are the predominant gases in the atmosphere. Their volumetric percentages (Table 2.1) are as follows:
  • Nitrogen ($N_2$): $78.08\%$
  • Oxygen ($O_2$): $20.94\%$
  • Argon ($Ar$): $0.93\%$
  • Carbon dioxide ($CO_2$): $0.036\%$
  • Other gases (Neon ($Ne$), Helium ($He$), Methane ($CH_4$), Krypton ($Kr$), Hydrogen ($H_2$), Xenon ($Xe$), and Ozone ($O_3$)): $0.014\%$
• Heavier gases like nitrogen, oxygen, argon, and carbon dioxide are largely confined to the lower $80-100\,km$ of the atmosphere. Beyond this height, gases become rarefied, and lighter gases like helium and hydrogen predominate. Water vapour and dust particles are also confined to the lower layers.
• Roles of specific gases:
  • Nitrogen: Chemically inactive; extracted by certain soil bacteria for plant use.
  • Oxygen: Chemically active gas essential for the energy of all living organisms through oxidation.
  • Argon: Chemically inactive; used in making electric bulbs and for welding works.
  • Carbon Dioxide: Significant in controlling atmospheric processes and climate. It facilitates photosynthesis in plants, while being released during animal exhalation. It absorbs solar radiation reflected from the earth, maintaining an average temperature of $15^\circ\text{C}$. Recent increases in $CO_2$ due to fossil fuel use and deforestation lead to Global Warming.
  • Ozone: Located primarily at an altitude of $40-50\,km$, it absorbs dangerous ultraviolet radiation.
  • Water Vapour: Absorbs solar radiation and maintains temperature balance. It condenses on dust particles to form clouds and rain.

Chemical Extent: Homosphere and Heterosphere

• Climatologists divide the atmosphere into two major layers based on chemical composition:
  • Homosphere: The lower layer extending to an altitude of approximately $80\,km$ where the chemical composition is almost uniform. Approximately $99\%$ of the mass of all atmospheric elements is confined to the first $32\,km$. Sub-divided into the Troposphere, Stratosphere, and Mesosphere.
  • Heterosphere: The upper atmosphere above $80\,km$ where chemical composition varies significantly. This region is divided into four sub-layers based on atomic weight:
  • Nitrogen Layer: $80\,km$ to $200\,km$.
  • Oxygen Layer: Up to $1125\,km$.
  • Helium Layer: Up to $3540\,km$.
  • Hydrogen Layer: Up to $10,000\,km$.
• Air density in the Heterosphere is extremely negligible.

Thermal Layers of the Atmosphere

• Troposphere: The lowermost layer. Average height is $12\,km$, reaching $16\,km$ in the equatorial region and $8\,km$ in polar regions. It contains three-fourths of all atmospheric gases and nearly all water vapour and dust. Temperature decreases with height at a rate of $6.5^\circ\text{C}$ per $km$ ascent (Lapse Rate). The upper limit is the Tropopause ($1.5\,km$ thick), where temperatures reach $-60^\circ\text{C}$.
• Stratosphere: Located above the Tropopause, extending up to $50\,km$ total height from the surface. It is cloudless and lacks weather phenomena, making it ideal for aeroplane routes. It contains the Ozone layer, which causes temperature to rise from $-60^\circ\text{C}$ in the lower region to $0^\circ\text{C}$ at its top boundary, the Stratopause.
• Mesosphere: Extends $30\,km$ above the Stratopause up to $80\,km$. Temperature decreases with altitude, reaching a minimum of $-100^\circ\text{C}$ at the Mesopause.
• Thermosphere: Extends from $80\,km$ to $400\,km$. Temperature increases with altitude, reaching up to $1650^\circ\text{C}$. It contains electrically charged ion particles, making it known as the Ionosphere; this layer reflects radio waves back to earth.
• Exosphere: The topmost layer starting from $400\,km$ up to $10,000\,km$. Hydrogen and helium dominate. Air is very thin, and temperatures rise to approximately $5550^\circ\text{C}$.

Atmospheric Pressure: Principles and Measurement

• Air has mass and thus exerts pressure. Atmospheric pressure is the extent of force exerted by air per unit area. It is measured at mean sea level.
• On average, pressure is $14.7\,\text{Pounds}$ per square inch or approximately $1\,\text{Kilogram}$ per square centimeter.
• Measurement Instruments:
  • Barometer: Measures air pressure. Normal sea level pressure is $29.92\,\text{inches}$ or $76\,\text{centimeters}$ of mercury ($760\,mm$).
  • Unit: The millibar ($mb$). Sea level pressure at $15^\circ\text{C}$ is $1013.2\,mb$. One inch of mercury corresponds to $33.9\,mb$.
  • Barograph: An instrument that automatically draws a pressure graph.
• Factors Influencing Pressure:
  • Air Temperature: Warm air expands, becomes less dense, and exerts low pressure. Moisture-laden warm air is even lighter. Cold air is dense and exerts high pressure.
  • Altitude: Pressure decreases with altitude as the depth and weight of the overhead atmosphere declines. Pressure falls by $1\,\text{inch}$ (or $34\,mb$) for every $900\,feet$ of ascent.

Global Pressure Belts

• Air movement (wind) is driven by pressure differences, moving from high pressure to low pressure. There are four major global pressure belts:
  • Equatorial Low Pressure Belt ($10^\circ\text{N}$ to $10^\circ\text{S}$): Characterized by high temperatures and vertical air movement, resulting in a calm zone known as the Equatorial Doldrum.
  • Sub-Tropical High Pressure Belt ($25^\circ-35^\circ\text{N}$ and $25^\circ-35^\circ\text{S}$): Formed by cold, heavy air descending from equatorial and polar regions. Navigators historically called this the Horse Latitude because they had to throw horses overboard to lighten ships in the calm, motionless air.
  • Sub-Polar Low Pressure Belt ($60^\circ-70^\circ\text{N}$ and $60^\circ-70^\circ\text{S}$): Created by the earth's rotation, which deflects air toward sub-tropical regions, thinning the air volume here.
  • Polar High Pressure Belt ($90^\circ\text{N}$ and $90^\circ\text{S}$): Extremely cold conditions and lack of water vapour result in permanent high pressure.

Mechanics of Wind: Origin and Motion Factors

• Wind is air in motion, generally moving parallel to the earth's surface. Vertical movement is referred to as a vertical wind or current.
• Factors determining wind direction and velocity:
  • Pressure Gradient Force: The rate of pressure difference between two places. Steeper gradients (greater difference over shorter distance) result in higher wind velocities.
  • Gravitational Force: Pulls the atmospheric envelope toward the center. It keeps air pressure variations vertical and contributes to the curved path of air movement.
  • Centrifugal Force (Coriolis Force): Discovered by Gasperd de Coriolis in 1844. It is an outward force generated by the earth's rotation. It is zero at the equator and maximum at the poles. It deflects winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere (Ferrell's Law).
  • Frictional Force: Acts as a resistance to wind. It is higher over rugged terrain or urban areas and lower over water or snow. Friction reduces wind velocity and, by extension, the Coriolis effect, slightly changing wind direction.

Measurement and Naming of Winds

• Naming: Winds are named after the direction from which they blow (e.g., a Westerly wind blows from the west).
• Terminology:
  • Windward: The direction from which the wind comes.
  • Leeward: The direction to which the wind blows.
• Instruments:
  • Wind Vane: Determines wind direction, expressed in degrees relative to magnetic north ($0^\circ$ for North, $90^\circ$ for East).
  • Anemometer: Measures wind velocity.
  • Anemograph: Automatically records direction and velocity.
• Scales:
  • Beaufort Scale: Developed by Sir Francis Beaufort in 1805. A $0-12$ scale describing wind nature and impact (e.g., $0$ is Calm with vertical smoke; $12$ is a Hurricane with violent destruction).
  • Unit of Velocity: The Knot. $1\,\text{Knot} = 1\,\text{nautical mile per hour} = 1.854\,km/h = 30.9\,m/min$.

Primary Wind Circulations (Planetary Winds)

• These are permanent circulation patterns covering the whole earth based on global pressure belts:
  • Trade Winds: Blow from sub-tropical highs ($30^\circ\text{N/S}$) toward the equatorial low. In the Northern Hemisphere, they blow North-East to South-West (North-East Trade Wind); in the Southern Hemisphere, they blow South-East to North-West (South-East Trade Wind). They meet at the Inter-Tropical Convergence Zone (ITCZ) or Doldrum.
  • Westerlies: Blow from sub-tropical highs toward sub-polar lows. They are more variable and intense than trade winds. In the Southern Hemisphere, due to oceanic dominance, they are extremely strong: $40^\circ-50^\circ$ (Roaring Forties), $50^\circ-60^\circ$ (Furious Fifties), and $60^\circ-70^\circ$ (Screaming Sixties).
  • Polar Wind (Polar Easterlies): Blow from polar high pressure toward sub-polar low pressure.
  • Jet Stream: A very high-velocity wind ($640\,km/h$) blowing east at high altitudes in the upper troposphere.

Secondary Wind Circulations: Air Masses and Fronts

• Secondary circulations result from variations in landforms, land-water distribution, and seasonal temperature-pressure changes.
• Air Mass: An immense body of air with homogeneous temperature and humidity. Classified as:
  • Tropical Continental ($cT$)
  • Tropical Maritime ($mT$)
  • Polar Continental ($cP$)
  • Polar Maritime ($mP$)
• Front: The boundary or line of discontinuity formed when two contrasting air masses (one cold and heavy, one warm and light) meet. Types include:
  • Cold Front: Cold air mass moves and shifts a warm air mass.
  • Warm Front: Advancing warm air mass moves over and shifts a cold air mass.
  • Stationary Front: Static condition where no shifting occurs.
  • Occluded Front: Formed by the mixing of cold and warm fronts.

Cyclonic and Anticyclonic Systems

• Cyclone: Atmospheric circulation with a low-pressure center and high-velocity cyclic winds. They rotate anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
  • Tropical Cyclone: Violent and destructive; formed over oceans in summer. Velocity: $120-280\,km/h$. Named Typhoon (East Pacific), Hurricane (West Indies), Cyclone (Indian Ocean), Bardoisila (Assam), and Willy-Willy (Australia).
  • Extra-Tropical Cyclone (Temperate Cyclone): Formed between $30^\circ$ and $65^\circ$ latitude at polar fronts. Diameter: $300-1500\,km$. Covers up to $1.6\,\text{million square kilometer}$.
• Anticyclone: Opposite of a cyclone; a high-pressure center with divergent cyclic winds. The term was coined by Sir Francis Galton in 1861. Rotating clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere.
  • Types: Sub-Tropical Warm Core and High-Latitude Cold Core.
  • Characteristics: Usually brings fair weather and low wind velocity. Cold core anticyclones can cause Blizzard conditions in North America and Europe, often called a "cold wave."

Seasonal and Local Wind Systems

• Monsoon Wind: Derived from Arabic "Mausim" and Malayan "Monsin." A seasonal surface wind that reverses direction between summer (moist, from sea to land) and winter (dry, from land to sea). Most distinct in South and South-East Asia.
• Local Winds (Tertiary Circulation):
  • Land Breeze: Blows from land to sea at night.
  • Sea Breeze: Blows from sea to land during the day.
  • Valley Wind: Occurs during the day as air moves up mountain slopes.
  • Mountain Wind: Occurs at night as cold, heavy air moves down slopes.
• Other Named Local Winds:
  • Chinook: Warm, dry wind on the eastern slopes of the Rockies (USA).
  • Foehn: Warm, dry wind on the northern slopes of the Alps (Europe).
  • Sirocco: Hot, dry southerly wind from the Sahara (called Khamsia in Egypt, Simoom in Arabia).
  • Loo: Hot, dry summer wind in the Gangetic Plain (India).
  • Mistral: Cold wind from the Alps toward the Mediterranean in winter.