Composition and Structure of Atmosphere

Introduction

The atmosphere is the thin envelope of gases surrounding the Earth, held in place by gravity. Without it, Earth would be lifeless — no air to breathe, no water cycle, no protection from solar radiation, and extreme temperature swings. The atmosphere extends to about 10,000 km above Earth's surface, but 99% of its mass is concentrated within the lowest 32 km.

Composition of the Atmosphere

The atmosphere is a mixture of several gases, water vapour, dust particles, and other matter:

• Permanent (constant) gases by volume (dry air):
• Nitrogen (N2): 78.08% — most abundant; relatively inert; essential for nitrogen cycle; dilutes oxygen
• Oxygen (O2): 20.95% — essential for respiration and combustion; absorbed and released by living organisms
• Argon (Ar): 0.93% — inert noble gas; third most abundant
• Carbon Dioxide (CO2): 0.04% (400 ppm) — absorbs outgoing longwave radiation (greenhouse gas); essential for photosynthesis
• Other trace gases: Neon, Helium, Krypton, Hydrogen, etc.

• Variable components:
• Water vapour (H2O): 0-4% by volume; highly variable; the most important greenhouse gas; drives weather; source of clouds and precipitation
• Ozone (O3): concentrated in the stratosphere (15-35 km); absorbs harmful ultraviolet (UV) radiation
• Dust and aerosols: condensation nuclei for cloud formation; affect radiation balance; from dust storms, volcanic eruptions, sea spray, pollen

Structure of the Atmosphere

The atmosphere is divided into layers based on temperature variation with altitude:

1. Troposphere

• Lowest layer: 0 to about 12 km (average); 8 km at poles, 16 km at equator (thicker at equator due to convection)
• Contains about 75-80% of total atmospheric mass and nearly all water vapour
• Temperature decreases with altitude at the Normal Lapse Rate (NLR) of about 6.5 degrees C per km
• ALL weather phenomena (clouds, rain, storms, fog) occur here
• The top of the troposphere is the tropopause (boundary layer)

2. Stratosphere

• 12 to 50 km above Earth's surface
• Temperature initially remains constant (isothermal), then increases with altitude due to absorption of UV radiation by ozone (O3)
• The ozone layer (ozonesphere) is concentrated here, mainly between 15-35 km
• Very stable (no weather); used by jet aircraft (smooth flying above weather)
• Bounded above by the stratopause

3. Mesosphere

• 50 to 80 km above Earth's surface
• Temperature decreases with altitude — the coldest layer of the atmosphere (~-90 degrees C at the top)
• Meteors burn up here due to friction with atmospheric gases
• Bounded above by the mesopause

4. Thermosphere (Ionosphere)

• 80 to 400+ km above Earth's surface
• Temperature increases sharply with altitude — can reach 1,500 degrees C or more (though it feels cold because molecules are so sparse)
• Contains ionised gas (plasma) — ions created by solar radiation (hence "ionosphere")
• The aurora borealis (Northern Lights) and aurora australis occur here when solar particles interact with atmospheric ions
• Radio wave reflection: the ionosphere reflects radio waves back to Earth, enabling long-distance radio communication
• The International Space Station (ISS) orbits in this layer

5. Exosphere

• Above 400-500 km — the outermost layer
• Transitions into outer space
• Hydrogen and helium atoms are so sparse that they can escape Earth's gravity

Important Boundary Layers (Pauses)

| Boundary | Location | Significance |
|---|---|---|
| Tropopause | ~12 km | Top of troposphere; temperature stops decreasing; marks limit of weather |
| Stratopause | ~50 km | Top of stratosphere; temperature stops increasing |
| Mesopause | ~80 km | Top of mesosphere; coldest point in atmosphere |

Significance of Atmospheric Layers

• Troposphere: weather, climate, all life-sustaining processes
• Stratosphere: ozone layer protects life from UV radiation; stable for jet flights
• Mesosphere: burns up meteors protecting Earth's surface
• Thermosphere: enables radio communication; auroras; satellite orbits
• Exosphere: transition to space; some satellite orbits

Ozone and Its Importance

• Ozone (O3) is a molecule of three oxygen atoms, found mainly in the stratosphere. It absorbs 95-99% of the Sun's harmful UV-B and UV-C radiation. Without the ozone layer:
• Increased skin cancer and cataracts in humans
• Damage to marine phytoplankton (base of ocean food chain)
• Disruption of plant growth

• Students often write that the atmosphere extends "indefinitely." It does NOT — it gradually thins out, with 99% of mass in the lowest 32 km.
• The thermosphere has very high temperatures but would feel cold to a human because the air is so thin — temperature is a measure of molecular speed, not heat content per unit volume.
• All weather occurs in the TROPOSPHERE, not the stratosphere or higher layers. Do not say "storms occur in the stratosphere."
• The Normal Lapse Rate (6.5 degrees C/km) applies only to the troposphere. Each layer has its own temperature pattern.
• Ozone depletion ≠ greenhouse effect: Ozone depletion is about UV radiation reaching Earth; Greenhouse effect is about trapping outgoing infrared radiation. These are different problems.

The atmosphere is a mixture of gases (N2 78%, O2 21%, CO2 0.04%, water vapour, ozone, dust) surrounding Earth. It is structured into five main layers (troposphere, stratosphere, mesosphere, thermosphere, exosphere) based on temperature patterns. The troposphere is where all weather occurs; the stratosphere houses the vital ozone layer; the mesosphere burns up meteors; the thermosphere enables radio communication and displays auroras. The atmosphere is essential for life, weather, and protecting Earth from harmful radiation.