Short Notes 01: Atmosphere

---

Heating of the Atmosphere

• Atmosphere is heated from below.
• Solar radiation heats Earth’s surface.
• Earth re-radiates heat upward (terrestrial radiation).
• Air is a poor conductor; heating mainly by radiation and convection.

---

Tropopause

• Boundary between troposphere and stratosphere.
• Height varies with latitude:
  • Equator: 16–18 km
  • Mid-latitudes – 50 N ( ISA ): ~11 km
  • Poles: 8–10 km
• Higher surface temperature → higher tropopause.
• Acts as a lid; most weather occurs below it.
• Important for flight planning, turbulence, jet streams.

---

Atmospheric Mass Distribution

• ~50% of atmospheric mass below 5.5 km (18,000 ft).
• ~75% below 10–11 km.
• ~99% below 35 km.
• Due to gravitational concentration near Earth’s surface.

---

Troposphere

• Lowest atmospheric layer.
• Contains >75% of atmospheric mass.
• Contains almost all weather.
• Temperature decreases with height.
• Generally unstable.

---

Stratosphere

• Extends from tropopause to ~50 km.
• Temperature increases with height (inversion).
• Heated by ozone absorption of UV radiation.
• Stable layer; minimal vertical motion.

---

Mesosphere

• Extends up to 80–90 km.
• Noctilucent clouds occur here (75–90 km).

---

Ozone Layer

• Maximum concentration at 20–25 km.
• Located in stratosphere.
• Absorbs UV radiation, warming the layer.

---

Water Vapour

• Most important atmospheric gas for weather.
• Mainly confined to lower troposphere.
• Exists as gas, liquid, and solid.
• Responsible for clouds, precipitation, latent heat, fog, icing.

---

Greenhouse Gases

• Major: H₂O, CO₂, CH₄, N₂O, O₃.
• Oxygen is not a greenhouse gas.
• Absorb infrared radiation and warm the atmosphere.

---

Atmospheric Composition (Dry Air)

• Nitrogen: 78%
• Oxygen: 21%
• Other gases: ~1%
• CO₂: ~0.03%
• O₂ : N₂ → 1:4 by volume, 1:3 by weight.

---

Heat Transfer in the Atmosphere

• Latent heat ≈ 77% of atmospheric heat transfer.
• Sensible heat ≈ 23%.
• Evaporation absorbs heat; condensation releases heat.

---

Oxygen Requirement in Aviation

• Supplemental oxygen recommended above 10,000 ft.
• Prevents hypoxia (fatigue, poor judgment, unconsciousness).

---

International Standard Atmosphere (ISA)

• Sea-level temperature: +15°C
• Sea-level pressure: 1013.25 hPa
• Lapse rate (below 11 km): 6.5°C/km ≈ 2°C/1000 ft
• Tropopause: 11 km, −56.5°C
• Isothermal layer: 11–20 km

---

Lapse Rate and Stability

• Lapse rate: temperature change with height.
• Normal: temperature decreases with height.
• Inversion: temperature increases with height.
• Isothermal: temperature constant.
• Environmental lapse rate is variable.

---

Tropopause – Key Facts

• Lapse rate becomes zero.
• Discontinuous near 40° and 60° latitude.
• Coldest over equator (−75 to −80°C).
• Over India: ~16–16.5 km.

---

Freezing Level

• Altitude where temperature is 0°C.
• Estimated using 2°C per 1000 ft.

---

Homosphere and Heterosphere

• Homosphere: Surface to ~80 km, uniform composition.
• Heterosphere: Above ~80 km, composition varies.

---