Aviation Meteorology Chapters

Met Chapter 1 – Atmosphere

Posted on by Aanand Keerthy

ATMOSPHERE

🌍 1.1 Introduction to Atmosphere

The atmosphere is a layer of gases surrounding the Earth, held in place by gravity. It moves along with the Earth and rotates at the same speed. Although its total thickness is about 480 km, nearly 80% of the atmosphere is concentrated within the first 16 km.

There is no definite upper boundary of the atmosphere—it gradually becomes thinner and merges into outer space.

Characteristics of Atmosphere

  • Has weight → exerts pressure
  • Compressible and expandable
  • Has no definite shape
  • Mobile → allows transfer of heat and moisture
  • Poor conductor of heat and electricity

🌬️ 1.2 Composition of Atmosphere

Air is a mixture of gases. The major constituents are:

Major Gases

  • Nitrogen (N₂): 78.09%
  • Oxygen (O₂): 20.95%
  • Argon: 0.93%
  • Carbon Dioxide (CO₂): 0.035%

👉 Nitrogen and Oxygen together form ~99% of air
👉 Ratio:

  • By volume → 4:1
  • By weight → 3:1

📌 Above 10,000 ft, oxygen decreases → supplementary oxygen required

Trace Gases

  • Neon, Helium, Krypton, Xenon
  • Hydrogen
  • Methane
  • Ozone
  • Nitrous Oxide
  • Carbon Monoxide

Also present:

  • Water Vapour (WV)
  • Dust particles (aerosols)

🌫️ 1.3 Variable Gases & Their Importance

Variable gases change with location and time (e.g., water vapour, CO₂).

  • More in industrial areas and near water bodies
  • Less in open and dry regions

Even in small quantities, they are very important for weather formation.

🌡️ 1.4 Greenhouse Effect

Greenhouse gases include:

  • Water Vapour
  • CO₂
  • Ozone
  • Methane

These gases:

  • Allow solar radiation (shortwave) to enter
  • Absorb and re-radiate terrestrial radiation (longwave)

👉 This keeps Earth warm
👉 Excess causes global warming

💧 1.5 Water Vapour & Humidity

Water exists in atmosphere as:

  • Solid → Snow, Ice, Hail
  • Liquid → Rain, Drizzle
  • Gas → Water vapour

Key points:

  • Decreases with height
  • Depends on temperature (warm air holds more moisture)
  • Up to 4% in tropical regions
  • Almost negligible near poles and above 30,000 ft

Definitions

  • Saturated Air → RH = 100%
  • Dry Air → RH < 100%

🌍 1.6 Homosphere & Heterosphere

  • Homosphere → up to ~80 km
    • Uniform composition
    • Due to mixing by gravity
  • Heterosphere → above 80 km
    • Gases arranged by weight

🌍 1.7 Ozone Layer

  • Located mainly between 10–50 km
  • Maximum concentration: 20–25 km

Importance:

  • Absorbs harmful UV radiation
  • Protects life on Earth (prevents skin cancer, etc.)

Ozone Hole:

  • Caused by pollution (CFCs, halocarbons)
  • Leads to breakdown of O₃ into O₂

🌫️ 1.8 Aerosols

Includes:

  • Dust
  • Smoke
  • Sea salt particles

Effects:

  • Reduce visibility
  • Affect temperature
  • Act as condensation nuclei → cloud formation

📊 1.9 Vertical Distribution of Air

  • Below 6 km → 50% of air mass
  • Below 10 km → 75%
  • Below 35 km → 99%

🌡️ 1.10 Heating of Atmosphere

Atmosphere is heated from below (Earth’s surface).

Heat Transfer Methods:

  • Sensible heat → 23%
    (conduction, convection, radiation)
  • Latent heat → 77%
    (evaporation, condensation)

🌍 1.11 Thermal Structure of Atmosphere

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1. Troposphere

  • Lowest layer
  • Extends:
    • 16–18 km (Equator)
    • 8–10 km (Poles)
  • Temperature decreases with height (6.5°C/km)
  • Weather occurs here
  • Contains:
    • 75% of air mass
    • 99% of water vapour

👉 Unstable layer

2. Tropopause

  • Boundary between troposphere & stratosphere
  • Temperature becomes constant

Height:

  • Equator → 16–18 km
  • Poles → 8–10 km
  • India → ~11.5 km

Importance:

  • Limits cloud height
  • Jet streams occur here
  • CAT occurs just below

3. Stratosphere

  • Extends up to 50 km
  • Temperature increases with height (inversion)
  • Due to ozone absorption of UV radiation

👉 Stable layer → no weather
👉 Contains nacreous clouds

4. Mesosphere

  • Up to 80 km
  • Temperature decreases with height
  • Very cold

👉 Coldest region (-90°C to -100°C)
👉 Noctilucent clouds occur

5. Thermosphere

  • Above mesosphere
  • Temperature increases sharply
  • Air is very thin

👉 Above 700 km → Exosphere

6. Ionosphere

  • Above 60 km
  • Important for radio communication
  • Affected by solar activity

🌍 1.12 Tropopause Types

  • Tropical → ~16–16.5 km (India)
  • Mid-latitude → ~11.5 km
  • Polar → lower height

👉 Jet streams occur at boundaries

🌍 1.13 ISA (International Standard Atmosphere)

Standard reference atmosphere used in aviation:

  • Temperature (MSL) → 15°C
  • Pressure → 1013.25 hPa
  • Density → 1.225 kg/m³
  • Lapse rate → 6.5°C/km

ISA Deviation

 

ISA Deviation=Actual Temperature−ISA Temperature\text{ISA Deviation} = \text{Actual Temperature} – \text{ISA Temperature}

✈️ 1.14 Jet Standard Atmosphere (JSA)

Used above 30,000 ft:

  • Lapse rate → 2°C/1000 ft
  • No tropopause assumed

📌 QUICK REVISION (Pilot18 Style)

  • Atmosphere heated from below
  • Troposphere = weather layer
  • Stratosphere = stable, ozone present
  • Mesosphere = coldest layer
  • Thermosphere = very high temperature
  • Tropopause height ↑ with temperature
  • 75% air mass in troposphere
  • Ozone peak → 20–25 km
  • ISA temp at MSL → 15°C

🧠 INFOGRAPHIC