AVN 2300: Module 1 Lecture

PART 1:

Atmosphere Basics

• Surrounds Earth, held by gravity.

• Provides air for breathing, protection from space, and retains moisture, gases, and particles.

Composition

• 78% Nitrogen, 21% Oxygen, ~1% other gases.

• Water vapor varies (0–4%) based on location and conditions.

Key Atmospheric Variables

1. Temperature: Measures molecular motion.

   • Higher movement = higher temperature.

   • Absolute zero (0 K) = no molecular motion.

2. Density: Mass of molecules in a volumes

   • Decreases with lower pressure, higher temperature, or increased humidity.

3. Pressure: Force from moving gas molecules.
   
   

Effects on Aircraft

• Lower air density reduces lift and engine performance.

• High temperature or humidity decreases density and performance.

Ideal Gas Law

• Pressure, density, and temperature are interrelated: P/DT=RP/DT = R.

• Changes in one variable adjust the others to maintain balance.

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Equalization Principles

• Temperature: Heat flows from high to low to balance.

• Density: High density moves to low density (like clowns spilling out of a packed car).

• Pressure: High pressure flows to low pressure to equalize.

Atmospheric Layers

1. Troposphere: Surface to ~65,000 ft.

2. Stratosphere: ~25,000–35,000 ft.

3. Mesosphere: Above the stratosphere.

4. Thermosphere: “Top” around 164,000 ft.

Special Layers

• Ozone Layer: ~80,000 ft, absorbs UV radiation.

  • Can impact aircrews/passengers at cruising altitudes.

• Ionosphere: Lower mesosphere to thermosphere.

  • Contains charged particles, affects radio communication.

Particulate Matter

• Suspended liquids/solids like water vapor, smoke, and dust.

• Reduces visibility and aids water condensation.

International Standard Atmosphere (ISA)

• Sea Level:

  • Pressure: 29.92 in Hg / 1013.2 mb.

  • Temperature: 59°F / 15°C.

• Lapse Rates:

  • Temperature: -2°C (3.6°F) per 1,000 ft.

  • Pressure: -1 in Hg (34 mb) per 1,000 ft.

Key Implications for Aviation

• Temperature and pressure changes with altitude significantly affect aircraft performance.

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PART 2: 

Seasons and Earth's Tilt

• Cause of Seasons: Earth's 23.5° axial tilt, not its elliptical orbit.

• Aphelion/Perihelion: Distance from the sun has minimal impact on seasons.

• Without tilt, solar radiation would be more uniform, leading to stable weather.

Solar Geometry

• Solar Elevation Angle (e): Angle of the sun above the horizon.

  • Directly overhead (90°) at 23.5° latitude during solstices.

  • Equation: e=90−(L−L(p))e = 90 - (L - L(p))e=90−(L−L(p)), where:

    • LLL = latitude of location.

    • L(p)L(p)L(p) = solar declination.

  • Example: At 40°N latitude (Denver/Columbus) during the summer solstice, e=73.5°e = 73.5°e=73.5°.

Importance of the Sun

• Sun's Role: Primary driver of:

  • Weather, wind, thermals, turbulence.

• Key takeaway: The sun is the reason for weather.

Energy Transfer Methods

1. Radiation:

   • Energy emitted as short-wave electromagnetic waves.

   • Absorbed radiation warms; emitted radiation cools.

2. Conduction: Heat transfer through direct contact.

3. Convection: Heat transfer through fluid movement (air or water).

Solar Radiation Variability

• Influenced by:

  • Time of day

  • Season

  • Latitude

Energy Distribution**

• Near Equator: Higher solar elevation angle → more energy per unit area.

• Near Poles: Lower angle → less energy per unit area.

Solar Radiation Breakdown

• 19% absorbed by the atmosphere.

• 51% absorbed by Earth's surface.

• 6% scattered by the atmosphere.

• 24% reflected (clouds/surface).

Albedo Effect

• Reflection reduces ground heating.

  • Example: Albedo 0.2 = 20% reflected.

Solar Heating and Cooling

Solar Heating

• Air: Poor heat conductor.

  • Sun warms ground, which heats air via:

    • Conduction, Convection, and minimal Radiation.

• Land vs. Water:

  • Land heats/cools faster than water, needing less heat energy.

Terrestrial Radiation (Cooling)

• At night:

  • Earth re-radiates heat, cooling the surface.

  • Water vapor absorbs heat, reducing cooling (e.g., humid areas stay warmer than deserts).

  • Clouds reduce cooling and prevent fog by limiting dew point cooling.

Conduction and Convection

• Day:

  • Ground heats → warms air → warm air rises (convection).

• Night:

  • Ground cools → cools air near surface (conduction).

Advection

• Horizontal heat transfer via air and ocean currents.

• Essential for balancing global temperatures between poles and equator.

Types of Temperatures

1. Surface Air: Measured 5 ft above ground, shaded to avoid direct solar effects.

2. Upper Air: Measured via balloons or PIREPs.

3. Indicated Air (IAT): Aircraft measurement with compression heating included.

4. Outside Air (OAT/TAT): Excludes compression effects.

Global Temperature Patterns

• January: Large seasonal changes, cooler toward poles.

• July: More uniform in Southern Hemisphere due to water coverage.