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.