Overview
This lecture provides an introduction to the fundamentals of aviation weather, focusing on the structure and behavior of the atmosphere, temperature, pressure, wind, moisture, cloud formation, and the practical implications of these elements for pilots and flight operations.
The Earth's Atmosphere
- Air is a mixture of gases: 78% nitrogen, 21% oxygen, and up to 5% variable water vapor.
- The atmosphere is layered; most weather and flight occur in the troposphere (surface to 7 miles).
- The tropopause marks the boundary to the stratosphere above, which is more stable.
- Air density decreases with altitude, impacting oxygen absorption and leading to hypoxia above 10,000 ft without supplemental oxygen.
Temperature
- Celsius and Fahrenheit are common temperature scales; conversion formulas are C = 5/9(F-32), F = 9/5C + 32.
- Heat is energy; temperature is the measure of that energy in a substance.
- Temperature varies daily (diurnal), seasonally, by latitude, topography, and altitude.
- Lapse rate: temperature generally decreases with altitude (~2°C per 1,000 ft); inversions occur when temperature increases with height.
Atmospheric Pressure and Altimetry
- Atmospheric pressure is the force of air's weight; measured with mercurial or aneroid barometers.
- Pressure decreases with altitude; rate varies with temperature.
- To compare pressures, station pressure is adjusted to sea level pressure; standard is 1013.2 mb or 29.92 inHg.
- Types of altitudes: indicated (what the altimeter reads), true (actual), pressure (standard), and density altitude (performance index).
- High density altitude (hot, low pressure, high elevation) reduces aircraft performance—longer takeoff/landing, reduced climb.
Wind
- Wind results from pressure differences; flows from high to low pressure.
- Pressure gradient force drives wind; closer isobars mean stronger winds.
- Coriolis force (from earth’s rotation) deflects wind right in the Northern Hemisphere, causing clockwise flow around highs and counterclockwise around lows.
- Surface friction slows wind and causes it to cross isobars toward lower pressure.
- Local winds include mountain/valley winds, katabatic winds (like Chinook), and land/sea breezes.
- Wind shear is a sudden change in wind speed/direction, creating turbulence.
Moisture, Cloud Formation, and Precipitation
- Water vapor in air varies; relative humidity is the percent saturation.
- Dew point is the temperature at which air becomes saturated.
- Condensation nuclei (dust, salt) aid cloud formation.
- Precipitation forms when particles grow large enough; cloud thickness >4,000 ft often needed for significant precipitation.
- Cooling, either by expansion (updrafts), contact with cold surfaces, or radiation, is primary for cloud and fog formation.
Key Terms & Definitions
- Hypoxia — deficiency of oxygen in the body from low air pressure.
- Lapse Rate — rate of temperature decrease with altitude.
- Inversion — a layer where temperature increases with altitude.
- Density Altitude — altitude in standard atmosphere where density equals observed density; affects aircraft performance.
- Coriolis Force — apparent deflection of moving air due to earth’s rotation.
- Relative Humidity — ratio of current to maximum possible water vapor in air at a given temperature.
- Condensation Nuclei — particles in air on which water vapor condenses to form clouds.
- Katabatic Wind — wind blowing downslope (e.g., Chinook).
Action Items / Next Steps
- Memorize standard atmosphere values (pressure, temperature, lapse rate).
- Practice converting Celsius and Fahrenheit.
- Review how altimeter settings and temperature affect true altitude.
- Be able to identify cloud types and their associated flying conditions.
- Prepare for discussion on stability, turbulence, and weather hazards in coming lectures.