Chapter 12: Weather Theory Introduction

Importance of Weather in Aviation

• Weather influences aircraft performance and flying safety.
• Defined by temperature, moisture, wind velocity, visibility, barometric pressure.
• Understanding weather theory helps pilots make sound decisions based on reports and forecasts.

Atmosphere

• A blanket of air composed of gases surrounding Earth.
• Supports life, absorbs solar energy, recycles water and chemicals.
• Protects life from radiation and space vacuum.

Composition

• Nitrogen: 78%
• Oxygen: 21%
• Argon, CO2, and other gases: 1%
• Water vapor varies from 0 to 5% and causes major weather changes.

Layers

1. Troposphere
   • Up to 20 km (48,000 feet near equator).
   • Weather, clouds, storms, temperature variances occur here.
   • Temperature decreases by 2°C every 1000 feet; pressure decreases by 1 inch per 1000 feet.
2. Stratosphere
   • Extends from tropopause to 50 km.
   • Stable air, occasional clouds.
3. Mesosphere
4. Thermosphere

Atmospheric Circulation

• Caused by uneven heating of Earth's surface.
• Warm air rises, cool air sinks, creating motion and pressure changes.
• Earth's rotation (Coriolis Force) affects air movement.

Pressure

• Air pressure decreases with altitude.
• Affects aircraft performance: takeoff, climb rate, landings.

Coriolis Force

• Affects large-scale air movement due to Earth's rotation.
• Deflects air to the right in Northern Hemisphere, creating distinct circulation cells.

Measurement of Atmospheric Pressure

• Historically measured in inches of mercury (Hg) using a barometer.
• Standard instruments: Aneroid barometer.
• Standard sea level pressure: 29.92 Hg, 1013.2 mb.

Altitude and Atmospheric Pressure

• Pressure decreases 1 Hg for every 1000 feet altitude.
• Changes in pressure and temperature affect density altitude.

Wind and Atmospheric Motion

• Wind results from pressure differences, Coriolis Force, friction, and temperature.
• Convection currents and winds affect flight operations.

Convective Currents

• Caused by uneven solar heating.
• Create turbulence at low altitudes.

Local and Global Wind Patterns

• High pressure: clockwise circulation (anticyclonic).
• Low pressure: counterclockwise circulation (cyclonic).

Wind Shear

• Sudden changes in wind speed/direction.
• Hazardous near the ground, associated with thunderstorms, temperature inversions.

Atmospheric Stability

• Stability affects vertical movement, turbulence, cloud formation.
• Adiabatic processes: cooling/heating of air as it rises/falls.

Inversion

• Temperature inversion traps pollutants, affects visibility and weather.

Moisture and Temperature

• Moisture in atmosphere depends on temperature; affects weather.
• Relative humidity: ratio of current to maximum moisture air can hold.

Dew Point

• Temperature at which air is fully saturated; leads to condensation.

Saturation Methods

• Air reaches saturation by cooling, mixing, contact with cool surfaces, or rising.

Clouds

• Indicators of weather conditions.
• Classified by height: low, middle, high, vertical development.

Fronts and Air Masses

• Air masses: large volumes of air uniform in temperature and humidity.
• Fronts: boundaries between air masses.
• Types: Warm, Cold, Stationary, Occluded.

Thunderstorms

• Three stages: Cumulus, Mature, Dissipating.
• Hazards: wind shear, turbulence, lightning, hail.

Surface Weather Maps

• Depict fronts, pressure systems, wind conditions.
• Isobars indicate pressure changes.

Conclusion

• Understanding weather principles is crucial for flight safety and planning.
• Flight operations must consider atmospheric conditions and potential hazards.