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esson 6.1 – The Atmosphere & Atmospheric Pressure
Weather occurs in the atmosphere. To understand weather, you must first understand the basic properties of the atmosphere, especially pressure and density.
Reference:
FAA Pilot’s Handbook of Aeronautical Knowledge (PHAK), Chapter 12 – Weather Theory
Composition of the Atmosphere
Composition of the Atmosphere
The atmosphere is a mixture of gases surrounding the Earth. Near the surface it is composed of approximately:
78% Nitrogen
21% Oxygen
1% Other gases (including argon, carbon dioxide, and water vapor)
Water vapor, although a small percentage, is extremely important because it is responsible for clouds, precipitation, and much of the weather we experience.
Layers of the Atmosphere (simplified)
Layers of the Atmosphere (simplified)
Troposphere – The lowest layer, where nearly all weather occurs. Temperature generally decreases with altitude.
Tropopause – The boundary at the top of the troposphere.
Stratosphere – Above the tropopause; temperature generally increases with altitude.
Atmospheric Pressure
Atmospheric Pressure
Atmospheric pressure is the weight of the column of air above a given point.
At sea level, standard pressure is 29.92 inches of mercury (inHg) or 1013.2 millibars.
Pressure decreases with altitude because there is less air above you.
Near the surface, pressure decreases approximately 1 inch of mercury for every 1,000 feet of altitude gain.
Density and Density Altitude
Density and Density Altitude
Air density is how closely the air molecules are packed together.
Three critical points every pilot must understand:
Pressure decreases with altitude — There is less air above you as you climb, so both pressure and density decrease.
Temperature and humidity affect density — Hot air is less dense than cold air. Humid air is less dense than dry air. Therefore, hot + high + humid conditions produce the lowest density (highest density altitude).
High density altitude reduces aircraft performance — When density altitude is high, you will experience:
Longer takeoff rolls
Reduced climb performance
Higher true airspeed for a given indicated airspeed
Reduced engine and propeller efficiency
Standard Atmosphere
Standard Atmosphere
Pilots use standard values for reference:
Sea level pressure: 29.92 inHg
Sea level temperature: 15°C (59°F)
Average temperature lapse rate: about 2°C (3.5°F) per 1,000 feet
These standards allow us to compare actual conditions to a common baseline and to calculate density altitude.
Key Points to Remember
Key Points to Remember
Nearly all weather occurs in the troposphere.
Pressure decreases about 1 inHg per 1,000 feet near the surface.
Hot, high, or humid conditions = high density altitude = reduced aircraft performance.
Always consider density altitude when planning takeoffs and climbs, especially on hot days or at high-elevation airports.
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