Aviation Meteorology - Atmospherics
The Atmosphere is a layer of gases that surrounds our planet, providing protection from ultraviolet rays and supporting life. It is primarily composed of Nitrogen (78%) and Oxygen (21%), with Argon, carbon dioxide, and trace gases making up the remaining 1%. The atmosphere is divided into several layers, each with distinct characteristics.
Layers of the Atmosphere
Troposphere: The lowest layer, extending from sea level up to 20,000 feet over the poles and up to 48,000 feet over the equator. Most weather phenomena occur here. Temperature decreases by about 3.6°F per 1,000 feet of altitude gain, and pressure decreases by about 1 inch of Hg per 1,000 feet.
At the top of the troposphere is the tropopause, which traps moisture and weather. Its altitude varies with latitude and season and is associated with the jet stream and clear air turbulence.
Stratosphere: Located above the troposphere and extending up to 180,000 feet. The pressure here is about 1/1000th of sea level pressure. This layer contains the ozone layer, which absorbs and scatters ultraviolet solar radiation. The stratosphere is stable and almost free of weather, making it the highest layer accessible by jet-powered aircraft.
Other layers include the Mesosphere, Thermosphere (where the International Space Station orbits), and the Exosphere (the outermost layer, merging with outer space and containing most satellites).
Atmospheric Pressure
Atmospheric pressure is the force exerted by the weight of air above a surface. At sea level, this pressure is 14.7 pounds per square inch. Pressure changes with altitude, temperature, and air density, affecting aircraft performance during launch, recovery, and climbing.
Pressure is typically measured in inches of mercury (in. Hg). The International Standard Atmosphere (ISA) provides a common reference for temperature and pressure, with standard sea level pressure defined as 29.92 in. Hg at 59°F (15°C). Atmospheric pressure is also reported in millibars, with 1 inch of mercury equaling approximately 34 millibars and standard sea level equaling 1013.2 millibars.
Weather stations convert local barometric pressure readings to mean sea level pressure for standardization. For example, a station at 3,000 feet above sea level with a reading of 24.92 inches of mercury reports a sea level pressure of 27.92 inches.
Altitude and Atmospheric Pressure
As altitude increases, atmospheric pressure decreases. On average, pressure drops by 1 inch of mercury for every 1,000 feet of altitude gain.
The Effects of Altitude on Flight
Altitude impacts all aspects of flight, from aircraft performance to human physiology. Higher altitudes mean lower atmospheric pressure, requiring longer takeoff and landing distances and reducing climb rates. Thin air at high altitudes necessitates higher speeds for sufficient lift, resulting in longer ground runs. Aircraft engines and propellers are also less efficient at higher altitudes due to decreased air density, leading to reduced climb rates and longer ground runs for obstacle clearance.
