The Atmosphere and Its Properties

1.1 Physical Properties of Air

• Air possesses several key physical properties that are fundamental to understanding flight:
•  Mass: Air has mass, meaning it has substance and is affected by gravity.
•  Compressibility: Air is a compressible fluid, able to flow or change its shape when subjected
• to even a minute pressure difference.
•  Viscosity: Air's viscosity is its internal friction or resistance to flow. It is very low, meaning
• small forces can move air molecules relative to each other. For example, water flows more
• easily than honey because honey has a much higher viscosity.

1.2 Static Pressure (SP)

• Static pressure is the pressure exerted by the air when it is stationary or not moving.
•  Definition and Unit: Static pressure is the result of the weight of the atmosphere pressing
• down on the air beneath it. Its unit is the

• Newton per meter square (N/m²), also known as a Pascal (Pa). 100 Pascals equal
• Hectopascal.
•  Formula: Static Pressure = Force / Area.
•  Effect of Altitude: Static pressure exerts the same force per square meter on all surfaces of
• an aeroplane at a given altitude. The lower the altitude, the greater the force per square
• meter, as there is more atmosphere pressing down. Pressure falls steadily with increasing
• altitude.

1.3 Temperature

• Temperature is a measure of the kinetic energy of the molecules in the air.
•  Units: The primary units are degrees Celsius (°C), measured relative to the freezing point of
• water, and Kelvin (K), measured relative to absolute zero.
• 0∘C=273∘K.
•  Absolute Zero: This is the lowest possible temperature where no heat energy remains in a
• substance. As seen in the comparison diagram, absolute zero is
• 0∘K, −273∘C, and −459∘F.
•  Lapse Rate: In the ICAO Standard Atmosphere (ISA), temperature falls steadily with
• increasing altitude up to about 36,000 ft, after which it remains constant through the lower
• stratosphere.

1.4 Density (ρ)

• Density is the mass of air per unit of volume.
•  Unit and Symbol: The unit is kilogram per cubic meter (kg/m³). The symbol for density
• is the Greek letter
• ρ (rho).
•  Relationship with Other Properties: Density varies with static pressure, temperature, and
• humidity. It is directly proportional to static pressure and inversely proportional to
• temperature and humidity.
•  Effect of Altitude: Density decreases with increasing altitude primarily because of the
• decreasing static pressure.

1.5 Dynamic Pressure (q)

• Dynamic pressure is the kinetic energy (energy of motion) of the air per unit volume.
•  Formula: The formula for dynamic pressure is:
• q = ½ ρ v²
• Where:
• q = Dynamic Pressure
• ρ = Air Density
• v = True Air Speed (TAS)
•  Total Pressure: The sum of static and dynamic pressures is known as Total Pressure or
• Pitot Pressure.
• Total Pressure = Static Pressure + Dynamic Pressure

1.6 Measuring Pressure and Airspeed

•  Sensing Pressures: An aircraft's instruments measure these pressures to determine airspeed.
• A
• Static Vent, a hole parallel to the airflow, senses static pressure.
• A
• Pitot Tube, facing into the airflow, senses Total Pressure (Static + Dynamic).
•  The Air Speed Indicator (ASI): The ASI is a pressure gauge that mechanically calculates
• the difference between Total Pressure and Static Pressure to find the Dynamic Pressure. The
• needle then indicates this dynamic pressure as airspeed. The instrument is calibrated to ISA
• density at sea level (
• 1.225kg/m3).

1.7 Airspeed Relationships

• The speed displayed on the ASI is not the true speed through the air. Several corrections must
• be applied.
• 1. Indicated Air Speed (IAS): The speed read directly from the ASI.
• 2. Calibrated Air Speed (CAS): IAS corrected for position and pressure error.
• 3. Equivalent Air Speed (EAS): CAS corrected for compressibility error, which becomes
• significant at higher speeds and altitudes.
• 4. True Air Speed (TAS): EAS corrected for density error. This is the actual speed of the
• aircraft through the air. 39TAS is the true measure of velocity; all the other "speeds" are
• actually pressure measurements.
• 5. Ground Speed (GS): TAS corrected for the effect of wind.