L1 Properties of the Atmosphere

Meteorology Introduction

• Meteorology is considered a complex but interesting subject in the PPL course.

• It requires understanding basic mechanisms rather than rote learning.

• The course equips pilots to interpret weather information for safe planning and assess actual weather conditions for sound decisions.

• The course focuses on practical aspects affecting flight planning and in-flight decisions, avoiding jargon and theoretical concepts.

• Weather forecasts and reports are used to illustrate weather phenomena.

• Weather systems are complex and unpredictable; small changes can significantly influence the weather.

• Treat weather forecasts with caution due to inherent unpredictability.

• Words like "tend," "may," "can," and "often" are used due to the difficulty of making definitive statements about weather.

Weather's Impact on Aviation Accidents

• Weather is a major factor in general aviation accidents.

• Dealing with bad weather relies on pilot judgment, not flying skill.

• Piloting ability cannot overcome fog, clouds, or thunderstorm turbulence.

• Weather accidents usually result from poor judgment: ignoring forecasts or continuing flight into poor weather.

• The weather is a powerful force, and taking it on is dangerous.

• Pilots who ignore weather risks become accident statistics.

Composition and Structure of the Atmosphere

• The atmosphere is a mixture of gases held to Earth by gravity.

• Approximate proportions of gases in a dry atmosphere:

  • 78% Nitrogen (N2)

  • 21% Oxygen (O2)

  • 1% Carbon Dioxide (CO2), Ozone (O3), and other gases

• The atmosphere is never completely dry; water vapor varies from close to 0% in polar regions to around 4% in humid tropical areas.

• Water vapor absorbs and releases energy (heat) and condenses into clouds, rain, snow, etc.

• Water Vapor (H2O), Carbon Dioxide (CO2), and Ozone (O3) have the greatest influence on weather.

Troposphere and Tropopause

• The atmosphere is divided into layers; the troposphere is closest to the Earth and most relevant to light-aircraft pilots.

• The troposphere extends from the surface to the tropopause.

• The troposphere contains almost all the atmosphere's water; hence, most weather occurs here (clouds, fog, mist, rain, snow).

• The tropopause is higher in summer than in winter.

• Typical tropopause heights for different latitudes:

  • 30°: 16 km / 52,000 ft (Summer and Winter)

  • 50°: 12 km / 39,000 ft (Summer), 9 km / 29,000 ft (Winter)

  • 70°: 9 km / 29,000 ft (Summer), 8 km / 26,000 ft (Winter)

• The troposphere is shallowest over the poles and deepest over the equator.

Stratosphere

• Above the tropopause is the stratosphere.

• The tropopause acts as a lid, keeping most water vapor and weather in the troposphere.

• The stratosphere extends from the tropopause to about 20 miles (120,000 ft) above the surface.

Properties of Gases: Temperature

• The main properties of a gas or gas mixture (like the atmosphere) are temperature, pressure, and density.

• Temperature: a measure of the degree of heat present.

• The ICAO standard unit of temperature is degrees Celsius (°C).

• Occasionally, degrees Fahrenheit (°F) are used.

Properties of Gases: Pressure

• Atmospheric pressure is the force exerted by the weight of the atmosphere.

• The ICAO unit for pressure is the Hectopascal (hPa); inches of mercury (inHg or Hg) are also used, especially in the US.

• The UK Met Office often uses millibars (Mb) as a measure of pressure; for aviation, 1 hPa = 1 Mb.

• Boyle's law: a gas's pressure and volume are inversely proportional when the temperature is constant.

Properties of Gases: Density

• Density: the amount of matter in a given volume.

• Air density is measured in grams per cubic meter (g/m^3).

• Dry air is denser than moist air (air containing water vapor).

Relationship Between Pressure, Temperature, and Density

• Air density is directly proportional to pressure and inversely proportional to temperature.

• At a constant temperature, if pressure reduces, density also reduces.

• At a constant pressure, if temperature increases, density decreases.

• This relationship is crucial for convection: warmer air is less dense and rises; colder air is denser and sinks.

Pressure and Altitude

• Air pressure at any level is the 'weight' of the air above that level.

• Most air molecules are held close to the Earth's surface by gravity.

• Higher air pressure exists at the surface due to more molecules.

• Air pressure decreases with increasing altitude.

Temperature, Density, and Altitude

• Temperature usually decreases with height in the troposphere, heated from below by the Earth.

• The temperature change is about 2°C per 1000 ft of altitude change.

• The rate of temperature change is called the lapse rate.

• The reduction in temperature continues up to the tropopause; above it, the temperature is constant.

• Density reduces as pressure reduces but increases as temperature reduces.

• Pressure change has a more significant effect on density than temperature change.

• Typical temperature at different altitudes:

  • Sea Level: +15°C

  • 5,000 ft: +5°C

  • 10,000 ft: -5°C

  • 15,000 ft: -15°C

  • 20,000 ft: -25°C

  • 25,000 ft: -35°C

  • 30,000 ft: -45°C

  • 35,000 ft: -55°C

Conversion of Temperature

• Temperature is commonly referred to in degrees Celsius in aviation.

• Conversion formulas:

  • Celsius to Fahrenheit: Divide by 5, multiply by 9, and add 32.

  • Fahrenheit to Celsius: Subtract 32, divide by 9, and multiply by 5.

• Quick rule of thumb: 10°C = 50°F; thereafter, allow 9°F for each 5°C.
  *Graphs and conversion tables are widely available to avoid mental arithmetic.

International Standard Atmosphere (ISA)

• The atmosphere's parameters (temperature, density, pressure) are constantly changing.

• The ISA provides an 'average' set of atmospheric conditions for calibration purposes.

• The mean altitude of the Tropopause in ISA is 11 km (approximately 35,000 ft).

• ISA sea-level conditions:

  • Temperature: +15°C

  • Pressure: 1013.25 mb/hPa (29.92 inHg or 760 mmHg)

  • Temperature Lapse Rate: 1.98°C/1000 ft (0.66°C / 100 m) until 36,090 ft, above which it remains at -56.5°C.

  • Density: 1.225 kg / cubic-metre

Variations from ISA and Aircraft Performance

• Real conditions differ from ISA; the actual air temperature is referred to as Outside Air Temperature (OAT).

• Aircraft performance tables may be marked