Notes Flashcards
The Atmosphere:
• The atmosphere is a layer of gases surrounding the Earth (or any planetary body) and is held in place by gravity.
• In its dry state, the atmosphere is a mixture of gases, with the following approximate composition:
• Nitrogen: 78.09%
• Oxygen: 20.95%
• Argon: 0.93%
• Carbon Dioxide (CO₂): ~0.03%
• Trace gases: Negligible
• Key Facts:
• Air is never completely dry as water vapor is always present.
• Nitrogen to Oxygen Ratio:
• By volume: 4:1
• By weight: 3:1
• Vertical Distribution of Air Mass:
• Below 6 km (20,000 ft): 50%
• Below 10 km: 75%
• Below 35 km: 99%
Layers of the Atmosphere:
• The atmosphere is divided into four layers:
1. Troposphere: Extends up to 8–10 km at the poles and 16–18 km at the equator.
2. Stratosphere: Extends from the tropopause up to 50 km.
3. Mesosphere: Extends from the stratopause up to 85 km.
4. Thermosphere: Above 85 km.
Troposphere (Ctropos = Turning):
• The lowest layer closest to Earth.
• The boundary between the troposphere and stratosphere is called the tropopause.
• Tropopause Height:
• Poles: 8–10 km
• Equator: 16–18 km
• Average height globally: 11 km (50% of Earth’s surface).
• Temperature Lapse Rates:
• 1.98°C per 1000 ft
• 6.5°C per km
• 3.5°F per 1000 ft
• Key Features:
• The troposphere is generally unstable and where all weather phenomena occur.
• The tropopause temperature varies with latitude and season:
• Equator: −70°C to −75°C
• Poles: −40°C to −45°C
Stratosphere:
• Layer above the tropopause, extending up to 50 km.
• The boundary is called the stratopause.
• Key Features:
• Ozone (O₃) is concentrated between 10–50 km, with a maximum concentration at 20–25 km.
• Temperature remains constant between 11–20 km, then increases with altitude.
• Very stable region with low humidity and no weather phenomena.
• Rare clouds like nacreous clouds (mother-of-pearl) are seen in the upper stratosphere.
Mesosphere:
• Extends from the stratopause to 85 km.
• Rare clouds like noctilucent clouds are sometimes seen in the upper mesosphere.
• The boundary is called the mesopause.
Thermosphere:
• Layer above the mesosphere.
• Air temperature increases rapidly with altitude.
International Standard Atmosphere (ISA):
• Air is dry.
• Temperature at MSL: 15°C
• Pressure at MSL: 1013.25 hPa or mb / 29.92 inches Hg
• Density at MSL: 1.225 kg/m³
Layers:
• Inversion Layer: Lapse Rate = Negative; Temperature increases with height
• Isothermal Layer: Lapse Rate = Zero; Temperature constant (-56.5°C)
• Normal Layer: Lapse Rate = 2°C/1000’ or 6.5°C/km
Formula: ISA Deviation = OAT - ISA
Jet Standard Atmosphere:
Same as ISA but there is no tropopause.
Atmospheric Pressure:
• Defined as the force exerted on a surface of unit area by air molecules.
Types of Pressure:
1. Static Pressure:
• When air is at rest, the pressure exerted on the body is called static pressure.
• Other names: Atmospheric Pressure, Barometric Pressure
• Concerned in Aviation Meteorology.
2. Dynamic Pressure:
• When air is in motion, the additional pressure exerted is called dynamic pressure.
• Held by gravitational force; decreases with height.
Measured by Aneroid or Mercury Barometer.
Pressure value varies at different heights.
Isobars:
• Lines joining places of equal pressure.
• Drawn at an interval of 2 hPa in India.
Pressure Change per hPa with Height:
• MSL: 27 feet/hPa
• 20,000 feet: 50 feet/hPa
• 40,000 feet: 100 feet/hPa
Note:
Pressure lapse rate is steeper in cold air.
At surface, pressure is more in cold air than warm air.
Semi-Diurnal Variation of Pressure:
• Maximum Pressure: 1000 & 2200 hours
• Minimum Pressure: 0400 & 1600 hours
• Variation highest in tropical regions, lowest at poles.
Pressure Systems:
1. Low Pressure (L):
• Circular isobars around the center.
• Pressure: Low; Weather: Bad; Visibility: Good.
• Winds:
• Anti-clockwise in Northern Hemisphere.
• Converge on the ground.
Wind Speed-Based Classification:
Depends on the speed of wind.
Nomenclature and Wind Speed
1. Low Pressure Area (LPA)
• Wind Speed: Up to 16 Kts
2. Depression (DD)
• Wind Speed: 17–27 Kts
3. Deep Depression (DD)
• Wind Speed: 28–33 Kts
4. Cyclonic Storm (CS)
• Wind Speed: 34–46 Kts
5. Severe Cyclonic Storm (SCS)
• Wind Speed: 47–63 Kts
6. Severe Cyclonic Storm with Core of Hurricane Winds
• Wind Speed: 64 Kts or More
Trough
• It is an elongated area of low pressure.
• Pressure: Lower along the trough line than on both sides.
• Weather: Bad (pressure rises on both sides of the trough).
High Pressure (H)
• A region of high pressure with widely spaced isobars, having the highest pressure at the center.
• Weather: High (Good)
• Visibility: Poor
• Winds:
• Clockwise in the Northern Hemisphere (NH).
• Diverges on the ground.
• Veers.
COL
• It is an area that lies between two lows and two highs.
• Pressure: Uniform (very little variation).
• Winds: Light & variable.
Altimeter
• Definition: A pressure instrument in an aircraft providing height from a selected datum.
QFE
• Definition: Actual pressure at the aerodrome.
• On the aerodrome, QFE is set on the altimeter; it will read zero.
• In air, it shows height above ground level (AGL).
QNH
• Aerodrome-level pressure reduced to MSL pressure in ISA conditions.
• On the aerodrome: Shows elevation from MSL.
• In air: Altimeter shows altitude (AMSL).
QNE
• CPS (Standard Pressure Setting):
• Standard pressure datum 1013.25 hPa or 29.92 inch Hg.
• Altimeter reads pressure altitude (FL).
OFF
• Aerodrome-level pressure reduced to MSL pressure in actual conditions.
• Used for drawing meteorological charts.
Comparison with QNH:
• If QNH > OFF → Positive pressure deviation.
• If QNH < OFF → Negative pressure deviation.
Variation of Standard Pressure According to Pressure Altitude
Pressure Altitude (ft) Pressure (hPa)
MSL 1013.25
5000 850
10000 700
18000 500
24000 400
30000 300
38000 200
53000 100
Effect of Pressure and Temperature on Altimeter Reading
• H → H: Higher pressure = higher altitude.
• L → L: Lower pressure = lower altitude.
Contours
• Definition: Lines joining places of equal height.
• On contour charts, drawn at intervals of:
• 40 gpm: 700 hPa & 500 hPa levels.
• 80 gpm: 300 hPa & 200 hPa levels.
Isallobars
• Definition: Lines joining places of equal pressure change.
Pressure Gradient
• Definition: Rate of fall of pressure horizontally.
Pressure Tendency
• Definition: Change of pressure with time.
