Comprehensive Meteorology Examination Guide (EASA Pilot Bank)

ATMOSPHERIC COMPOSITION, EXTENT, AND VERTICAL DIVISION

• Tropopause Height Variation: In the northern hemisphere, the height of the tropopause normally decreases from south to north (i.e., it is higher at the equator and lower at the poles).

• Equatorial Tropopause: The average height of the tropopause over the equator is approximately $16\,km$.

• Humidity Distribution: Most atmospheric humidity is concentrated in the troposphere. More than $90\%$ of all water vapor is contained in this layer.

• Vertical Boundaries: The boundary layer between the troposphere and the stratosphere is called the tropopause.

• Cloud Projection: Cumulonimbus (CB) clouds are the only type that can project up into the stratosphere.

• Tropopause Characteristics:

  • It separates the troposphere from the stratosphere.

  • It is often defined as an isothermal layer (temperature remains constant with height).

  • It is the level at which temperature ceases to fall with increasing height.

• Troposphere Thickness factors: The thickness of the troposphere varies primarily with latitude (thickest at the equator, thinnest at the poles).

• Standard Dry Air Composition: By volume, the troposphere consists of approximately $21\%$ oxygen, $78\%$ nitrogen, and $1\%$ other gasses.

• Temperature and Height at Extremes:

  • Over the equator: Approx. $16\,km$ and $-75^{\circ}C$.

  • Over the poles: Approx. $8\,km$ and $-40^{\circ}C$.

• Stratosphere Extent: In mid-latitudes, the stratosphere extends on average from $11\,km$ to $50\,km$.

VERTICAL DISTRIBUTION OF TEMPERATURE AND HEAT TRANSFER

• Standard Lapse Rate Calculations: The ICAO standard atmosphere uses a temperature lapse rate of $0.65^{\circ}C$ per $100\,m$ ($1.98^{\circ}C$ per $1000\,FT$).

• Temperature Calculations examples:

  • If temperature at $FL\,140$ is $-12^{\circ}C$, at $FL\,110$ ($3000\,FT$ lower) it will be approx. $-6^{\circ}C$.

  • If temperature at $FL\,80$ is $+6^{\circ}C$, at $FL\,130$ ($5000\,FT$ higher) it will be approx. $-4^{\circ}C$.

• ISA Deviation: If the outside air temperature (OAT) at $FL\,200$ is $-35^{\circ}C$, it is $10^{\circ}C$ colder than the Standard Atmosphere (ISA), which is $-25^{\circ}C$ at that level.

• Physical Processes in Warming: Convection and condensation contribute the most to atmospheric warming.

• Convective Activity: Over land in mid-latitudes, convection is greatest in the summer during the afternoon.

• Advection: Defined as the horizontal motion of air.

• Solar Radiation: The sun's radiation primarily heats the surface of the earth, which in turn heats the air in the troposphere.

LAPSE RATES, STABILITY, AND INSTABILITY

• Environmental Lapse Rate (ELR): This is the actual temperature change measured in the atmosphere and varies with time and location.

• Dry Adiabatic Lapse Rate (DALR): Has a fixed value of $1^{\circ}C$ per $100\,m$ ($3^{\circ}C$ per $1000\,FT$).

• Saturated Adiabatic Lapse Rate (SALR): Generally lower than DALR because heat is released during condensation. Its value is closest to DALR in very cold air (e.g., cirrus clouds/high altitudes).

• Stability Definitions:

  • Absolute Instability: Exists when the ELR exceeds the DALR (> 1^{\circ}C/100\,m).

  • Conditional Instability: Exists if the air is unstable for saturated air but stable for dry air (ELR is between SALR and DALR).

  • Absolute Stability: ELR is less than the SALR.

• Stability Indicators: An air mass is "stable" when the vertical motion of rising air tends to become weaker and disappears. Stability increases if warm air is advected in the upper part of a layer and cold air in the lower part.

• Inversions: An inversion is a layer where temperature increases with height. It is always absolutely stable.

• Isothermal Layer: A layer where the temperature remains constant with height.

TEMPERATURE NEAR THE EARTH'S SURFACE

• Diurnal Variation: The variation between day and night temperatures is largest when the sky is clear and winds are weak.

• Minimum Temperature Timing: On a clear day with calm winds, the minimum temperature is reached approximately half an hour after sunrise.

• Surface Effects: Cloud cover (e.g., stratus) during the night acts as a blanket, keeping surface temperatures slightly higher than they would be under a clear sky.

BAROMETRIC PRESSURE AND ALTIMETRY

• Isobars: Lines on a weather chart connecting positions with the same air pressure reduced to sea level (QFF).

• Standard Altimeter Settings:

  • QFE: Pressure at field elevation/station level. Altimeter reads zero on the ground.

  • QNH: Pressure reduced to mean sea level (MSL) using ICAO standard atmosphere values. Altimeter reads airfield elevation on the ground.

  • QFF: Pressure reduced to MSL using actual atmosphere values (temperature included). Generally used on surface synoptic charts.

  • QNE: Standard pressure setting of $1013.25\,hPa$.

• Pressure Variation with Height:

  • At Mean Sea Level (MSL), $1\,hPa \approx 27\,FT$ ($8\,m$).

  • At $5500\,m$ ($18000\,FT$), $1\,hPa \approx 50\,FT$ ($15\,m$).

• Altimeter Errors (Temperature):

  • When flying from warm air to cold air, the altimeter will overread (indicate a higher altitude than the true altitude).

  • True altitude is lower than pressure altitude in cold air masses.

• Atmospheric Mass: Half the total mass of the atmosphere is found within the first $5\,km$.

INTERNATIONAL STANDARD ATMOSPHERE (ISA) AND PRESSURE LEVELS

• Standard Values at MSL: $15^{\circ}C$ and $1013.25\,hPa$.

• Temperature Change: Decreases by $0.65^{\circ}C$ per $100\,m$ up to $11\,km$.

• Lowest ISA Temp: $-56.5^{\circ}C$.

• Standard Pressure Heights (Moderate Regions):

  • $850\,hPa \approx FL\,50$

  • $700\,hPa \approx FL\,100$

  • $500\,hPa \approx FL\,180$

  • $300\,hPa \approx FL\,300$

  • $200\,hPa \approx FL\,390$

WIND AND GENERAL CIRCULATION

• Wind Cause: Horizontal pressure differences (Pressure Gradient Force).

• Geostrophic Wind: A theoretical wind that balances the Pressure Gradient Force and the Coriolis Force. It applies when isobars are straight and no friction is involved.

• Gradient Wind: Takes the curvature of isobars into account.

  • Around a low pressure (cyclone): Geostrophic wind speed > Gradient wind speed.

  • Around a high pressure (anticyclone): Gradient wind speed > Geostrophic wind speed.

• Coriolis Force: Prevents air from flowing directly from high to low pressure; deviates air to the right in the Northern Hemisphere and to the left in the Southern.

• Northern Hemisphere Wind Behavior:

  • Ascending from surface to $2000\,FT$: Wind veers (turns clockwise) and increases in speed.

  • Descending to surface: Wind backs (turns counter-clockwise) and decreases in speed due to friction.

• Global Belts:

  • Subtropical High-Pressure Belt: Found between $25^{\circ}$ and $35^{\circ}$ latitude.

  • Travelling Low Pressure Systems: Found between $35^{\circ}$ and $55^{\circ}$ latitude.

  • Trade Winds: NE Trade winds (Northern Hemisphere) and SE Trade winds (Southern Hemisphere) blow toward the ITCZ.

TURBULENCE AND WIND SYSTEMS

• Moderate Turbulence (ICAO): Occupants feel strain against seat belts; loose objects move; food service and walking are difficult; accelerometer changes of $0.5$ to $1.0\,g$.

• Clear Air Turbulence (CAT): Most severe CAT is likely in a curved jet stream near a deep trough. The degree of CAT is proportional to the intensity of vertical and horizontal windshear.

• Local Winds:

  • Sea Breeze: Occurs during the day, blowing from sea to land.

  • Land Breeze: Occurs at night, blowing from land to sea (usually weaker than sea breeze).

  • Anabatic Wind: Wind blowing up a slope during the day due to surface heating.

  • Katabatic Wind: Cold wind blowing down a slope at night (e.g., Bora, Mistral, Chinook).

  • Foehn: A warm, dry wind on the leeward side of a mountain range caused by moisture loss on the windward side.

  • Harmattan: A dry, sand-laden NE wind blowing in winter over North West Africa.

JET STREAMS

• Definition: Minimum speed of $60\,kt$.

• Polar Front Jet Stream: Main cause is the north-south horizontal temperature gradient at the polar front. Found in the warm air mass, just below the tropopause.

• Core Heights:

  • Arctic Jet: Approx. $20000\,FT$.

  • Polar Front Jet: Approx. $30000\,FT$.

  • Subtropical Jet: Approx. $40000\,FT$.

• Equatorial Easterly Jet: Occurs only in the summer of the Northern Hemisphere at approx. $45000\,FT$.

• Turbulence in Jets: Highest probability is found on the cold air side of the core (to the left of the core in the Northern Hemisphere when looking downstream).

MOISTURE, CLOUDS, AND PRECIPITATION

• Dewpoint: The temperature to which air must be cooled to become saturated at a given pressure.

• Relative Humidity (RH): The ratio of actual water vapor pressure to saturation vapor pressure. It decreases as temperature increases (if moisture remains constant).

• Adiabatic Processes: An ascending unsaturated parcel cools at $1^{\circ}C/100\,m$; a saturated parcel cools slower (approx. $0.6^{\circ}C/100\,m$) because latent heat is released.

• Cloud Classification:

  • High (CI, CS, CC): Composed of ice crystals. Bases approx. $15000$ to $35000\,FT$.

  • Medium (AS, AC): Bases approx. $7000$ to $15000\,FT$.

  • Low (ST, SC, NS): Bases surface to $6500\,FT$.

• Precipitation Types:

  • Drizzle (DZ): Associated with Stratus (ST).

  • Continuous Rain (RA): Associated with Nimbostratus (NS).

  • Showers (SH): Associated with Cumulonimbus (CB) or Towering Cumulus.

  • Hail (GR): Associated with Cumulonimbus (CB).

  • Freezing Rain (FZRA): Occurs when rain falls from warm air aloft into a layer of cold air (below $0^{\circ}C$).

FOG AND VISIBILITY

• Radiation Fog: Forms over flat land on clear nights with calm or light winds (< 5\,kt). Caused by terrestrial radiation cooling. Extent is approx. $500\,FT$.

• Advection Fog: Forms when warm, moist air flows over a colder surface. Can form suddenly day or night and is persistent with wind speeds up to $15\,kt$.

• Steam Fog (Arctic Sea Smoke): Forms when cold air moves over warm water.

• Frontal Fog (Mixing Fog): Forms when warm humid air meets cold humid air.

• Meteorological Visibility vs. RVR: Visibility is often reduced by haze (dust trapped under an inversion) or precipitation. Runway Visual Range (RVR) is reported when visibility is less than $1500\,m$.

AIRMASSES AND FRONTAL SYSTEMS

• Airmass Stability: In unstable air, an ascending parcel continues to rise. In stable air, vertical motion weakens.

• Polar Front: Boundary between polar air and tropical air.

• Warm Front: Warm air overrides a cold air mass. Inclined plane approx. $1/150$. Sequence: CI then CS then AS then NS.

• Cold Front: Cold air pushes under a warm air mass. Associated with CB clouds and showers in summer. Wind veers sharply upon passage.

• Occlusions:

  • Cold Occlusion: The coldest air is behind the occlusion.

  • Warm Occlusion: The coldest air is ahead of the occlusion (original warm front).

• Frontal Movement: Frontal depressions typically move in the direction of the $2000\,FT$ wind in the warm sector.

DEPRESSIONS AND ANTICYCLONES

• Subsidence: The vertically downward motion of air in a high-pressure area. Leads to cloud dissipation and subsidence inversions.

• Blocking Anticyclone: A warm, quasi-stationary high situated in high latitudes ($50^{\circ}N-70^{\circ}N$), such as over Scandinavia, that stays for long periods.

• Thermal Depression: Forms over land in summer due to intense heating. Associated with surface convergence and upper-level divergence.

• Tropical Revolving Storms (Hurricanes/Typhoons):

  • Main energy source: Latent heat from condensing water vapor.

  • Danger zone: The wall of clouds around the eye.

  • Track: Usually Move West in early stages, then turn North-East.

METEOROLOGICAL HAZARDS

• Icing:

  • Rime Ice: Small supercooled droplets freeze rapidly upon impact; white and opaque.

  • Clear Ice: Large supercooled droplets spreading before freezing; transparent and heavy; most dangerous; found in CB and NS clouds between $0^{\circ}C$ and $-15^{\circ}C$.

  • Hoar Frost: Water vapor turns directly to ice (sublimation) on the aircraft surface.

• Windshear: Vertical or horizontal variation in wind speed/direction. Often found at the top of a surface-based inversion or near thunderstorms.

• Microburst: A concentrated downdraft (< 4\,km diameter) lasting $1-5\,minutes$. Highly dangerous during takeoff and landing.

• Thunderstorms: Requires moisture, conditional instability, and a lifting action. Stages: Cumulus (updrafts), Mature (up/downdrafts and precipitation), Dissipating (downdrafts).

METEOROLOGICAL REPORTS AND CODES

• METAR: Actual weather report, usually issued every half hour.

• TAF: Aerodrome forecast valid for a stated period (e.g., $9$ or $24\,hours$).

• TREND: A short landing forecast (valid for $2\,hours$) added to a METAR.

• SIGMET: Information/warnings regarding significant weather hazardous to all aircraft (e.g., severe turbulence, icing, TS).

• CAVOK: Visibility $\ge 10\,km$; no cloud below $5000\,FT$ or MSA (whichever is higher); no CB/TCU; no significant weather.

• Cloud Coverage Codes:

  • FEW: $1-2$ oktas.

  • SCT (Scattered): $3-4$ oktas.

  • BKN (Broken): $5-7$ oktas.

  • OVC (Overcast): $8$ oktas.