met 4th edition without edits

Meteorology

The science that examines the Earth’s atmosphere and the dynamic processes that shape weather

Water Vapor Concentration

Variable component making up 0% to 4% of air by volume depending on location and weather

Troposphere

The lowest atmospheric layer reaching from the surface to an average altitude of 11 km (36,090 ft), containing over 75% of total atmospheric mass

Lapse Rate

The rate at which temperature typically decreases with altitude within the troposphere

Tropopause

The isothermal boundary between the troposphere and stratosphere where temperature stops decreasing and remains constant

Height of Tropopause (Equator)

Approximately 53,000 ft due to air heating and expansion

Height of Tropopause (Poles)

Approximately 23,000 ft due to cold air density and contraction

Tropopause Breaks

Sudden drops in tropopause height occurring typically around 40° and 60° latitude, which generate jet streams

Stratosphere

Layer extending from the tropopause to approximately 50 km, characterized by a temperature inversion caused by the ozone layer

Stratopause

The boundary above the stratosphere where temperature rises back to approximately 0°C

Aviation Atmosphere

The combined region of the troposphere and the lower stratosphere

Mesosphere

Layer between 50 km and 80-90 km where temperature decreases with height to approximately -90°C

Thermosphere

Outer layer where temperatures rise with altitude, reaching 600°C at 200 km and up to 2000°C during sunspot activity

International Standard Atmosphere (ISA)

A universal baseline defining standard day conditions of temperature, pressure, and density used for performance calculations

ISA MSL Temperature

15°C

ISA MSL Pressure

1013.25 hPa

ISA MSL Density

1225 g/m³

ISA Temperature Lapse Rate (MSL to 11 km)

1.98°C per 1000 ft

ISA Temperature (11 km to 20 km)

Constant at -56.5°C

ISA Temperature Lapse Rate (20 km to 32 km)

Rises at 0.3°C per 1000 ft

ISA Deviation

The difference between actual outside air temperature (OAT) and the standard ISA temperature for a specific altitude

ISA Deviation Formula

ΔISA = T actual - T ISA

Atmospheric Pressure

The force exerted per unit area by the weight of the vertical column of air above a surface

Standard Pressure (mmHg)

760 mmHg

Standard Pressure (inHg)

29.92 inHg

Pressure Unit Conversion (hPa to mmHg)

(Pressure in hPa / 1013.25) * 760

Diurnal Pressure Variation (Temperate)

Typically small, averaging about 1 hPa

Diurnal Pressure Variation (Tropics)

More significant, can be as much as 3 hPa

Pressure at 10,000 ft (FL100)

700 hPa

Pressure at 20,000 ft (FL200)

500 hPa

Pressure at 30,000 ft (FL300)

300 hPa

Pressure at 40,000 ft (FL400)

200 hPa

Rate of Pressure Change (MSL to 10,000 ft)

Drop of approximately 313 hPa

Rate of Pressure Change (30,000 ft to 40,000 ft)

Drop of approximately 100 hPa

Primary Cause of Weather

Uneven heating of the Earth's surface by solar radiation

Pressure Gradient Force

The force driving air from high pressure to low pressure; its steepness determines wind speed

Coriolis Force (Northern Hemisphere)

Apparent force caused by Earth's rotation that deflects moving air to the right

Coriolis Force (Southern Hemisphere)

Apparent force caused by Earth's rotation that deflects moving air to the left

Geostrophic Wind

Wind flowing parallel to isobars when Pressure Gradient Force and Coriolis Force are in balance

High-Pressure Circulation

Sinking air flowing outward in a clockwise spiral in the Northern Hemisphere

Low-Pressure Circulation

Rising air flowing inward in a counter-clockwise spiral in the Northern Hemisphere

QNE

Standard pressure setting of 1013 hPa used for Flight Levels

QFE

Pressure at the aerodrome datum; altimeter reads height above aerodrome and zero on the ground

QNH

QFE reduced to MSL using ISA conditions; altimeter reads altitude above MSL and aerodrome elevation on the ground

QFF

Meteorological sea-level pressure using actual OAT; used for meteorology but never for altimetry

Depression (Low-Pressure Center)

Region of minimum atmospheric pressure characterized by ascending air, surface convergence, and high-altitude divergence

Weather in a Depression

Thick cloud cover, continuous light/moderate precipitation, mild temperatures, and strong winds

Anticyclone (High-Pressure Center)

Region of maximum atmospheric pressure characterized by descending air, surface divergence, and high-altitude convergence

Weather in an Anticyclone

Clear skies, no precipitation, light winds, potential for haze (summer) or fog (winter)

Latent Heat

Heat used to alter the state of a substance without changing its temperature

Saturation

The limit of water vapor air can hold at a given temperature and pressure; warmer air holds more vapor

Evaporation

Liquid to gas phase change; latent heat is absorbed

Condensation

Gas to liquid phase change; latent heat is released

Sublimation

Solid directly to gas phase change; latent heat is absorbed

Deposition

Gas directly to solid phase change; latent heat is released

Relative Humidity

Ratio of actual water vapor content to the maximum capacity (saturation) at a given temperature and pressure

Dewpoint

The temperature to which air must be cooled to become saturated

Air Density

Mass per unit volume (g/m³); decreases as altitude increases because pressure reduction outweighs temperature effects

Density Reduction with Altitude

At 20,000 ft density is 50% of surface; 40,000 ft is 25%; 60,000 ft is 10%

Density Altitude

Pressure altitude corrected for non-standard temperature; differs by ~120 ft per 1°C deviation from ISA

Density Altitude Calculation

Pressure Altitude + (120 x ISA deviation)

Effect of Low Air Density

Reduced engine thrust/power and reduced lift; requires higher TAS or lower aircraft weight for takeoff/landing

Terrestrial Radiation

Main method of atmospheric heating; Earth absorbs solar radiation and retransmits it as long-wave radiation (4-80 microns)

Conduction

Heat transfer through direct contact between the ground and the air layer touching it

Convection

Vertical movement of air caused by heating/cooling; heated air becomes less dense and rises

Advection

Process by which air moves horizontally, carrying temperature and humidity characteristics

Temperature Inversion

An atmospheric layer in which temperature increases with altitude

Environmental Lapse Rate (ELR)

The actual lapse rate of the stationary surrounding air; average value is 1.98°C/1000 ft

Dry Adiabatic Lapse Rate (DALR)

Rate at which unsaturated air cools when rising; constant at 3°C/1000 ft

Saturated Adiabatic Lapse Rate (SALR)

Rate at which saturated air cools when rising; average value is 1.5°C/1000 ft due to latent heat release

Absolute Stability

Condition where ELR is lower than both DALR and SALR; forced rising air remains colder than surroundings and sinks

Absolute Instability

Condition where ELR is greater than both DALR and SALR; forced rising air remains warmer than surroundings and continues to rise

Conditional Stability

Condition where stability depends on saturation; air is stable if unsaturated (DALR > ELR) and unstable if saturated (SALR < ELR)

Neutral Stability (Unsaturated)

Occurs when ELR is exactly 3°C/1000 ft

Cloud Formation

Results from air reaching its dew point, typically through adiabatic cooling of rising air

High-Level Clouds (Height Band)

16,500 ft to 45,000 ft; consists of ice crystals

Cirrus (CI)

Thin wispy high cloud; no icing or precipitation; found 400-600 nm ahead of a warm front

Cirrostratus (CS)

Sheet-like high cloud producing the halo phenomenon; associated with warm fronts

Medium-Level Clouds (Height Band)

6,500 ft to 23,000 ft; includes Altocumulus and Altostratus

Altocumulus Castellanus (ACC)

Medium cloud with castle turret appearance indicating mid-level instability and potential CB formation

Altocumulus Lenticularis (ACSL)

Lens-shaped cloud associated with mountain waves; icing can be severe

Low-Level Clouds (Height Band)

Surface to 6,500 ft

Stratus (ST)

Layer cloud with base often below 1000 ft; depth 1000-1500 ft; can produce drizzle or snow grains

Nimbostratus (NS)

Thick, dark layer cloud producing moderate to severe icing/turbulence and continuous precipitation

Cumulonimbus (CB)

Towering instability cloud with anvil top; hazardous with severe turbulence, icing, hail, and thunderstorms

Convection Lifting

Surface heating causes air to rise; forms cumuliform clouds

Orographic Uplift

Air forced to rise over high ground

Turbulence Cloud Formation

Requires a stable layer (inversion/isothermal) and wind speeds greater than 10 kt

Air Mass

Large volume of air with uniform temperature and humidity; forms in stable high-pressure regions

Tropical Air Mass

Originates in warm, sub-tropical regions

Maritime Air Mass

Forms over oceans; characterized by high moisture content

Air Mass Modification (Warmer Surface)

Air becomes warmer, more unstable, and relative humidity reduces

Front

The boundary zone separating two different air masses

Indicator of Frontal Passage (NH)

The wind will always shift (veer) to the right

Warm Front

Warm air replaces cold air along a gentle slope; produces CI -> CS -> AS -> NS cloud sequence

Warm Front Precipitation

Starts as virga from AS, becoming widespread continuous moderate/heavy rain or snow from NS

Cold Front

Cold air replaces warm air by undercutting it; moves at geostrophic wind speed

Cold Front Weather

Rapid unstable lift producing vertically developed CU and CB clouds; severe but moves rapidly

Occlusion

Fast-moving cold front catches a slow-moving warm front

Warm Occlusion

Air ahead of the warm front is colder than air within the cold front; cold front rides up over warm front