PCAA Meteorology flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Standard atmosphere sea level

ISA defines 1013.25 hPa pressure, 15 °C temperature and 1.225 kg/m³ density at mean sea level, with a lapse rate of 1.98 °C per 1000 ft up to 11 km.

Tropopause height and temperature

In ISA the tropopause lies at 11 km (36,090 ft) with a temperature of -56.5 °C; above it the lower stratosphere is isothermal before temperature rises due to ozone.

Density altitude formula

Density altitude equals pressure altitude plus 120 ft for every degree Celsius by which the actual temperature exceeds ISA temperature at that level; high values degrade performance.

Dry adiabatic lapse rate

Unsaturated air cools by 3 °C per 1000 ft (9.8 °C/km) when lifted adiabatically; this rate is constant with height and used to compare with the environmental lapse rate.

Conditional instability

An environmental lapse rate between the saturated and dry adiabatic rates means the air is stable for unsaturated parcels but unstable for saturated ones, favouring deep convective cloud.

Lifting condensation level estimate

Cloud base height is approximately 400 ft per degree Celsius of surface temperature-dewpoint spread; a 5 °C spread gives a base near 2000 ft AGL.

QNH versus QFE

QNH set on the subscale makes the altimeter indicate elevation above mean sea level; QFE makes it indicate height above the aerodrome reference point.

Transition altitude and level

Transition altitude is the height where climb changes from QNH to standard 1013.25 hPa; transition level is where descent changes back to QNH; the gap is the transition layer.

Cold temperature altimeter error

Flying in air colder than ISA causes the altimeter to over-read, so true altitude is lower than indicated, roughly 4% per 10 °C of ISA deviation.

QNE and flight levels

QNE is the standard altimeter setting 1013.25 hPa (29.92 inHg); the altitude indicated with this setting is a flight level such as FL350.

Geostrophic wind balance

Above the friction layer, wind blows parallel to straight isobars where the pressure gradient force balances the Coriolis force; closer isobars mean stronger wind.

Buys Ballot's law

In the Northern Hemisphere, standing with your back to the wind puts low pressure on your left and high pressure on your right; used to locate pressure systems.

Sea breeze mechanism

Daytime heating of land makes it warmer than the sea; air rises over land and cooler air flows from sea to land, strongest in the afternoon.

Katabatic wind

Cold dense air flows downslope at night under gravity, often pooling in valleys and producing low-level wind shear and cold-air fog pockets.

Radiation fog conditions

Forms by nocturnal cooling under clear skies with light wind and a dewpoint spread near 2 °C or less; typically dissipates after sunrise as the surface warms.

Advection fog

Warm moist air moving over a colder surface cools to saturation; it can persist day and night with winds of about 5-15 kt and is common over coastal waters.

Cumuliform versus stratiform cloud

Cumuliform clouds with strong vertical development indicate instability and give showers; layered stratiform clouds indicate stability and give steady, widespread precipitation.

Cold front passage signs

Steep slope, fast movement, narrow band of heavy showers, gusty veering wind, pressure rising after passage, temperature falling and visibility improving behind the front.

Warm front characteristics

Shallow slope, slow movement, wide band of steady rain, veering wind, pressure falling then steady, poor visibility in precipitation, and possible frontal fog ahead.

Occluded front

A cold front overtakes a warm front and lifts the warm sector off the ground; weather is mixed, with rain, low cloud and poor visibility near the triple point.

Squall line

A line of active thunderstorms often ahead of a cold front, producing severe gusts, hail, heavy rain and strong wind shear along its length.

Structural icing temperature range

Airframe icing requires visible moisture and temperatures between 0 °C and -20 °C, with the most rapid accumulation typically between -2 °C and -10 °C.

Clear versus rime ice

Clear ice forms from large supercooled droplets in cumuliform cloud at 0 to -10 °C; rime ice forms from small droplets in stratiform cloud at -10 to -20 °C.

Microburst hazard

A downdraft under 4 km across lasting 5-15 minutes that can cause a sudden 30-90 kt headwind loss, making it lethal during approach and departure.

Clear air turbulence location

CAT is most frequent near jet streams, especially on the cold side of the core, near tropopause folds and in mountain wave regions; it is not visible on radar.

METAR cloud amount codes

FEW means 1-2 oktas, SCT 3-4, BKN 5-7 and OVC 8; a ceiling is defined by the lowest broken or overcast layer.

TAF change group codes

BECMG indicates a gradual change, TEMPO a temporary fluctuation, and FM a rapid change beginning at a specified time; TAF validity is usually 24-30 hours.

SIGMET validity

SIGMETs warn of severe turbulence, severe icing, mountain waves, volcanic ash and tropical cyclones; they are valid for 4 hours, or 6 hours for volcanic ash.

Upper air chart levels

Standard constant-pressure charts are drawn for 850, 700, 500, 300 and 250 hPa, corresponding roughly to 5000, 10,000, 18,000, 30,000 and 34,000 ft.

Thickness chart use

The 1000-500 hPa thickness is proportional to mean layer temperature; low thickness indicates cold air, a lower snow level and possible icing at lower altitudes.

Western disturbances in Pakistan

Winter low-pressure systems from the Mediterranean and Caspian bring rain and snow to northern and western Pakistan, and can cause fog and low cloud in Punjab.

Pakistan monsoon season

From July to September, moist southwesterly flow from the Arabian Sea and Bay of Bengal brings heavy rain, with the heaviest falls on windward Himalayan slopes, causing flooding.