CAA NZ PPL Meteorology flashcards

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

ISA sea-level values

International Standard Atmosphere at mean sea level: temperature 15 °C, pressure 1013.25 hPa, density 1.225 kg/m³, with a temperature lapse rate of 1.98 °C per 1000 ft up to the tropopause at 36,090 ft.

QNH versus QFE

QNH is the altimeter subscale setting that makes the altimeter read elevation above mean sea level on the ground. QFE makes it read height above the aerodrome reference point.

Density altitude

Pressure altitude corrected for non-standard temperature. It increases with high temperature, high elevation and low pressure, degrading takeoff, climb and landing performance.

Pressure lapse rate

In the lower atmosphere pressure decreases by about 1 hPa for every 30 ft of ascent near sea level. This converts altimeter setting errors into altitude errors.

Coriolis force

Apparent force caused by the Earth's rotation that deflects moving air to the left in the Southern Hemisphere and to the right in the Northern Hemisphere, producing geostrophic flow aloft.

Buys Ballot's law

In the Southern Hemisphere, stand with your back to the wind and low pressure lies on your right. This helps locate depressions and anticyclones from surface wind.

Southern Hemisphere circulation

Air circulates clockwise around a low pressure centre and anticlockwise around a high pressure centre in the Southern Hemisphere, with surface friction causing inflow toward the low.

Dry adiabatic lapse rate

The rate at which a rising unsaturated air parcel cools, about 3 °C per 1000 ft. It is used to assess stability and convective cloud development.

Saturated adiabatic lapse rate

The rate at which a rising saturated air parcel cools, about 1.5 °C per 1000 ft. It is less than the dry rate because latent heat is released on condensation.

Convective cloud base

Estimated by multiplying the surface temperature-dewpoint spread in °C by 400 ft. A spread of 5 °C gives a convective cloud base near 2000 ft above ground.

Radiation fog

Forms on clear, calm nights when the surface cools by radiation and the air reaches its dewpoint. It usually dissipates after sunrise as the surface warms.

Advection fog

Forms when warm moist air moves over a colder surface and is cooled below its dewpoint. It can persist through the day and is common over cold sea areas.

Carburettor icing conditions

Can occur at outside air temperatures up to about 20 °C when humidity is high, because fuel vaporisation and venturi cooling drop the temperature below freezing inside the carburettor.

Clear versus rime ice

Clear ice forms from large supercooled droplets and is heavy and irregular; rime ice forms from small droplets and is rough and milky. Both occur between 0 °C and about −20 °C.

Cold front characteristics

A fast-moving boundary with a steep slope, often producing a narrow band of heavy showers, squall lines and a sharp wind shift with temperature fall behind it.

Warm front characteristics

A slowly moving boundary with a gentle slope, producing a wide band of stratiform cloud and prolonged steady rain ahead of the surface front.

METAR element order

Wind, visibility, present weather, cloud, temperature and dewpoint, QNH, then trend. CAVOK replaces visibility, weather and cloud when conditions meet the criteria.

CAVOK criteria

Reported when visibility is 10 km or more, there is no cloud below 5000 ft or below the highest minimum sector altitude, no cumulonimbus, and no significant weather.

TAF change groups

FM indicates a rapid change from a stated time, BECMG a gradual change over a period, TEMPO temporary fluctuations, and PROB a percentage probability of occurrence.

SPECI report

A special aerodrome weather report issued when conditions change significantly, such as visibility crossing a threshold, cloud base change, or onset of thunderstorms.

SIGMET purpose

Warns of significant en-route weather hazardous to aircraft, including severe turbulence, severe icing, volcanic ash, tropical cyclones and severe squall lines.

Cloud amount in eighths

FEW is 1-2 oktas, SCT is 3-4, BKN is 5-7 and OVC is 8. Heights in reports are given in feet above aerodrome level.

Wind gust notation

A gust is shown after the mean wind speed with a G, for example 27015G30KT. It indicates a peak wind significantly above the mean.

Crosswind component formula

Crosswind equals wind speed multiplied by the sine of the angle between the wind direction and the runway heading. Headwind uses the cosine of the same angle.

Föhn effect

Air that loses moisture rising over a range warms at the dry adiabatic rate as it descends the lee side, producing warmer, drier and often clearer conditions downwind.

Mountain wave signs

Stable air crossing a ridge with increasing wind with height can produce lenticular cloud, rotor turbulence and strong downslope winds on the lee side.

Sea breeze formation

Daytime heating of land causes air to rise and cooler air to flow onshore from the sea, producing a coastal wind shift and sometimes a sea breeze front.

Tropical cyclone structure

A warm-core low with a calm eye, a violent eyewall of strongest winds and spiral rainbands. In the Southern Hemisphere circulation is clockwise.

Volcanic ash hazard

Ash damages engines, abrades windscreens and reduces visibility. SIGMET and NOTAM provide warnings; avoid the ash cloud entirely and report any encounter.

Freezing level

The height of the 0 °C isotherm. It indicates where airframe icing becomes possible in visible moisture and is forecast on upper-level and area charts.

Anticyclone weather

A high pressure system with subsiding air, light winds and often clear skies, but an inversion can trap haze, smoke or fog near the surface.

VFR weather decision rule

If actual or forecast conditions trend below the visibility and cloud clearance minima for the airspace class, divert or hold rather than continuing into deteriorating weather.