NCAA PPL Meteorology key terms

A set of average sea-level values used for aircraft and altimeter calibration: 29.92 inHg (1013.25 hPa), 15 °C, and a lapse rate of about 2 °C per 1,000 ft. It gives a common reference so pressure readings can be converted to altitude.

The sea-level pressure value dialed into an altimeter so it shows field elevation when the aircraft is on the ground. It comes from a nearby station or ATIS and must be updated as pressure changes or when flying between distant areas.

The altitude shown when the altimeter is set to 29.92 inHg. It is the basis for aircraft performance charts and is found by adding or subtracting the difference between current pressure and standard pressure to the indicated altitude.

Pressure altitude corrected for nonstandard temperature. High density altitude means thin air, which lengthens takeoff rolls, reduces climb rates, and lowers engine and propeller performance. It is the figure to check before operating on hot days or at high elevations.

The actual atmospheric pressure measured at a given location, without adjustment to sea level. It falls as elevation increases, so mountain stations report much lower values than coastal airports at the same weather conditions.

The speed shown on the airspeed indicator, driven by the difference between pitot (ram) pressure and static pressure. It is what the pilot flies by for stall margins and aircraft limitations, though it differs from true airspeed as altitude and temperature change.

The actual speed of the aircraft through the surrounding air mass. It is higher than indicated airspeed at altitude because thinner air produces less ram pressure; roughly, it increases about 2 percent per 1,000 feet of altitude.

The aircraft plumbing that supplies ram air pressure from the pitot tube and ambient (static) pressure from the static port to the airspeed indicator, altimeter, and vertical speed indicator. Blockages of either source cause specific, recognizable instrument errors.

The rate at which temperature decreases with increasing height in the atmosphere. The standard average is about 2 °C per 1,000 feet in the troposphere; actual rates vary and are steeper in unstable air and shallower in stable or inverted layers.

A layer where temperature increases with height instead of decreasing. Inversions trap moisture, smoke, and haze beneath them, produce smooth air but often poor visibility, and can create wind shear at their boundaries.

The tendency of an air parcel to continue rising or to settle back after being displaced. Warm air below cold air is unstable and favors cumuliform clouds, turbulence, and showers; cold air below warm air is stable and favors smooth air and layered clouds.

The drop in temperature that occurs as a parcel of air expands while rising into lower pressure, without any heat being added or removed. It explains why rising air eventually condenses into clouds and why mountain slopes and updrafts cool so quickly.

The temperature to which air must be cooled, at constant pressure and moisture content, to become saturated. The smaller the spread between temperature and dew point, the higher the relative humidity and the more likely fog, low clouds, and precipitation become.

The amount of water vapor in the air expressed as a percentage of the maximum the air could hold at that temperature. Warm air can hold more moisture, so a given vapor content yields a lower percentage as temperature rises.

The condition in which air holds all the water vapor it can at a given temperature and pressure. Further cooling or added moisture then forces condensation onto nuclei, forming cloud droplets, fog, or precipitation.

The change of water vapor into liquid water, occurring when saturated air cools further. In the atmosphere it forms cloud droplets and fog, usually on tiny particles called condensation nuclei; the release of latent heat during condensation helps fuel thunderstorms.

Energy absorbed or released when water changes phase, without changing temperature. Evaporation absorbs heat from the surface; condensation and freezing release heat into the surrounding air, which can strengthen updrafts and slow the cooling of a rising parcel.

A visible body of water droplets or ice crystals suspended in the atmosphere, formed by cooling of moist air to saturation. Cloud type reveals stability: layered stratus suggests stable air, while towering cumulus and cumulonimbus suggest instability and convective activity.

A massive, vertically developed cloud with a flat anvil top, associated with thunderstorms, heavy rain, hail, lightning, and severe turbulence. It is one of the most hazardous clouds to general aviation and should be avoided by a wide margin.

The height above ground of the lowest broken or overcast cloud layer, or of an obscuring phenomenon such as fog. It is a key element of flight category and instrument approach minimums, and its absence means unlimited ceiling.

A cloud in contact with the ground that reduces visibility below one statute mile. It forms when air cools to its dew point or moisture is added, and is common with radiation cooling on clear calm nights and with warm moist air over cold surfaces.

A classification of current conditions by ceiling and visibility, used in aviation weather products: VFR, MVFR, IFR, and LIFR. It gives pilots a quick sense of whether visual or instrument conditions prevail at a station.

The greatest distance at which prominent objects can be seen and identified. In aviation weather it is reported in statute miles and is reduced by fog, haze, smoke, precipitation, and blowing dust or snow, directly affecting VFR legality.

A routine coded surface weather observation issued at least hourly, giving wind, visibility, clouds, temperature, dew point, and altimeter setting. Pilots decode it to confirm conditions at departure, destination, and alternate airports.

A terminal aerodrome forecast covering expected conditions at an airport, typically for 24 to 30 hours. It includes prevailing conditions plus temporary and probability groups for wind, visibility, clouds, and precipitation, and is used for flight planning.

A change in wind speed or direction over a short distance, either horizontally or vertically. It can occur with fronts, inversions, thunderstorms, and terrain, and at low altitude it is a serious threat during takeoff and approach.

A localized downdraft from a thunderstorm that spreads out horizontally, producing a sudden headwind followed by a strong downdraft and tailwind near the surface. It can cause a fatal loss of airspeed and altitude on approach or just after takeoff.

Wind shear occurring below about 2,000 feet above ground, often reported as a nonconvective alert in terminal forecasts. It is especially dangerous because there is little altitude to recover from an abrupt airspeed change.

A convective storm produced by a cumulonimbus cloud, with three stages: cumulus, mature, and dissipating. Hazards include turbulence, hail, lightning, icing, microbursts, and tornadoes; the mature stage is the most violent.

Vertical air motions caused by uneven surface heating, producing rising columns of warm air and descending cooler air. Pilots feel them as turbulence and lift; they also form cumulus clouds and are the engine of thunderstorm development.

Irregular, gusty air motion that bumps the aircraft, caused by convection, wind flow over terrain, fronts, or wind shear. It is described as light, moderate, severe, or extreme and is a common cause of injury in light aircraft.

A standing wave in the airflow downstream of a mountain ridge, often marked by lenticular clouds and strong vertical currents on the lee side. It can produce severe turbulence, downdrafts, and altimeter errors near the ridge.

Forced rising of air as it is pushed up a sloping terrain feature. It cools the air adiabatically, often forming clouds and precipitation on the windward side and drier conditions downwind.

Air moving up a gentle land slope, such as a coastal plain, which cools and frequently produces low stratus, fog, or steady precipitation. It is a common cause of poor flying conditions on the windward side of a front.

A thunderstorm formed by local surface heating and instability rather than a front or strong dynamic forcing. These typically develop in the afternoon, are scattered and short-lived, and dissipate after sunset when heating ends.

A line of active thunderstorms, often ahead of or along a cold front, that can extend for hundreds of miles. It produces strong gusty winds, heavy rain, hail, and severe turbulence, and should be avoided rather than crossed.

The leading edge of a cooler air mass advancing under warmer air. It lifts the warm air steeply, producing cumuliform clouds, showers, thunderstorms, and a wind shift with a temperature drop as it passes.

The boundary where a warm air mass rides up over a retreating cooler air mass. It brings a gradual sequence of layered clouds, lowering ceilings, and steady precipitation, with poor visibility and low ceilings well behind the surface front.

The accumulation of ice on aircraft surfaces when flying through supercooled water droplets. It adds weight, reduces lift, and can block pitot tubes and control surfaces; structural icing is most likely in visible moisture between 0 °C and about −20 °C.

Precipitation that falls as liquid but freezes on contact with surfaces at or below freezing. In flight it produces rapid, severe airframe icing, making it one of the most dangerous conditions a pilot can encounter.