Aviation Weather Information (AC 00-45G & AC 00-6A)

Standard sea level temperature and pressure

15°C (59°F) and 29.92" Hg (1013.2 mb)

What do closely spaced isobars indicate?

A strong pressure gradient and strong winds.

How do surface winds flow relative to isobars?

At an angle to the isobars due to surface friction.

How do winds aloft flow relative to isobars?

More or less parallel to the isobars due to Coriolis force.

Weather characteristics of stable air

Stratiform clouds, poor visibility, smooth air, steady/continuous precipitation.

Weather characteristics of unstable air

Cumuliform clouds, good visibility, turbulence, showery precipitation.

General characteristics of a low pressure area

Cyclone: counterclockwise and rising air.

General characteristics of a high pressure area

Anti-cyclone: clockwise and descending air.

What is a ridge?

An extended area of high pressure with descending air.

What is a trough?

An extended area of low pressure with rising air.

Three requirements for a thunderstorm to form

Unstable atmosphere, sufficient moisture, and a lifting action.

How is an unstable atmosphere identified via lapse rate?

Ambient lapse rate is greater than the standard 2°C per 1000'.

How is moisture content evaluated for thunderstorms?

By the temperature/dew point spread; closer spread means closer to saturation.

Examples of thunderstorm lifting actions

Heating from below (thermals), orographic lifting, or frontal lifting.

Cumulus stage of a thunderstorm

The building stage characterized by updrafts only.

Mature stage of a thunderstorm

Characterized by updrafts, downdrafts, violent turbulence, and the start of rain.

Dissipating stage of a thunderstorm

Characterized by downdrafts only, and often a recognizable anvil top.

What does a thunderstorm's anvil top indicate?

The direction in which the storm is moving.

What is wind shear and why is it hazardous?

Rapid change in wind direction/velocity; causes airspeed changes and severe turbulence.

What is the dew point?

The temperature at which the air becomes saturated.

Advection fog

Forms when moist, warm air moves over colder land or water.

Radiation fog

Forms on clear nights with little to no wind, only over land.

Upslope fog

Forms as moist, unstable air is cooled as wind pushes it up a slope.

Precipitation-induced fog

Forms when warm rain falls through cool air, evaporating and saturating it.

Induction icing

Ice blocking air intake manifold; includes carburetor ice.

Instrument icing

Icing of pitot tube or static ports causing inaccurate instrument readings.

Structural icing

Ice forming on the aircraft surface (propeller, tail, wings).

Rime ice

Small droplets freeze on impact; traps air making it white/opaque.

Clear ice

Large droplets freeze slowly, sliding back; smooth, glossy, and hard to see.

Mixed ice

Forms when water droplets vary in size.

Standard weather briefing

Request when planning a flight with no previous weather data gathered.

Abbreviated weather briefing

Used to supplement mass data or update a previous briefing.

Outlook weather briefing

Request when proposed departure time is 6 or more hours away.

What is EFAS (Flight Watch)?

Enroute Flight Advisory Service; provides up-to-date enroute weather advisories.

What is a METAR?

Aviation Routine Weather Report; issued hourly, valid for the hour.

What is a TAF?

Terminal Aerodrome Forecast; issued 4 times daily, valid 24-30 hours.

Area of coverage for a TAF

Within a 5 statute mile (sm) radius of the airport.

Aviation definition of a ceiling

The height above ground of a broken or overcast layer.

Does a TAF report cloud ceilings in MSL or AGL?

AGL (Note: Source text incorrectly says MSL).

TAF terms: FROM vs BECMG

FROM is rapid change (<1 hr); BECMG is gradual change over 1 hr.

TAF terms: TEMPO vs PROB

TEMPO means temporary (<1 hr); PROB indicates probability percentage.

Four sections of an Area Forecast (FA)

Header, Synopsis, VFR Clouds/Weather, and Outlook.

Does an Area Forecast report ceilings in MSL or AGL?

MSL, unless preceded by AGL or CIG (ceiling).

AIRMET Sierra

Issued for mountain obscuration and widespread IFR conditions.

AIRMET Tango

Issued for moderate turbulence and surface winds >30 knots.

AIRMET Zulu

Issued for moderate icing.

SIGMET (Significant Weather Information)

Issued for severe turbulence, widespread dust/volcanic ash, and severe icing.

Convective SIGMET

Issued for thunderstorm activity, winds >50 kts, or hail >3/4".

Winds and Temperatures Aloft Forecast (FB)

Issued twice daily (every 12 hours) for specific altitudes.

How are temperatures above 24,000 feet identified on an FB?

They are negative (the minus sign is omitted).

Decode FB entry '710556' at 30,000 feet

Winds 210° at 105 knots, temperature -56°C.

Altitude limits for winds/temps forecasts on FB

No winds within 1500' of station; no temps within 2500'.

Surface Analysis Chart

Issued 8 times daily; shows pressure systems, fronts, and local weather.

Weather Depiction Chart

Issued 8 times daily; graphical depiction of METAR surface conditions.

Significant Weather Prognostic Chart

Forecast of hazards (icing, freezing levels, turbulence) issued 4 times daily.

Radar Summary Chart

Issued hourly (35 past); displays precipitation areas and echo tops.