Weather

Question 1

Convective SIGMET (WST)

Answer 1

Issued hourly at 55 minutes past the hour
Valid for 2 hours

Contains either an observation and a forecast or only a forecast

Convective SIGMETs are issued for any of the following:

o Severe thunderstorms due to:
i. Surface winds greater or equal to 50 knots
ii. Hail at the surface greater than 3/4 inch in diameter
o Tornadoes
o Embedded thunderstorms
o A line of thunderstorms at least 60 miles long affecting 40% of its length
o Thunderstorms producing heavy or greater precipitation affecting more than 40% of an area of at least 3000 square miles. o

Convective SIGMETs always implies severe or greater turbulence, severe icing, or low level wind shear.

Question 2

SIGMET (WS)

Answer 2

A non-scheduled inflight advisory
maximum forecast period of 4 hours

Advises of non-convective weather potentially hazardous to all types of aircraft

A SIGMET is issued when the following is expected to occur:

o Severe icing not associated with thunderstorms
o Severe or extreme turbulence or Clear Air Turbulence (CAT) not associated with thunderstorms.
o Dust storms, sandstorms lowering surface visibility below 3 miles.
o Volcanic ash

Question 3

AIRMET (WA)

Answer 3

Valid for 6 hours.

An advisory of significant weather phenomena at lower intensities than those which require the issuance of SIGMETs

o AIRMET (T) - describes moderate turbulence, sustained surface winds of 30 knots or greater, and/or non-convective low-level wind shear. o AIRMET (Z) - describes moderate icing and provides freezing level heights.
o AIRMET (S) - describes IFR conditions and/or extensive mountain obscurations.
o Graphical AIRMETs (AIRMET G) – found at http:// aviationweather.gov

Question 4

METAR

Answer 4

Scheduled METARs are published every hour.

METARS (SPECI) are issued when there is a significant change in one or more reported element since the last scheduled METAR.

Aviation routine weather show surface weather observations in a standard international format.

Question 5

TAF

Answer 5

Terminal Aerodrome Forecast. Weather forecast for 5SM radius area around the station. Issued 4 times a day, every six hours and normally cover a 24
or 30 hour forecast period. TAF amendments (TAF AMD) supersede the previous TAF.

Question 6

Aviation Area Forecast (FA)

Answer 6

A forecast of weather condition over an area of several states. When there isn’t a TAF available for your route, check the FAs
together with SIGMETs, AIRMETs and other information. Area forecasts are issued 3 times a day (or 4 times for the Caribbean, Alaska and Hawaii regions

Question 7

Surface analysis chart

Answer 7

Generated from surface station reports. Shows pressure systems, isobars, fronts, airmass boundaries (such as: drylines and outflow
boundaries) and station information (such as: wind, temperature/dew point, sky coverage, and precipitation). Issued every 3 hours. (Hawaii, tropical and Oceanic
regions every 6 hours)

Question 8

Weather depiction chart

Answer 8

Just like the surface analysis chart, it is generated from surface station observations.

Depicts areas of VFR (at least 3000’ ceiling and 5SM visibility), Marginal VFR (1000’-3000’ ceiling and/or 3-5SM visibility, shown as contoured areas) and IFR (less than 1000’ ceiling and/or 3SM visibility, shown as shaded areas). It also shows basic METAR information at selected stations (visibility, sky coverage, ceilings and obstructions to visibility).

Issued every 3 hours, starting at 01:00Z.

Question 9

Radar summary chart (SD)

Answer 9

Depicts precipitation type, intensity, coverage, movement, echoes, and maximum tops.

Issued hourly

Question 10

Wind & temp aloft forecasts (FB)

Answer 10

Issued 4 times daily for different altitudes and flight levels.

Winds within 1500’ AGL and temperatures within 2500’ AGL
are not shown.

Format: DDff±tt where DD=wind direction; ff=wind speed tt=temperature.

Light and variable winds: 9900.

Winds between 100-199 Kt are coded by adding 5 to the first digit of the wind direction.

Examples:1312+05 = winds 130 at 12 kt temperature =5° C. 7525-02 = winds 250 at 122 kt temperature -02° C.

Above FL240 temperatures are negative and the minus sign (-) is omitted.

Question 11

Convective outlook (AC)

Answer 11

Available in both graphical and textual format. A 3-day forecast of convective activity. Convective areas are classified as slight (SLGT), moderate (MDT), and high (HIGH) risk for severe thunderstorms.

Issuance:
day 1 – 5 times a day,
day 2 – twice a day,
day 3 – once a day.

Question 12

Low level significant weather chart

Answer 12

Forecasts significant weather conditions for a 12 and 24 hour period from the surface to 400 mb level (24,000 ft). Issued 4 times a day. Depicts weather categories (IFR, MVFR and VFR), turbulence and freezing levels.

Question 13

Mid-level significant weather chart

Answer 13

Depict forecasts of significant weather at various altitudes and flight levels from 10,000’ MSL.to FL450. Shows: thunderstorms, jet streams, tropopause height, tropical cyclones, moderate and severe icing conditions, moderate or severe turbulence, cloud coverage and type, volcanic ash and areas of released radioactive materials. Issued 4 times a day for the North Atlantic Region.

Question 14

High-level significant weather charts

Answer 14

Depicts forecasts of significant weather phenomena for FL250 to FL630. Shows: coverage bases and tops of thunderstorms and CB clouds, moderate and severe turbulence, jet streams, tropopause heights, tropical cyclones, severe squall lines, volcanic eruption sites, widespread sand and dust storms.

Issued 4 times a day.

Question 15

Conditions necessary for the formation of thunderstorms

Answer 15

1. Sufficient water vapor (humidity)
2. An unstable temperature lapse rate
3. An initial uplifting force (such as: front passage, mountains, heating from below, etc.)

Question 16

Thunderstorm hazards -

Answer 16

Limited visibility
wind shear
strong updrafts and downdrafts
icing
hailstones
heavy rain
severe turbulence
lightning strikes 
tornadoes.

Question 17

Life cycle of a thunderstorm

Answer 17

Cumulus stage (3-5 mile height) – lifting action of the air begins. Growth rate may exceed 3000 fpm.

Mature stage (5-10 miles height) – begins when precipitation has become to fall from the cloud base. Updraft at this stage may exceed 6000 fpm. Downdrafts may exceed 2500 fpm. All thunderstorm hazards are at their greatest intensity at the mature stage.

Dissipating stage (5-7 miles height) – characterized by strong downdrafts and the cell is dying rapidly.

Question 18

Fog

Answer 18

A cloud that begins within 50 ft of the surface. Occurs when the air temperature near the ground reaches its dew point, or when the dew point is raised to the existing temperature by added moisture to the air.
Radiation fog – Occurs at calm clear nights when the ground cools rapidly due to the release of ground radiation.

Advection fog – warm, moist air moves over a cold surface. Winds are required for advection fog to form.

Ice fog – Forms when the temperature is much below freezing and water vapor turns directly into ice crystals. Common in the arctic regions but also occurs in
mid-latitudes.

Upslope fog – moist, stable air is forced up a terrain slope and cooled down to its dew point by adiabatic cooling.

Steam fog – Cold, dry air moves over warm water. Moisture is added to the airmass and steam fog forms.

Question 19

Icing (AC 91-74, AC 00-6A)

Answer 19

Structural ice – Two conditions for formation: 1.Visible moisture (clouds, fog, precipitation)
2. Aircraft surface temperature below freezing.

Clear ice– most dangerous type. Heavy, hard and difficult to remove. Forms when water drops freeze slowly as a smooth sheet of solid ice. Usually occurs at temperatures close to the freezing point (-10° to 0° C) by large supercooled drops of water

Rime ice – Opaque, white, rough ice formed by small supercooled water drops freezing quickly. Occurs at lower temperatures then clear ice does.
o Mixed ice – Clear and rime ice formed simultaneously.

Instrument ice – structural ice forming over aircraft instruments and sensors, such as pitot and static.

Induction ice – ice reducing the amount of air for the engine intake.

o Intake ice – Blocks the engine intake.

o Carburetor ice – May form due to the steep temperature drop in the carburetor venturi.

Typical conditions are outside air temperatures of -7° to 21° C and a
high relative humidity (above 80%).

Frost – Ice crystals caused by sublimation when both the temperature and the dew point are below freezing.