Chapter 2


A. Weather Briefing Services

1. What is the primary means of obtaining a weather briefing? (AIM 7-1-2)

The primary source of preflight weather briefings is an individual briefing obtained from a briefer at the AFSS/FSS. These briefings, which are tailored to your specific flight, are available 24 hours a day through the use of the toll free number (1-800-WX BRIEF).

2. What are some examples of other sources of weather information? (AIM 7-1-2)

a. Telephone Information Briefing Service (TIBS) (AFSS)
b. Transcribed Weather Broadcasts (TWEB)
c. Telephone Access to TWEB (TEL-TWEB)
d. Weather and aeronautical information from numerous private industry sources
e. The Direct User Access Terminal System (DUATS)

3. Where can you find a listing of FSS and weather information numbers? (AIM 7-1-2)

Numbers for these services can be found in the Airport/Facility Directory under “FAA and NWS Telephone Numbers” section. They are also listed in the U.S. Government section of the local telephone directory.

4. What type of weather briefings are available from an FSS briefer? (AIM 7-1-4)

Standard Briefing – Request anytime you are planning a flight and you have not received a previous briefing or have not received preliminary information through mass-dissemination media; e.g., TIBS, TWEB, etc.

Abbreviated Briefing – Request when you need information to supplement mass-disseminated data, update a previous briefing, or when you need only one or two items.

Outlook Briefing – Request whenever your proposed time of departure is six or more hours from the time of the briefing. This is for planning purposes only.

In flight Briefing – Request when needed to update a preflight briefing.

5. What pertinent information should a weather briefing include?

a. Adverse Conditions
b. VFR Flight Not Recommended
c. Synopsis
d. Current Conditions
e. En route Forecast
f. Destination Forecast
g. Winds Aloft
h. Notices to Airmen
i. ATC Delay
j. Pilots may obtain the following from AFSS/FSS briefers upon request: Information on Special Use Airspace (Alert Areas, MOAs, Prohibited Areas, Restricted Areas, Military Training Routes, etc.), a review of printed NOTAMs, approximate density altitude information, information on air traffic services and rules, customs/immigration procedures, ADIZ rules, search and rescue, LORAN-C NOTAMs, GPS RAIM availability, and other assistance as required.

6. What is EFAS? (AIM 7-1-5)

En route Flight Advisory Service is specifically designed to provide en route aircraft with timely and meaningful weather advisories pertinent to the type of flight intended, route of flight and altitude. EFAS is also a central collection and distribution point for pilot-reported weather information (PIREPs). EFAS provides communications capabilities for aircraft flying at 5,000 feet AGL to 17,500 feet MSL on a common frequency of 122.0 MHz. Also known as “Flight Watch”.

7. What is HIWAS? (AIM 7-1-10)

Hazardous In-flight Weather Advisory Service (HIWAS) is a continuous broadcast of in-flight weather advisories including summarized Aviation Weather Warnings, SIGMETs, Convective SIGMETs, Center Weather Advisories, AIRMETs, and urgent PIREPs. HIWAS is an additional source of hazardous weather information which makes this data available on a continuous basis. NAVAIDs with HIWAS capability are depicted on sectionals by an “H” in the upper right corner of the identification box.

8. What is ATIS? (AIM 4-1-13)

Automatic Terminal Information Service (ATIS) is the continuous broadcast of recorded non-control information in selected high activity terminal areas. Its purpose is to improve controller effectiveness and to relieve frequency congestion by automating the repetitive transmission of essential but routine information. The information is continuously broadcast over a discrete VHF radio frequency or the voice portion of a local NAVAID, and updated upon the receipt of any official hourly and special weather.

9. What type of information is provided in an ATIS broadcast? (AIM 4-1-13)

Information includes the time of the latest weather sequence, ceiling, visibility, obstruction to visibility, temperature, dew point (if available), wind direction (magnetic), and velocity, altimeter, other pertinent remarks, instrument approach and runway in use.

B. Weather Reports and Forecasts

1. What is a METAR? (AC 00-45E)

A METAR is an aviation routine weather report ; a weather observer’s interpretation of the weather conditions at a given site and time. METARs contain the following elements in this order: Type of report, ICAO station identifier, date and time of report, modifier (as required), wind, visibility, runway visual range (RVR) (as required), weather phenomena, sky condition, temperature/dew point group, altimeter and remarks (RMK) (as required).

R35L/4500V6000FT-RA BR BKN030 10/10 A2900 RMK AO2

2. Describe the various types of weather observing programs now in use. (AIM 7-1-12)

a. Manual Observations – Reports made from airport locations staffed by FAA or NES personnel.
b. AWOS – Automated Weather Observing System; consists of various sensors, a processor, a computer-generated voice sub-system, and a transmitter to broadcast local, minute-by-minute weather data directly to the pilot. Observations will include the prefix AUTO in data.
c. AWOS Broadcasts – In addition to the AWOS computer-generated voice, some systems are configured to permit station personnel to append the automated report with an operator-generated message.
d. ASOS / AWSS– Automated Surface Observing System (ASOS) / Automated Weather Sensor System (AWSS), the primary surface weather observing system of the U.S. provides the continuous minute-by-minute observations necessary to generate METARs and other aviation weather information. The information may be transmitted over a discrete VHF radio frequency or the voice portion of a local NAVAID.

3. What are PIREPs (UA), and where are they usually found? (AC 00-45E)

These reports contain information concerning weather as observed by pilots en route. Required elements for all PIREPs are message type, location, time (in UTC), flight level (altitudes are MSL), type of aircraft, and at least one weather element encountered (visibility in SM, distances in NM). A PIREP (abbreviation for “Pilot Reports”) is usually transmitted as an individual report but can be appended to a surface aviation weather report or placed into collectives. Also referred to in coded reported as “UA”.

4. What are Radar Weather Reports (SD)? (AC 00-45E)

These include the type, intensity, and location of the echo top of the precipitation observed by radar, reported each hour at H+35. All heights are reported above MSL. SDs should be used along with METARs, satellite photos, and forecasts when planning a flight, to help in thunderstorm area avoidance; once airborne, depend on Flight Watch.

5. What are Terminal Aerodrome Forecasts (TAFs)?

A TAF is a concise statement of the expected meteorological conditions within a 5-SM radius from the center of an airport’s runway complex during a 24-hour time period. TAFs use the same weather code found in METAR weather reports, in the following format: Type
(routine or amended), ICAO station identifier, date and time of origin, valid period date and time, and forecasts. Issued four times daily and valid for 24 hours.

6. What is an Aviation Area Forecast (FA)? (AC 00-45E)

An FA is a forecast of VMC, clouds, and general weather conditions over an area the size of several states. It is used to determine forecast en route weather and to interpolate conditions at airports which do not issue TAFs. FAs are issued 3 times a day by the Aviation Weather Center (AWC) for each of the 6 areas in the contiguous 48 states. The weather forecast office (WFO) in Honolulu issues FAs for Hawaii. The Alaska Aviation Weather Unit (AAWU) in Anchorage, Alaska produces the FA for the entire state of Alaska.

7. What are In-Flight Aviation Weather Advisories (WST, WS, WA)? (AC 00-45E)

In-flight Aviation Weather Advisories are forecasts to advise en route aircraft of development of potentially hazardous weather. All heights are referenced to MSL, except in the case of ceilings (CIG) which indicates above ground level. The advisories are of three types: Convective SIGMENT (WST), SIGMET (WS), and AIRMET (WA).

8. What is a Convective SIGMET? (AC 00-45E)

Convective SIGMETs (WST) imply severe or greater turbulence, severe icing and low-level wind shear. They may be issued for any convective simulation which the forecaster feels is hazardous to all categories of aircraft. Bulletins are issued hourly at H+55. Special bulletins are issued at any time as required and updated at H+55. The text of the bulletin consists of either an observation and a forecast, or just a forecast. The forecast is valid for up to 2 hours.

a. Severe thunderstorm due to:
• Surface winds greater than or equal to 50 knots.
• Hail at the surface greater than or equal to ¾ inches in diameter.
• Tornadoes
b. Embedded thunderstorms
c. A line of thunderstorms
d. Thunderstorms producing greater than or equal to heavy precipitation that affects 40% or more of an area at least 3,000 square miles.

9. What is a SIGMET (WS)? (AC 00-45E)

A SIGMET (WS) advises of non-convective weather that is potentially hazardous to all aircraft. SIGMETs are issued for the six areas corresponding to the FA areas. The maximum forecast period is four hours. In the conterminous U.S. SIGMETs are issued when the following phenomena occur or are expected to occur:

a. Severe icing not associated with a thunderstorm;
b. Severe or extreme turbulence or clear air turbulence (CAT) not associated with thunderstorms;
c. Dust storms or sandstorms lowering surface or in flight visibilities to below 3 miles;
d. Volcanic ash.

10. What is an AIRMET (WA)? (AC 00-45E)

An AIRMET is an advisory of significant weather phenomena that describe conditions at intensities lower than those which required for issuance of SIGMETs, issued every 6 hours beginning at 0145 UTC (Central Daylight Time). Pilots should use AIRMETs in the preflight and en route phase of flight to enhance safety. Each AIRMET Bulletin includes an outlook for conditions expected after the AIRMET valid period, and contain details about IFR, extensive mountain obscuration, turbulence, strong surface wins, icing, and freezing levels.

11. What is a Winds and Temperatures Aloft Forecast (FD)? (AC 00-45E)

Winds and temperature aloft are forecasted for specific locations in the contiguous U.S., and also prepared for a network of locations in Alaska and Hawaii. Forecasts are made twice daily based on the 00Z and 12Z data for use during specific time intervals. In an FD, a 4-digit code group shows wind direction, in reference to true north, and wind speed in knots. A 6-digit code group shows forecast temperatures in degree Celsius.

12. What valuable information can be determined form Winds and Temperatures Aloft Forecasts (FD)? (AC 00-45E)

Most favorable altitude – based on winds and direction of flight.
Areas of possible icing – by noting air temperatures of +2°C to -20°C.
Temperature inversions.
Turbulence – by observing abrupt changes in wind direction and speed at different altitudes.

13. What are Center Weather Advisories (CWA)? (AC 00-45E)

Issued by a CWSU (Center Weather Service Unit), this is an aviation warning for use by aircrews to anticipate and avoid adverse weather conditions in the en route and terminal environments. The CWA is a short-term “now cast”, pinpointing hazardous weather already causing an impact or expected to cause an impact within a 2-hour period; therefore it is an in-flight advisory rather than a flight planning tool. CWAs are valid for a maximum of 2 hours; if conditions are expected to continue beyond the 2-hour valid period, a statement will be included in the CWA.

14. What is a Convective Outlook (AC)? (AC 00-45E)

The AC is a flight planning tool used to avoid thunderstorms, issued in Day 1 and Day 2 time-period parts, which described the prospects for general thunderstorm activity during the following 24-hour periods. Areas in which there is high, moderate or slight risk of severe thunderstorms are included as well as where thunderstorms may approach severe limits.

C. Aviation Weather Charts

1. What is a Surface Analysis Chart? (AC 00-45E)

This is a computer-prepared chart transmitted every three hours that covers the contiguous 48 states and adjacent areas. Provides a ready means of locating pressure systems and fronts. It also gives an overview of winds, temperatures and dew point temperatures at chart time. Since weather moves and conditions change, using the surface analysis chart in conjunction with other information gives a more complete weather picture.

2. What are Radar Summary Charts? (AC 00-45E)

This is a computer-generated graphical display of a collection of automated radar weather reports, available hourly with a valid time of 35 minutes past each hour. Displays areas of precipitation as well as information about type, intensity, configuration, coverage, echo top, and cell movement of precipitation (and severe weather watches if in effect when the chart is valid). It aids in preflight planning by identifying general areas and movement of precipitation and/or thunderstorms; however, it displays drops or ice particles of precipitation size only – it does not display clouds and fog. Therefore, since the absence of echoes does not guarantee clear weather, and cloud tops will most likely be higher than the tops of the precipitation echoes detected by radar, the radar summary chart must be used along with other charts, reports, and forecasts for best effectiveness.

3. What is a Winds and Temperatures Aloft Chart (FD)? (AC 00-45E)

This chart is a computer-generated from FB (Previously called FD) reports, depicting both observed and forecast winds and temperatures aloft. Forecast winds and temperatures aloft are prepared for eight levels on eight separate panels; 6,000, 9,000, 12,000, 18,000, 24,000, 30,000, 34,000 and 39,000 feet MSL. They are available daily as 12-hour progs valid at 1200Z and 0000Z. FB charts are typically used to determine winds at a proposed altitude or to select the best altitude for a proposed flight; also, temperatures can be determined from the forecast charts. Interpolation must be used to determine winds and temperatures at a level between charts and data when the time period is other than the valid time of the chart.

4. What are Significant Weather Prognostic Charts? (AC 00-45E)

These “Progs” charts portray forecasts of selected weather conditions at specified valid times (12, 24, 36 and 48 hour progs). Each valid time is the time at which the forecast conditions are expected to occur, made from a comprehensive set of observed weather conditions. The observed conditions are extended forward in time and become forecasts by considering atmospheric and environmental processes. Forecast information for the surface to 24,000 feet is provided by the low-level significant weather progchart. Forecast information from above 24,000 to 60,000 feet is provided by the high-level significant weather prog chart.

5. What is a Convective Outlook Chart? (AC 00-45E)

This chart depicts the convective outlook (AC) presented in two panels. The left-hand panel is the Day 1 Convective Outlook, and the right-hand panel is the Day 2 Convective Outlook. “Day 1” is issued five times daily (0600Z, 1300Z, 1630Z, 2000Z, and 0100Z) and all Day 1 outlooks are valid till 12Z the following day. The outlook issued qualifies the level of risk (i.e., SLGT, MDT, HIGH) as well as the areas of general thunderstorms. The Day 2 chart contains the same information as the Day 1, and is issued twice a day (0830Z and 1730Z) in a period from 12Z the following day to 12Z the next day.


1. What are “NOTAMs”? (AIM 5-1-3)

Notices To Airmen (NOTAM) – time-critical aeronautical information which is of either a temporary nature, or not sufficiently known in advance to permit publication on aeronautical charts or in other operational publications receives immediate dissemination via the National NOTAM System. It includes such information as airport or primary runway closures, changes in the status of navigational aids. ILS’s radar service availability and other information essential to planned en route, terminal, or landing operations.

2. What are the three categories of NOTAMs? (AIM 5-1-3)

There are three types of NOTAMs generated by the FAA:
a. NOTAM (D) – A NOTAM given (in addition to local dissemination) distant dissemination beyond the area of responsibility of the Flight Service Station. These NOTAMs will be stored and available until canceled. NOTAM Ds contain information on all civil public use airports and navigational facilities that are part of the National Airspace System, and are serious enough to affect whether or not an airport or a certain facility is usable.
b. NOTAM (L) – A NOTAM given local dissemination by voice and other means to satisfy local user requirements. It includes such data as taxiway closures, personnel and equipment near or crossing runways, and airport lighting aids that do not affect instrument approach criteria, such as VASI.
c. FDC NOTAM – The National Flight Data Center will issue these NOTAMs when it becomes necessary to disseminate information that is regulatory in nature, and they contain such things as amendments to published IAPs and other current aeronautical charts. They are also used to advertise temporary flight restrictions caused by such things as natural disasters or large scale public events that may generate congestion of air traffic over a site

3. What particular types of NOTAMs will be omitted in a pilot briefing if not specifically requested by the pilot? (AIM 7-1-4)

NOTAM (D) information and FDC NOTAMs published in the Notices to Airmen Publication (NTAP) are not included in pilot briefings unless the pilot specifically requests it. Also, NOTAM (L) information is distributed locally only and is not attached to the hourly weather reports. A separate file of local NOTAMs is maintained at each FSS for facilities in their area only. NOTAM (L) information for other FSS areas must be specifically requested directly from the FSS responsible for the airport concerned.

4. Where can NOTAM information be obtained? (AIM 5-1-3)

a. Nearest FSS
b. DUATs vendors
c. NTAP printed NOTAMs. Not normally provided in a briefing so pilots must make specific request for them during briefing. The NTAP is also available online at the FAA website.

E. Aviation Weather Hazards

1. What are the two major classifications of thunder storms? (AC 00-6A)

Air mass thunderstorms – Most often result from surface heating. They occur at random in unstable air and last for only an hour or two. They reach maximum intensity and frequency over land during middle and late afternoon. Off-shore they reach a maximum during late hours of darkness when land temperature is coolest and cool air flows off the land over the relatively warm water.

Steady-state thunderstorms – Usually form inlines, last for several hours, dump heavy rain and possibly hail, and produce strong gusty winds and possibly tornadoes. They are normally associated with weather systems. Fronts, converging winds, and troughs aloft force upward motion spawning these storms which often form into squall lines. They are intensified by afternoon heating.

2. What procedures should be followed when avoiding turbulence around thunderstorms? (AIM 7-1-30)

Never regard any thunderstorm lightly even when radar observes report the echoes are to light intensity. Avoiding thunderstorms is the best policy, but these are some “do’s and don’ts”:
a. Don’t land or take off in the face of an approaching thunderstorm. A sudden gust front of low-level turbulence could cause loss of control.
b. Don’t attempt to fly under a thunderstorm even if you can see through to the other side. Turbulence and wind shear under the storm could be disastrous.
c. Don’t fly without airborne radar into a cloud mass containing scattered embedded thunderstorms. Scattered thunderstorms not embedded can usually be visually circumnavigated.
d. Don’t trust the visual appearance to be a reliable indicator of the turbulence inside a thunderstorm.
e. Do avoid, by at least 20 miles, any thunderstorms identified as severe or giving an intense radar echo. This is especially true under the anvil of a large cumulonimbus cloud.
f. Do clear the top of a known or suspected severe thunderstorm by at least 1,000 feet altitude for each 10 knots of wind speed at the cloud top. (Note: This should exceed the altitude capability of most aircraft.)
g. Do circumnavigate the entire area if the area has 6/10 thunderstorm coverage.
h. Do remember that vivid and frequent lightning indicates the probability of a severe thunderstorm.
i. Do regard as extremely hazardous any thunderstorm with tops of 35,000 feet or higher whether the top is visually sighted or determined by radar.

3. Can ATC provide in-flight assistance in avoiding thunderstorms and severe weather? (AIM 7-1-15)

Yes, to the extent possible, controllers will issue pertinent information on weather or chaff areas and assist pilots in avoiding such areas when requested. Pilots should respond to a weather advisory by either acknowledging the advisory or by requesting an alternate course of action as appropriate.
However, the controller’s primary responsibility is to provide safe separation between aircraft. Additional services such as weather avoidance assistance can only be provided to the extent that it doesn’t interfere with their primary function. ATC radar limitations and frequency congestion may also limit the controller’s capability to assist.

4. Give some examples of charts and reports useful in determining the potential for and location of thunderstorms along your route. (AC 00-45E)

a. A convective outlook (AC) describes the prospects for general thunderstorm activity during the following 24 hours. Areas in which there is a high, moderate, or slight risk of severe thunder storms are included, as well as areas where thunderstorms may approach severe limits.
b. A composite moisture stability chart outlines areas of stable / unstable air, moisture, and possible aviation hazards.
c. A radar summary chart graphically displays a collection of radar reports. The chart displays the type of precipitation echoes and indicates their intensity, intensity trend, configuration, coverage, echo tops and bases, and movement.
d. A connective outlook chart depicts areas of probable thunderstorm activity.
e. Pilot Reports (PIREPs) help in determining the actual conditions along your planned route of flight.

5. What are microbursts? (AIM 7-1-27)

Microbursts are small-scale intense downdrafts which, on reaching the surface, spread outward in all directions from the downdraft center. This causes the presence of both vertical and horizontal wind shears that can be extremely hazardous to all types and categories of aircraft, especially at low altitudes. Due to their small size, short life span, and the fact that they occur over areas without surface precipitation, microbursts are not easily detectable using conventional weather radar or wind shear alert systems.

6. Where are microbursts most likely to occur? (AIM 7-1-27)

Microbursts can be found almost anywhere there is convective activity. They may be embedded in heavy rain associated with a thunderstorm or in light rain in benign-appearing virga. When there is little or no precipitation at the surface accompanying the microburst, a ring of blowing dust may be the only visual clue of its existence.

7. What are some basic characteristics of a microburst? (AIM 7-1-27)

Size: less than 1 mile in diameter as it descends from the cloud base; can extend 2 1/2 miles in diameter near ground level.
Intensity: downwards as strong as 6,000 feet per minute; horizontal winds near the surface can be as strong as 45 knots resulting in a 90-knot wind shear (headwind to tail wind change for traversing aircraft).
Duration: an individual microburst will seldom last longer than 15 minutes from the time it strikes the ground until dissipation. Sometimes microbursts are concentrated into a line structure, and under these conditions activity may continue for as long as an hour.

8. How can microburst encounters be avoided? (AIM 7-1-27)

The following should be examined for clues of potential microburst/wind shear conditions affecting the flight:

TAFs – examine the terminal forecast for convective activity.

METARs – inspect for wind shear clues (thunderstorms, rain showers, blowing dust).
Severe weather watch reports – check for issuance since severe convective weather is a prime source for microbursts.

LLWAS reports – Low Level Wind shear Alert System, designed to detect wind shifts between outlying stations and a reference station.

SIGMETs and convective SIGMETs – may provide essential clues.

Visual clues from the cockpit – heavy rain (in a dry or moist environment) which can be accomplished by curling outflow, a ring of blowing dust or localized dust in general, flying debris, virga, a rain core with rain diverging away horizontally from the rain core, or tornadic features (funnel clouds, tornadoes). At night, lightning may be the only visual clue.

PIREPS – reports of sudden airspeed changes in the airport approach or landing corridors provide indication of the presence of wind shear.

Airborne weather radar – to detect convective cells.

9. Define wind shear and state the areas in which it is likely to occur. (AC 00-54)

Wind shear is the rate of change of wind velocity (direction and / speed) per unit distance; conventionally expressed as vertical or horizontal wind shear. It may occur at any level in the atmosphere but three areas are of special concern:
a. Wind shear with a low-level temperature inversion;
b. Wind shear in a frontal zone or thunderstorm; and
c. Clear air turbulence (CAT) at high levels associated with a jet stream or strong circulation.

10. Why is wind shear an operational concern to pilots? (AC 00-6A)

Wind shear is an operational concern because unexpected changes in wind speed and direction can be potentially very hazardous to aircraft operations at low altitudes on approach to and departing from airports.

11. What airplane characteristics will be observed in the following wind shear situations?
– A sudden increase in head wind.
– A sudden decrease in head wind.

Increased head wind – As a tail wind shears to a constant head wind, an increase in airspeed and altitude occurs along with a nose-up pitching tendency. The usual reaction is to reduce both power and pitch. This reaction can be dangerous if the aircraft suddenly encounters a downdraft and tailwind. Now the situation demands the exact opposite of the pilot’s initial reaction: a need for more performance from the airplane instead of less (more power / increased pitch altitude).
Decreased head wind – As a head wind shears to a calm or tail wind, pitch attitude decreases, airspeed decreases, and a loss of altitude occurs. The required action is more power and higher pitch altitude to continue a climb or remain on the glide slope.

12. Concerning wind shear detection, what does the abbreviation LLWAS indicate? (AIM 4-3-7)

Low-Level Wind Shear Alert System (LLWAS) is a computerized system that detects the presence of a possible hazardous low-level wind shear by continuously comparing the winds measured by sensors installed around the periphery of an airport with the wind measured at the center of the airport. If the difference between the center field wind sensor and a peripheral wind sensor becomes excessive, a thunderstorm or thunderstorm gust front wind shear is possible.