U.S. Department

of Transportation


Federal Aviation



Air Traffic


A Communication from the  

Vice President,
  System Operations Services


   Issue # 2004-1
                  *** SPECIAL ***

   March 2004                                                                                             

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/*TEFR/ Spring and summer means warmer weather is on the way. In many parts of the country, it also means that pilots and controllers must contend with thunderstorms. Spring often commences with a vengeance because severe thunderstorms usually occur then when there are more intense clashes of cold air with warm, moist air. We are cognizant of the destructive potential that thunderstorms pose to aircraft during all stages of flight. It is important to understand the dynamics of thunderstorms and the tools that are being used throughout the National Airspace System (NAS) to detect their movement and the effect on traffic flow. This Air Traffic Bulletin addresses thunderstorm development and tools available to detect thunderstorm movement.

If something is dynamic, there is movement. In the case of thunderstorms, movements may be horizontal (strong, gusty wind) or vertical (downbursts). The vertical movement may also be upward to form hail. The dynamic nature of the storms also produces turbulence. Even a stationary thunderstorm that is not moving across the earth's surface is dynamic. Contending with a thunderstorm in motion just makes the controller's job more difficult.

Thunderstorms may form when certain atmospheric conditions are met. The air must be unstable and there must be a sufficient moisture supply. Instability and moisture are not enough; a lifting mechanism is also necessary. With sufficient meteorological data, a weather forecaster can objectively determine stability and moisture content of the air. Judging the lifting mechanism is a greater challenge. Meteorologists evaluate all conditions when deciding whether convective significant meteorological information (SIGMET) should be issued. These data are also used when preparing a terminal area forecast (TAF), and a center weather advisory (CWA).

The atmosphere can be in states of stable, neutral, or unstable. Visualize a fairly large volume of air (approximately 1,000 to 2,000 feet on each side); meteorologists refer to the air volumes as parcels. "Lifting" the parcel causes it to cool as it rises upward. The temperature of the parcel may become different from that of the surrounding air as it rises. The temperature of the parcel as compared to the surrounding air at this point is important. If the temperature is the same, the atmosphere is neutral, and the parcel remains where it was lifted. If the parcel is cooler than the surrounding air, it will sink; the atmosphere is stable. If the parcel is warmer than the surrounding air, it is unstable and will continue to rise, condensate to form clouds, and possibly develop into thunderstorms. The more unstable the atmosphere, the more likely thunderstorms will develop.

There are many lifting mechanisms to force parcels of air upward to generate thunderstorms: Heating, orographic lifting, sea breeze, and frontal lifting. Heating to cause lifting occurs over land that is heated by the sun. It is the same phenomenon as heating water in a pot over a burner. Thunderstorms formed in this way are often called air mass thunderstorms and they often show little movement over the ground. Orographic lifting occurs when air flows from lower to higher terrain to get lifted. Thunderstorms formed in this way will often be encountered along the tops of ridges or mountain ranges, and they generally remain over the ridges or migrate to the leeward side. Sea breeze generated thunderstorms are most common over Florida a few miles inland from the coast. Fronts, the boundary between air masses of contrasting temperature and moisture content, are the most vigorous source of lifting. Most severe thunderstorms occur in a warm, moist air mass before a fast-moving cold front being pushed by a cold air mass.

Understanding how thunderstorms develop and how future development is forecast is but one aspect of dealing with them. How about those that already exist? How do we detect their existence? How widespread are they? How intense are they? How do we know how and where they are moving? How tall are they? Are they producing dangerous gust fronts, wind shear, and microburst?

One of the greatest tools available to all aviation interests (pilots, controllers, and meteorologists) for detecting, measuring, and following thunderstorms is weather radar. Radar can show where they are, how widespread they may be, and how tall they are. Observing them over spans of time allows one to determine their movements and trends. Trend refers to development and dissipation, for all thunderstorms have a life cycle; they form, develop, mature, and die. The weather radars developed for general public use are called next generation radar (NEXRAD) and are best at providing coverage, movement, trend, and height information. The weather radars operated by the FAA are more attuned to detecting gust fronts, wind shear, and microbursts; hazards that impact the terminal environment. The FAA operates terminal doppler weather radar (TDWR) and the weather system processor (WSP) which is an enhancement of the weather channel of an airport surveillance radar (ASR). The low-level windshear alert system (LLWAS) detects the hazards, but it is not radar.

Newly developed processing and display systems are coming into the NAS to process other weather sensor information to help controllers in all environments contend with thunderstorms. They are the integrated terminal weather system (ITWS), medium intensity airport weather system (MIAWS), corridor integrated weather system (CIWS), and weather and radar processor (WARP). It is important for all controllers to realize that the four systems process weather information for display; they are not detection systems.

ITWS will be located at all TDWR-equipped airports. It processes TDWR, NEXRAD, ASR-9 Weather Channel, lightning, LLWAS, Automated Surface Observing System, and weather elements from airborne sources to provide more than just the TDWR wind shear and microburst alerts. It has predictive algorithms that can tell when advancing gust fronts and microbursts may impact the terminal environment. It assists controllers in deciding which runway to use and helps them decide when to "turn the airport around." ITWS is being modified to provide a one-hour forecast for thunderstorm development and dissipation.

MIAWS will be located at airports that have LLWAS for wind shear and microburst detection. Its source of data will be NEXRAD; it will display current precipitation levels and will extrapolate storm positions for planning purposes.

CIWS is in reality ITWS adapted to large geographical areas. It is located in the traffic management unit positions in several large terminal radar approach controls (TRACON) and the en route centers throughout the corridor. Its sources of information are ASR-9 and NEXRAD. It provides cloud tops information and a two-hour forecast for movement and trend. It is a valuable tool to enhance safety and capacity.

WARP depicts the weather on the en route controller's situation displays. Its source of information is NEXRAD.

A review of the following references will provide more information about procedures and phraseology: FAAO 7110.65, Air Traffic Control, Paragraph 2-6-4, Weather and Chaff Services; Paragraphs 3-9-1, Departure Information, and 3-10-1, Landing Information. Other paragraphs for refresher training include procedures contained in Paragraph 3‑1-8, Low Level Wind Shear Advisories. This will be helpful as we approach the thunderstorm months.

The information contained in the Aeronautical Information Manual (AIM) is periodically updated to include the various newer weather detection technology and the products they provide. References from the AIM include paragraphs 4-3-7 for LLWAS and TDWR; paragraph 7-1-26 for detection of microbursts, wind shear, and gust fronts; paragraph 7‑1-28 for thunderstorms; and paragraph 7-1-29 for thunderstorm flying (from a pilot's perspective).

An additional source of weather information can be obtained from the pilot. Responsibility for soliciting pilot weather reports for AFSS/FSS is contained in FAAO 7110.10, paragraph 9-2-3, and for controllers at ATCTs, TRACONs, and ARTCCs in FAAO 7110.65, paragraph 2-6-3.

Hazardous inflight weather advisory service (HIWAS) is a continuous broadcast of inflight weather advisory recorded by flight service.  HIWAS includes  summarized aviation weather warning (AWW), SIGMETs, convective SIGMETs, CWA, airmen's meteorological information (AIRMET), and urgent pilot weather reports (UUA). Controllers shall advise pilots of hazardous weather that may impact operations within 150 NM of their sector or area of jurisdiction. Facilities shall review alert messages to determine the geographical area and operational impact for hazardous weather information broadcasts. The broadcast is not required if aircraft on your frequency(s) will not be affected. HIWAS procedures are contained in FAAO 7110.10, paragraph 2-5.

Controllers within commissioned HIWAS areas shall broadcast a HIWAS alert on all frequencies, except emergency frequency, upon receipt of hazardous weather information. Controllers are required to disseminate data based on the operational impact on the sector or area of control jurisdiction.

Controllers outside of commissioned HIWAS areas shall advise pilots of the availability of hazardous weather advisories. Pilots requesting additional information should be directed to contact the nearest flight watch or flight service. Controllers shall also apply the same procedure when HIWAS outlets, or outlets with radio coverage extending into your sector or airspace under your jurisdiction, are out of service. Further information on HIWAS is contained in FAAO 7110.65, paragraph 2-6-2. 

Questions/comments about content should be addressed to ATP-100