5-92 Table 5-16. TAF Fog Terms
TERM DESCRIPTION Freezing Fog (FZFG) Any fog (visibility less than 5/8 SM) consisting predominantly of water droplets at temperatures less than or equal to 32 ˚F / 0 ˚C, whether or not rime ice is expected to be deposited. FZBR is not a valid significant weather combination and will not be used in TAFs.
Shallow Fog (MIFG) The visibility at 6 feet above ground level is greater than or equal to 5/8 sm and the apparent visibility in the fog layer is less than 5/8 sm.
Patchy Fog |(BCFG) Fog patches covering part of the airport. The apparent visibility in the fog patch or bank is less than 5/8 sm, with the foggy patches extending to at least 6 feet above ground level.
Partial Fog (PRFG) A substantial part of the airport is expected to be covered by fog while the remainder is expected to be clear of fog (e.g., a fog bank). Note: MIFG, PRFG and BCFG may be forecast with prevailing visibility of P6SM.
Examples:
1/2SM FG Fog is reducing visibilities to less than 5/8 sm, therefore FG is used to encode the fog.
3SM BR Fog is reducing visibilities to between 5/8 and 6 sm, therefore BR is used to encode the fog.
5.13.2.8 Vicinity (VC).
In the United States, vicinity (VC) is defined as a donut-shaped area between 5 and 10 sm from the center of the airport’s runway complex. The FAA requires TAFs to include certain meteorological phenomena, which may directly affect flight operations to and from the airport. Therefore, NWS TAFs may include a prevailing condition forecast of fog, showers, and thunderstorms in the airport’s vicinity. A prevailing condition is defined as a greater than or equal to 50 percent probability of occurrence for more than half of the subdivided forecast time period. VC is not included in TEMPO or PROB groups.
The significant weather phenomena in Table 5-17, TAF Use of Vicinity (VC), are valid for use in prevailing portions of NWS TAFs in combination with VC:
5-93 Table 5-17. TAF Use of Vicinity (VC)
PHENOMENON CODED Fog* VCFG Shower(s)** VCSH Thunderstorm VCTS *Always coded as VCFG regardless of visibility in the obstruction, and without qualification as to intensity or type (frozen or liquid) **The VC group, if used, should be the last entry in any significant weather group (w’w’)
5.13.2.9 Cloud and Vertical Obscuration Groups (NsNsNshshshs or VVhshshs or SKC).
The initial time period and any subsequent FM groups include a cloud or obscuration group (NsNsNshshshs or VVhshshs or SKC), used as appropriate to indicate the cumulative amount (NsNsNs) of all cloud layers in ascending order and height (hshshs), to indicate vertical visibility (VVhshshs) into a surface-based obstructing medium, or to indicate a clear sky (SKC). All cloud layers and obscurations are considered opaque.
5.13.2.9.1 Cloud Group (NsNsNshshshs).
The cloud group (NsNsNshshshs) is used to forecast cloud amount in Table 5-18, TAF Sky Cover.
Table 5-18. TAF Sky Cover
SKY COVER CONTRACTION SKY COVERAGE SKC 0 oktas FEW 0 to 2 oktas SCT 3 to 4 oktas BKN 5 to 7 oktas OVC 8 oktas
5-94 When 0 oktas of sky coverage is forecast, the cloud group is replaced by SKC. The contraction CLR, which is used in the METAR code, is not used in TAFs. TAFs for sites with ASOS/AWOS contain the cloud amount and/or obscurations, which the forecaster expects, not what is expected to be reported by an ASOS/AWOS.
Heights of clouds (hshshs) are forecast in hundreds of feet AGL.
The lowest level at which the cumulative cloud cover equals 5/8 or more of the celestial dome is understood to be the forecast ceiling. For example, VV008, BKN008, or OVC008 all indicate an 800 ft ceiling.
5.13.2.9.2 Vertical Obscuration Group (VVhshshs).
The vertical obscuration group (VVhshshs) is used to forecast, in hundreds of feet AGL, the vertical visibility (VV) into a surface-based total obscuration.
VVhshshs is this ceiling at the height indicated in the forecast. TAFs do not include forecasts of partial obscurations (i.e., FEW000, SCT000, or BKN000).
Example:
1SM BR VV008 Ceiling is 800 ft due to vert ical visibility into fog
5.13.2.9.3 Cloud Type (CB).
The only cloud type included in the TAF is CB. CB follows cloud or obscuration height (hshshs) without a space whenever thunderstorms are included in significant weather group (w’w’), even if thunderstorms are only forecast in the vicinity (VCTS). CB can be included in the cloud group (NsNsNshshshs) or the vertical obscuration group (VVhshshs) without mentioning thunderstorm in the significant weather group (w’w’). Therefore, situations may occur where nearly identical NsNsNshshshs or VVhshshs appear in consecutive time periods, with the only change being the addition or elimination of CB in the forecast cloud type.
Examples :
1/2SM TSRA OVC010CB Thunderstorms are forecast at the airport
5.13.2.10 Low-Level Wind Shear Alert System (LLWAS).
The LLWAS system was originally developed by the FAA in the 1970s to detect large-scale wind shifts (sea breeze fronts, gust fronts, and cold and warm fronts). It was developed by the FAA in response to an accident at JFK Airport in New York. The aircraft (Eastern 66) landed during a wind shift caused by interacting sea breeze and thunderstorm outflows.
5-95 Figure 5-50. Microburst Effect on Landing Aircraft
Artist rendition of a microburst and its effect on a landing aircraft (Image Credit:
National Center for Atmospheric Research (NCAR)).
This Phase-1 LLWAS was very simple. It compared a center field wind to five other sensors around the airport. When there was a 15-knot vector difference, it would flash the wind data to the air traffic controller and the controller would read the raw winds (e.g., 120/35 (120 degrees at 35 kts), 110/20, 350/15, etc.) from each sensor to the pilot landing or about to take off and the pilot had to do the vector addition in his head to determine the headwind/tailwind components.
This simple system worked for large-scale weather features, but it also had a serious false alarm problem and the sensors were too far apart to capture small, but intense windshear events important to aircraft. Also, the center field wind could be variable and this would trigger windshear alarms at all the outer sensors since all the other sensor winds were compared to center field.
Research conducted at the NCAR in the 1980s indicated that microburst windshear was very dangerous to aircraft below 1,000 ft. Several major accidents during the 1980s also implicated windshear as a factor.
5.13.2.10.1 LLWAS History.
In 1983, the FAA asked NCAR to develop a version of LLWAS that could detect microbursts. Between 1983 and 1988, NCAR developed and tested a new LLWAS system, called enhanced LLWAS, or LLWAS–Network Expansion, that detected microbursts, determined the strength in terms of headwind/tailwind gains or losses (in knots), and located the event (on the runway, at 1, 2, or 3 NM on departure or arrival). The system was designed to provide alerts specific to each runway operation. It was designed to have a probability of detection of 90 percent or greater and a false alarm rate of 10 percent or less.
5-96 This system was later improved and is now called the Phase-3 LLWAS. A typical Phase-3 LLWAS will have enough sensors to be spaced 2 km apart (∼1 NM apart) and cover out to 2 NM from the end of each major runway.
The largest LLWAS is at Denver International Airport, which has 32 wind sensors. Most Phase-3 systems have between 12 and 16 wind sensors. A siting evaluation is done for each airport to determine the network geometry since it depends on terrain, number of runways, obstructions, etc.
The Phase-3 LLWAS alert information is described below.
If a pilot is landing on runway 08, and there is a microburst on his path, the controller would have a display that reads: 08A MBA 30K–3MF 350/25.
This is read to a pilot arriving on runway 08 (08A) by a final controller as “microburst alert (MBA), expect a thirty knot loss (30K–) at three miles final (3MF), threshold wind three–five–zero at 25 (knots).”
If there was a windshear with a wind speed gain at 1 mile departure (headwind gain), for a pilot departing runway 25 left, the final controller’s LLWAS display would show: 25LD WSA 15k+ 1MD. This would be read as “windshear alert, expect a fifteen knot gain at one mile departure.” There are Phase-3 LLWAS systems at 9 U.S. airports and Phase-2 LLWAS at more than 100 airports. Taiwan, Korea, Singapore, Saudi Arabia, and Kuwait are now implementing Phase-3 LLWAS systems. Note, the FAA also has Terminal Doppler Weather Radars (TDWR) for windshear detection at 45 airports and has Airport Surveillance Radar-9 (ASR-9)-based windshear detection systems at another 37 airports. The FAA originally had 110 Phase-1 LLWAS systems, which were upgraded to Phase-2 systems.
A Phase-2 LLWAS has the same number of sensors (5 to 6) as a Phase-1 system (described above), but the windshear algorithm was upgraded to significantly decrease the number of false alarms. As mentioned above, a Phase-1 or Phase-2 LLWAS was not designed to detect microbursts per se, but if the flow is large and strong, it may send an alert.
When NCAR developed the Phase-3 LLWAS, it gave the specifications to the FAA. The University Corporation for Atmospheric Research (UCAR) Foundation owned the intellectual property for the windshear algorithm during the lifetime of the patent. A license agreement was required for companies to implement LLWAS technology until early 2013 when the patent exclusion expired. A license from UCAR Foundation is no longer required to utilize the LLWAS algorithm. The UCAR Foundation does, however, provide technical materials such as test datasets, test airport configuration files, test alert outputs, etc. to aid companies in the implementation and testing of the Phase-3 LLWAS algorithm.
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5.13.2.10.2 Frequently Asked Questions (FAQ).
1. Q. What is LLWAS?
A. A typical Phase-3 LLWAS system includes a network of anemometers (wind sensors) atop tall poles located around the airport (i.e., remote stations) out to no more than 3 NM from the end of the runways, a master station that processes system data and communicates with the remote stations, an archiving system, operator console, alphanumeric alarm displays, and in some instances, graphical displays.
2. Q. How does it work?
A. The wind data at each remote station is processed every 10 seconds to determine if there is divergence or convergence within the network, or station-to-station wind differences between stations aligned with the runways. The divergence/convergence information is processed and if the intensity of the event is large enough, the system will calculate the strength of the along-runway wind losses or gains and generate windshear or microburst alerts (depending on strength), and identify the location of the event.
3. Q. What is a microburst?
A. A microburst is an intense windshear. By definition:
Microburst n: A small, very intense downdraft that descends to the ground resulting in a strong wind divergence. The size of the event is typically less than 4 km across. Microbursts are capable of producing winds of more than 100 mph, causing significant damage. The life span of a microburst is around 5 to 15 minutes.
4. Q. What causes microbursts?
A. Microbursts are strong windshears (greater than 30-knot wind speed losses over distances of 1 to 4 km) that are primarily generated by evaporative cooling and rain loading. When precipitation (rain or snow) descends below the cloud base or is mixed with dry air, it begins to evaporate and this evaporation process cools the air. The cool air descends and accelerates as it approaches the ground. When the cool air approaches the ground, it spreads out in all directions and this divergence of the wind is the signature of the microburst. In humid climates, microbursts can also generate from heavy precipitation. The weight and drag associated with the falling precipitation can result in a downdraft that will descend to the ground and spread out.
5. Q. Why is it a problem for airplanes?
A. Windshear is a rapid change of wind speed or direction over a short distance. In general, windshear becomes a hazard for aircraft if the wind changes more than 20 kts over a distance of 1 to 4 km
(0.5 to 2.5 NM). On either takeoff or landing, aircraft are near stall speeds. When going through a windshear, the headwind decreases
5-98 resulting in a loss of lift. If the aircraft is near stall, then a little loss of lift can make all the difference to whether the aircraft can continue the flight.
6. Q. How and what kind of decisions do airports or airlines make using LLWAS?
A. LLWAS provides information on windshear type, location, and intensity. Windshear alerts are issued via radio to arriving and departing aircraft by final air traffic controllers. In the United States, most airlines require that pilots not continue their arrival or departure if there is a microburst alert valid for their operation.
Although most aviation authorities do not close the runways when microbursts are occurring, the air traffic controllers will work with the pilots to reroute aircraft away from the event to a runway that is not impacted by the windshear.
7. Q. How and what kind of decisions do airports make using LLWAS?
A. LLWAS provides information on windshear type, location, and intensity. The pilots get the alert from the controller and the pilot is supposed to determine if they feel comfortable continuing the operation. In the United States, airlines require that pilots do not continue if there is a microburst alert.
8. Q. Where have LLWAS systems been implemented?
A. Phase-3 LLWAS systems have been implemented throughout the United States and at numerous international airports including, but not limited to, Taiwan, Korea, Saudi Arabia, Hong Kong, Kuwait, Italy, Singapore, and Spain. Additional LLWAS procurements are in progress.
9. Q. What are some of the windshear problems associated with airports?
A. Any airport that has thunderstorms will be exposed to Convective windshear. Any airport near mountains will experience terrain-induced windshear from time to time. Any airport near a coast will experience windshear due to sea breezes. In the drier climates, even a light shower (or virga) can produce severe windshears, so in some places, the convection does not have to be as strong as a thunderstorm (e.g., Denver, Phoenix, Reno, Albuquerque, etc.).
Discussion Credit: NCAR.
For m ore information readers can go to:
https://www.ral.ucar.edu/projects/low-level-wind-shear-alert-system-llwas.
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5.13.2.11 Non-Convective Low-Level Wind Shear (LLWS) Group (WShwshwshws/dddffKT).
Windshear (WS) is defined as a rapid change in horizontal wind speed and/or direction, with distance and/or a change in vertical wind speed and/or direction with height. A sufficient difference in wind speed, wind direction, or both can severely impact airplanes, especially within 2,000 ft AGL because of limited vertical airspace for recovery.
Forecasts of LLWS in the TAF refer only to Non-Convective LLWS from the surface up to and including 2,000 ft AGL. LLWS is always assumed to be present in Convective activity. LLWS is included in TAFs on an “as-needed” basis to focus the aircrew’s attention on LLWS problems that currently exist or are expected. Non-Convective LLWS may be associated with the following: frontal passage, inversion, low-level jet, lee side mountain effect, sea breeze front, Santa Ana winds, etc.
When LLWS conditions are expected, the Non-Convective LLWS code WS is included in the TAF as the last group (after cloud forecast). Once in the TAF, the WS group remains the prevailing condition until the next FM change group or the end of the TAF valid period if there are no subsequent FM groups. Forecasts of Non-Convective LLWS are not included in TEMPO or PROB groups.
The format of the Non-Convective LLWS group is:
WShwshwshws/dddffKT WS Indicator for Non-Convective LLWS Hwshwshws Height of the top of the WS layer in hundreds of feet AGL Ddd True direction in 10-degree increments at the indicated height -- VRB is not used for direction in the Non-Convective LLWS forecast Ff Speed in knots of the forecast wind at the indicated height KT Unit indicator for wind Example:
TAF…13012KT…WS020/27055KT Windshear from the surface to 2,000 ft. Surface winds from 130 (southeast) at 12 kts changes to 270 (west) at 55 knots at 2,000 ft.
In this example, the indicator WS is followed by a three-digit number that is the top of the windshear layer. LLWS is forecast to be present from the surface to this level. After the solidus (/), the five-digit wind group is the wind direction and speed at the top of the windshear layer. It is not a value for the amount of shear.
5-100 A Non-Convective LLWS forecast is included in the initial time period or a FM group in a TAF whenever:
• One or more Pilot Weather Reports (PIREP) are received of Non-Convective LLWS within 2,000 ft of the surface, at or in the vicinity of the TAF airport, causing an indicated air speed loss or gain of 20 kts or more, and the forecaster determines the report(s) reflect a valid Non-Convective LLWS event rather than mechanical turbulence; or
• When Non-Convective vertical WS of 10 kts or more per 100 ft in a layer more than 200 ft thick are expected or reliably reported within 2,000 ft of the surface at, or in the vicinity of, the airport.
5.13.2.12 Forecast Change Indicator Groups.
Forecast change indicator groups are contractions that are used to subdivide the forecast period (24 hours for scheduled TAFs; less for amended or delayed forecasts) according to significant changes in the weather.
The forecast change indicators FM, TEMPO, and PROB are used when a change in any or all of the elements forecast is expected.
5.13.2.12.1 From (FM) Group (FMYYGGgg).
The change group FMYYGGgg (voiced as “from”) is used to indicate when prevailing conditions are expected to change significantly over a period of less than 1 hour. In these instances, the forecast is subdivided into time periods using the contraction FM, followed, without a space, by six digits, the first two of which indicate the day of the month and the final four, which indicate the time (in hours and minutes Z) the change is expected to occur.
While the use of a four-digit time in whole hours (e.g. 2100Z) is acceptable, if a forecaster can predict changes and/or events with higher resolution, then more precise timing of the change to the minute will be indicated. All forecast elements following FMYYGGgg relate to the period of time from the indicated date and time (YYGGgg) to the end of the valid period of the terminal forecast, or to the next FM if the terminal forecast valid period is divided into additional periods.
The FM group will be followed by a complete description of the weather
(i.e., self-contained), and all forecast conditions given before the FM group are superseded by those following the group. All elements of the TAF (surface wind, visibility, significant weather, clouds, obscurations, and when expected, Non-Convective LLWS) will be included in each FM group, regardless if they are forecast to change or not. For example, if forecast cloud and visibility changes warrant a new FM group but the wind does not, the new FM group will include a wind forecast, even if it is the same as the most recently forecast wind.
The only exception to this involves the significant weather group. If no significant weather is expected in the FM time period group, then significant
5-101 weather group is omitted. A TAF may include one or more FM groups, depending on the prevailing weather conditions expected. In the interest of clarity, each FM group starts on a new line of forecast text, indented five spaces.
Examples:
TAF KDSM 022336Z 0300/0324 20015KT P6SM BKN015 FM030230 29020G35KT 1SM +SHRA OVC005 TEMPO 0303/0304 30030G45KT 3/4SM -SHSN FM030500 31010G20KT P6SM SCT025...
A change in the prevailing weather is expected on the 3rd day of the month at 0230 UTC and the 3rd day of the month at 0500 UTC.
TAF KAPN 312330Z 0100/0124 13008KT P6SM SCT030 FM010320 31010KT 3SM -SHSN BKN015 FM010500 31010KT 1/4SM +SHSN VV007...
Note the wind in the FM010500 group is the same as the previous FM group, but is repeated since all elements are required to be included in a FM group.
5.13.2.12.2 TEMPO (YYGG/YeYeGeGe) Group.
The change-indicator group TEMPO YYGG/YeYeGeGe is used to indicate temporary fluctuations to forecast meteorological conditions that are expected to:
• Have a high percentage (greater than 50 percent) probability of occurrence;
• Last for one hour or less in each instance; and
• In the aggregate, cover less than half of the period YYGG to YeYeGeGe.
The first two digits (YY) are the day of the month for the start of the TEMPO. The next two digits (GG) are the starting hour (UTC). After the solidus (/), the next two digits (YeYe) are the ending day of the month, while the last two digits (GeGe) are the ending hour (UTC) of the TEMPO period.
Each TEMPO group is placed on a new line in the TAF. The TEMPO identifier is followed by a description of all the elements in which a temporary change is forecast. A previously forecast element that has not changed during the TEMPO period is understood to remain the same and will not be included in the TEMPO group. Only those weather elements forecast to temporarily change are required to be included in the TEMPO group.
5-102 TEMPO groups will not include forecasts of either significant weather in the vicinity (VC) or Non-Convective LLWS.
Examples:
TAF KDDC 221130Z 2212/2312 29010G25KT P6SM SCT025 TEMPO 2215/2217 30025G35KT 1 1/2SM SHRA BKN010...
In the example, all forecast elements in the TEMPO group are expected to be different than the prevailing conditions. The TEMPO group is valid on the 17 from 1500 UTC to 1700 UTC.
TAF KSEA 091125Z 0912/1012 19008KT P6SM SCT010 BKN020 OVC090 TEMPO 0912/0915 -RA SCT010 BKN015 OVC040...
In this example the visibility is not forecast in the TEMPO group. Therefore, the visibility is expected to remain the same (P6SM) as forecast in the prevailing conditions group. Also, note that in the TEMPO 0912/0915 group, all three cloud layers are included, although the lowest layer is not forecast to change from the initial time period.
5.13.2.12.3 PROB30 (YYGG/YeYeGeGe) Group.
The probability group, PROB30 YYGG/YeYeGeGe, is only used by NWS forecasters to forecast a low probability occurrence (30 percent chance) of a thunderstorm or precipitation event and its associated weather and obscuration elements (wind, visibility, and/or sky condition) at an airport.
The PRO B30 group is the forecaster’s assessment of probability of occurrence of the weather event that follows it. The first two digits (YY) are the day of the month for the start of the PROB30. The next two digits (GG) are the starting hour (UTC). After the solidus (/), the next two digits (YeYe) are the ending day of the month, while the last two digits (GeGe) are the ending hour (UTC) of the PROB30 period. PROB30 is the only PROB group used in NWS TAFs. Note that U.S. military and international TAFs may use the PROB40 (40 percent chance) group as well.
The PROB30 group is located within the same line of the prevailing condition group, continuing on the line below if necessary.
NWS TAFs do not use the PROB30 group in the first 9 hours of the TAF’s valid period, including amendments. Also, only one PROB30 group may be used in the initial forecast period and in any subsequent FM groups. Note that U.S. military and international TAFs do not have these restrictions.
5-103 PROB30 groups do not include forecasts of significant weather in the vicinity (VC) or Non-Convective LLWS.
Example:
FM012100 18015KT P6SM SCT050 PROB30 0123/0201 2SM TSRA OVC020CB In this example, the PROB30 group is valid on the 1st day of the month at 2300 UTC to the 2nd day of the month at 0100 UTC.
5.13.2.12.4 TAF Examples.
TAF KPIR 111140Z 1112/1212 13012KT P6SM BKN100 WS020/35035KT TEMPO 1112/1114 5SM BR FM111500 16015G25KT P6SM SCT040 BKN250 FM120000 14012KT P6SM BKN080 OVC150 PROB30 1200/1204 3SM TSRA BKN030CB FM120400 14008KT P6SM SCT040 OVC080 TEMPO 1204/1208 3SM TSRA OVC030CB TAF Terminal Aerodrome Forecast KPIR Pierre, South Dakota 111140 Prepared on the 11th at 1140 UTC 1112/1212 Valid from the 11th at 1200 UTC until the 12th at 1200 UTC 13012KT Wind 130 at 12 kts P6SM Visibility greater than 6 sm BKN100 Ceiling 10,000 broken WS020/35035KT Windshear at 2,000 ft, wind from 350 at 35 kts TEMPO 1112/1114 Temporary conditions between the 11 th day of the month at 1200 UTC and the 11th day of the month at 1400 UTC 5SM Visibility 5 statute miles BR Mist FM111500 From the 11th day of the month at 1500 UTC 16015G25KT Wind 160 at 15 kts gusting to 25 kts P6SM Visibility greater than 6 sm SCT040 BKN250 4,000 scattered, ceiling 25,000 broken FM120000 From the 12th day of the month at 0000Z 14012KT Wind 140 at 12 kts P6SM Visibility greater than 6 sm BKN080 OVC150 Ceiling 8,000 broken, 15,000 overcast PROB30 1200/1204 30 percent probability between the 12 th day of the month at 0000 UTC and the 12th day of the month at 0400 UTC 3SM Visibility 3 sm TSRA Thunderstorm with moderate rain showers