Tutorial

4. Weather Data
When an airplane takes flight,the type and intensity of the weather determine the Federal Aviation Regulations (FARs), the Federal Laws, usedfor that flight. Conditions where the ceiling is more than 1,000 feet and the visibility is greater than threemiles are known as Visual Meteorological Conditions (VMC) and Visual Flight Rules (VFR) are used. Conditions wherethe ceiling is less than 1,000 feet and/or the visibility is less than three miles are known as Instrument MeteorologicalConditions (IMC) and Instrument Flight Rules (IFR) are in effect. A ceiling is the height above the Earth's surfaceof the lowest layer of clouds or obscuring phenomena that is reported as "broken," "overcast,"or "obscuration," but is not classified as "thin" or "partial."

The US Weather Service andmany private organizations report and forecast the weather. It is important for pilots and air traffic managementpersonnel to know that a weather report or a weather map is history and a weather forecast is a computer-generatedbest guess. They must use all available weather data from real time to forecasts before and during the flight tomake decisions that maintain safety in the skies. There are many sources of weather data, all of which are coveredbriefly below. For convenience they are categorized into printed reports and forecasts, graphic weather analyses,and briefings and advisories. Not all are used all the time, but each provides valuable information. For short,local flights a pilot may only need to be informed of general weather conditions. For extended or cross-countryflights, a pilot will need more detailed information covering a greater area and perhaps at various altitudes,too.

NASA Research Weather is a factor in approximately 30% of aviation accidents and contributes directly or indirectly to nearly 80 % of fatal general aviation accidents. Experts working with the NASA Aviation Safety Program (AvSP) ranked weather data dissemination as the most critical priority to reduce weather-related aviation accidents. NASA AvSp programs are working to improve communication of weather information. NASA Glenn Research Center's WINCOMM (Weather Information Communications) program is developing advanced communications and information technologies to enable high quality and timely dissemination of intuitive aviation weather information. These aeronautical communications technologies will benefit both the national and global aviation communities. Led by NASA Langley Research Center, the goal of the AWIN (Aviation Weather Information) program is to provide improved weather information (not simply data) to users in the National Airspace System, and to foster the improved usage of this information. While the emphasis of this project is to provide this information to the flight deck, other weather information users in the National Airspace are also considered. WINCOMM and AWIN researchers collaborated in flight test and evaluation of a worldwide weather data-link capability using satellites. This successful flight testing moves NASA closer to its goal of developing advanced communications and information technologies to enable high-quality and timely dissemination of aviation weather information to all relevant users on the aviation information network and is a positive step toward solving communications-specific issues associated with the dissemination of weather data directly to the cockpit.

Printed Reports and Forecasts

METAR is the Aviation Routine Weather Report and is more or less standard around the world. The temperaturesare given in Celsius degrees. The atmospheric pressure however is reported in hecto pascals everywhere, but theUS where the atmospheric pressure is reported in inches of mercury. For aviation purposes the standard temperatureand atmospheric pressure are 59° F (15°C) and 29.92 in. Hg (1013.2 hPa). Follow this link to examinea sample METAR.

TAF is an airport forecast. TAF is more or less standard around the world. TAF forecasts significantweather changes, temporary changes, probable changes and expected changes in weather conditions. Follow this linkto examine a sample TAF.

• Station designator (airport or other type of aviation facility)
• Time of report and type (this could be a record observation or a special report)
• Sky condition and ceiling
• Visibility
• Weather and obstructions to vision
• Sea level pressure (in millibars)
• Temperature and dew point
• Wind information
• Altimeter setting
• Remarks and coded data

Follow this link to examine an SA report.

Radar Weather Reports (RAREPs) are issued by most radar stations each hour with special reports providedas needed. This report provides information regarding general areas of precipitation, noting thunderstorm activity.It also includes type of precipitation, its intensity, direction and speed of activity as well as height of rain-bearingclouds. These reports are regularly transmitted over weather service circuits and can be found in Flight Servicestation weather broadcasts.

Terminal Forecast (FT) is one of the best sources for predicting what the weather at a particular airport(terminal) will be in the future. These forecast predictions cover an area within 5 nautical miles of the centerof the runway field. FTs are usually issued 3 times a day and are valid for accuracy up to 24 hours. The firstsection of the FT covers the expected weather for the upcoming 18-hour period. This includes information aboutceiling, visibility, vision obstructions, wind and expected weather changes at that facility. Follow this linkto examine a sample FT report.

An Area Forecast (FA) reports the general weather conditions of a particular region. The contiguous statesare divided into 6 regions for which area forecasts are prepared. This is a good source of weather informationfor en route weather and helps to determine the weather conditions for airports that do not provide terminal forecasts.The FA gives a 12-hour forecast plus a 6-hour outlook with information describing 5 specific weather conditions:

• Hazards/flight precautions (H)
• Synopsis (S)
• Icing (I)
• Turbulence (T)
• Significant clouds/weather (C )

Follow this link to examine a sample FA report.

The Winds and Temperatures Aloft Forecast (FD) gives an estimate of wind direction as it relates to truenorth, the wind speed (in knots) and the temperature (degrees Celsius) for selected levels of altitude. Followthis link to examine an FD report.

Severe Weather Reports and Forecasts include convective outlook (AC) and severe weather watch bulletin(WW). The AC forecasts general thunderstorm activity for the 24-hour period after the report is issued. The WWdefines areas where thunderstorms or tornadoes have the possibility of developing.

Perhaps the most useful information to a pilot planning a flight during preflight is the Weather Depiction Chart. This is actually is a simplified version of the weather surface chart. This chart is also generated every 3 hours. It contains information on frontal activity and has an abbreviated version of station models. It does not contain as much detailed information as the surface weather chart, but offers pilots a way to easily and quickly scan for adverse weather conditions.


Click the image to view the entire chart.

Special weather radar systems can detect certain types of weather phenomena. These radar systems emit pulsesof radar energy from a rotating antenna. If the signals encounter precipitation, they are reflected back to theantenna as echoes. These are then graphically depicted displaying the strength and location of the precipitation.This radar detection does not detect all cloud formations, only frozen or liquid forms of precipitation. It willnot detect fog or the actual tops of rain clouds. The radar summary chart has limitations in that it only displaysthe conditions that existed at the time recorded on the chart. Remember that thunderstorms can develop rapidlyand recently developed thunderstorms might not be displayed.


Click the image to view the entire chart.

The low level significant weather prognostic chart is different from the three previous charts described in that it is a forecast chart (not an observation chart). The "prog" chart is issued 4 times daily and provides a 12-hour and a 24-hour weather forecast for a given region. It also only covers from the surface up to the 400-millibar pressure level (24,000 feet in altitude). It is to be used only in planning flights below 24,000 feet.


Click the image to view the entire chart.

Briefings and Advisories

Whether a commercial, military or private pilot, all must obtain a formal weather briefing during preflight. Basedupon the following pilot's information a briefing is given:

• type of flight (VFR or IFR)
• aircraft type
• departure and destination airport
• flight route
• flight altitudes
• estimated time of departure (ETD)
• estimated time of arrival (ETA)

There are three types of briefings that are offered to pilots by "pilot weather briefers" who have been certificated by the National Weather Service. These "briefers" can be contacted at local Flight Service Stations and Weather Service Offices. Pilots need to know which weather briefing to request when contacting "briefers." There are standard, abbreviated and outlook briefings.

The Standard Briefing is the most complete weather briefing and for the pilot who does not have an opportunity to gather any other weather data, gives the following information:

• adverse conditions
• VFR (visual flight rules) flight not recommended
• synopsis
• current conditions
• en route forecast
• destination forecast
• winds aloft
• notices to airmen (NOTAM)
• ATC (air traffic control) delays

The Abbreviated Briefing is for pilots who need to update previous weather data they have gathered. Pilots also use this briefing when they only need a few bits of weather data to complete their weather picture.

The Outlook Briefing is requested by pilots whose departure time is 6 or more hours away. The briefer then gives forecast information appropriate to the proposed flight route. This will assist the pilot in making an initial decision about the feasibility of such a flight.

Because of the quickly growing number of flight operations daily there is a tremendous demand for weather information services. Because of this demand, many weather information services are recorded. Pilots can use a telephone to get preliminary weather information and decide, based upon that information, if he or she needs to speak with a briefer. There are various automated systems available with one such system , the Pilots Automatic Telephone Weather Answering Service (PATWAS), offers a continuous weather recording that gives a summary of weather data for an area within 50 nautical miles of the station. This information will help pilots in their decision-making process for a "go/no-go" decision.

While en route on a flight there are many in-flight weather advisories available to pilots via radio frequencies.The weather information provided helps pilots during their en route decision-making processes. The pilot uses theweather information to determine whether or not the conditions may be hazardous to the flight. It also helps thepilot working with air traffic management personnel to decide on a change in the flight plan.

AIRMET (WA) is broadcasted for weather phenomena that potentially affects all aircraft. For pilots of light aircraft, AIRMET (acronym for "AIRman's METeorological information) gives valuable information about the following conditions: moderate icing, moderate turbulence, sustained winds 30 knots or greater at the surface, widespread area with a ceiling of less than 1,000 feet and/or visibility less than 3 miles and extensive obscurement of mountains. These are important to light aircraft, as they have limited flight capabilities due to lack of equipment and/or instrumentation and less pilot flight experience. Follow this link to examine a sample AIRMET.

SIGMETs (WSs) are broadcasted for hazardous weather that is considered of extreme importance to all aircraft. SIGMETs (acronym for "SIGnificant METeorological information") warn of the following weather hazards: severe icing, severe and extreme turbulence, duststorms, sandstorms or volcanic ash lowering visibility to less than 3 miles. A Convective SIGMET (WST) is issued for hazardous convective weather (such as tornadoes, thunderstorms, hail) and covers severe or great turbulence, severe icing, and low-level wind shear. Follow this link to examine a sample SIGMET.

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