Tutorial
6. Weather Services for Pilotage
When
an airplane takes flight, the type and intensity of the weather determine the
Federal Aviation Regulations (FARs), Federal Laws, used for that flight. Conditions
where the ceiling is more than 1,000 feet and the visibility is greater than
three miles are known as Visual Meteorological Conditions (VMC) and Visual Flight
Rules (VFR) are used. Conditions where the ceiling is less than 1,000 feet and/or
the visibility is less than three miles are known as Instrument Meteorological
Conditions (IMC) and Instrument Flight Rules (IFR) are in effect. A ceiling
is the height above the Earth's surface of the lowest layer of clouds or obscuring
phenomena that is reported as "broken," "overcast," or "obscuration,"
and is not classified as "thin" or "partial."
The US Weather Service and many private organizations report and forecast the weather. It is important for pilots
and air traffic management personnel to know that a weather report or a weather map is history and a weather forecast
is a computer-generated best guess. They must use all available weather data from real time to forecasts before
and during the flight to make decisions that maintain safety in the skies. There are many sources of weather data,
all of which are covered briefly 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-country flights, a pilot will need more detailed information covering a greater area and perhaps
at various altitudes.
Printed Reports and Forecasts
METAR is the Aviation Routine Weather Report and is more or less standard around the world. The temperatures
are given in Celsius degrees. The atmospheric pressure however is reported in hecto pascals everywhere, but the
US where the atmospheric pressure is reported in inches of mercury. For aviation purposes the standard temperature
and atmospheric pressure are 59° F (15°C) and 29.92 in. Hg (1013.2 hPa). Follow this link to examine
a sample METAR.
TAF is an airport forecast. TAF is more or less standard around the world. TAF forecasts significant
weather changes, temporary changes, probable changes and expected changes in weather conditions. Follow this link
to examine a sample TAF.
Follow this link to examine an SA report.
Radar Weather Reports (RAREPs) are issued by most radar stations each hour with special reports provided as 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-bearing clouds. These reports are regularly transmitted over weather service circuits and can be found in Flight Service station weather broadcasts.
Terminal Forecasts (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 center of the runway field. FTs are usually issued 3 times a day and are valid for accuracy up to 24 hours. The first section of the FT covers the expected weather for the upcoming 18-hour period. This includes information about ceiling, visibility, vision obstructions, wind and expected weather changes at that facility. Follow this link to examine a sample FT report.
An Area Forecast (FA) reports the general weather conditions of a particular region. The contiguous states are divided into 6 regions for which area forecasts are prepared. This is a good source of weather information for 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 five specific weather conditions:
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 true north, the wind speed (in knots) and the temperature (degrees Celsius) for selected levels of altitude. Follow this 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 the 24-hour period after the report is issued. The WW defines 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 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 pulses
of radar energy from a rotating antenna. If the signals encounter precipitation, they are reflected back to the
antenna 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 will
not detect fog or the actual tops of rain clouds. The radar summary chart has limitations in that it only displays
the conditions that existed at the time recorded on the chart. Remember that thunderstorms can develop rapidly
and 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 four 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. Based
upon the following pilot's information a briefing is given:
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". These 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:
The Abbreviated Briefing is for pilots who need to update previous weather data they have gathered or who 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 many weather information services are recorded. Pilots can use a telephone to obtain 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. 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 the
weather information to determine whether or not the conditions may be hazardous to the flight. It also helps the
pilot 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 (WS) 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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