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Aviation Weather Services – Overview

There are lots of sources for aviation weather and I cover them in some detail in this post. The National Weather Service publishes weather observations for aviators and publishes most of them at the Standard Briefing page. Most of them are covered in the FAA publication Aviation Weather Services AC 00-45F. The 2007 version of this document can be found here. Some of the observations and forecasts are easy to understand and some take a bit of work to decipher. The Aviation Weather Services booklet does a good job of explaining how to interpret the weather products and I’d recommend that you read it if the legends on the web site aren’t clear.

The FAA Knowledge Tests on the other hand, are often less than straightforward. For example, one question on the private pilot exam is:

From which primary source should information be obtained regarding expected
weather at the estimated time of arrival if your destination has no Terminal Forecast?
      A. Low-Level Prognostic Chart.
      B. Weather Depiction Chart.
      C. Area Forecast.

All of these products will give you some information on the weather at your destination when you arrive. However, only a careful reading of the question and knowledge of the exact wording of what is contained in the weather product will give you the correct answer.

As described in the AWS document, the Weather Depiction Chart is based on METARs and gives weather at the time the chart was made. It is updated eight times a day, so reviewing it several times before a flight will give a good indication of what is happening at your destination. This is probably not the right answer since the question is probably looking for a forecast product.

Short-Range Surface Prognostic (Prog) Charts provide a forecast of surface pressure systems, fronts and precipitation for a 2-day period. The forecast area covers the 48- contiguous states, the coastal waters and portions of adjacent countries. The forecasted conditions are divided into four forecast periods, 12-, 24-, 36-, and 48-hours. Each chart depicts a “snapshot” of weather elements expected at the specified valid time. Reviewing these forecasts would give an excellent overview of what to expect at your destination. It is missing temperatures and wind. which would be available on a TAF.

The FA [Area Forecast] contains forecast information for VFR/MVFR clouds and weather for a 12-hour period with a 12- to 18-hour categorical outlook forecast for IFR, MVFR, and/or VFR. The following weather elements are included in the 12-hour forecast:

  • Thunderstorms and precipitation;
  • Sky condition (cloud height, amount, and tops) if bases are at or below (AOB) FL180 MSL. (Tops will only be forecast for broken (BKN) or overcast (OVC) clouds);
  • Obstructions to visibility (fog, mist, haze, blowing dust, etc.) if surface visibilities are three (3) to six (6) miles; and
  • Sustained surface wind speed of 20 knots or greater.

This product contains the detailed information for 12 hours and more general information (VFR/IFR) for the next six hours. It contains precipitation, sky cover, and significant wind so it covers most of the information for a TAF.

The best answer, in my opinion, would be to check the TAFs at nearby airports, especially ones that share weather patterns. That isn’t an option, so I’d go with the Area Forecast since it has the most information contained in a TAF.

However, page 7-1 of the Aviation Weather Services book says:

To understand the complete weather picture, the FA must be used in conjunction with the AIRMETs and SIGMETs. Together, they are used to determine forecast en route weather and to interpolate conditions at airports for which no Terminal Aerodrome Forecasts (TAFs) are issued.

The point here is that the questions on the test don’t necessarily reflect how the products will be used, but often hinge on obscure differences between the weather products and a single phrase in the book. This the first of a series of posts that summarize the information in the Aviation Weather Services booklet. I hope it will make it easier to answer the test questions. It might even help you make sense of the different products that are available.

Aviation Weather Center
A. The development of new aviation weather products is an evolutionary process with distinct stages of product maturity. The growing demand for new weather products and the corresponding increase in research and development to meet that demand, along with relatively unfettered access to weather information via the public Internet, created confusion within the aviation community regarding the relationship between regulatory requirements and new weather products. Consequently, the FAA finds it necessary to differentiate between those weather products that may be used to comply with regulatory requirements and those that may only be used to improve situational awareness. To clarify the proper use of aviation weather products to meet the requirements of the regulations, the FAA developed the following definitions:

Primary Weather Product. An aviation weather product that meets all the regulatory requirements and safety needs, for use in making flight-related aviation weather decisions.

Supplementary Weather Product. A aviation weather product that may be used for enhanced situational awareness. If used, a supplementary weather product must only be used in conjunction with one or more primary weather products. In addition, the FAA may further restrict the use of supplementary weather products through limitations described in the product label.

NOTE: An aviation weather product produced by the Federal Government is a primary product unless designated as a supplementary product by the FAA.

B. In developing the definitions of primary and supplementary weather products, it is not the intent of the FAA to change or increase the regulatory burden upon certificate holders. Rather, the definitions are meant to eliminate confusion by differentiating between products that may be used to meet regulatory requirements and other products that may only be used to improve situational awareness.

C. All flight-related, aviation weather decisions must be based on primary weather products. Supplementary weather products augment the primary products by providing additional weather information, but may not be used as stand-alone products to meet aviation weather regulatory requirements or without the relevant primary products. When discrepancies exist between primary and supplementary products pertaining to the same weather phenomena, users must base flight-related decisions on the primary weather product. Furthermore, multiple primary products may be necessary to meet all aviation weather regulatory requirements.

D. As previously noted, the FAA may choose to restrict certain weather products to specific types of usage or classes of user. Any limitations imposed by the FAA on the use of a product will appear in the product label.

Types of Aviation Weather Information

The FAA has identified the following three distinct types of weather information that may be needed to conduct aircraft operations: observations, analyses, and forecasts.


Observations are raw weather data collected by some type of sensor(s). The observations can either be in situ (e.g. surface or airborne) or remote (e.g. weather radar, satellite, profiler, and lightning).


Analyses of weather information are an enhanced depiction and/or interpretation of observed weather data.


Forecasts are the predictions of the development and/or movement of weather phenomena based on meteorological observations and various mathematical models.

In-flight weather advisories, including Significant Meteorological Information (SIGMET), Convective SIGMETs, Airman’s Meteorological Information (AIRMET), Center Weather Advisories (CWA), and Meteorological Impact Statements (MIS), are considered forecast weather information products.

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