Table of Contents

An international weather reporting code is used for weather reports (METAR) and forecasts (TAFs) worldwide. The reports follow the format shown in Figure 7-1.

For aviation purposes, the ceiling is the lowest broken or overcast layer, or vertical visibility into an obscuration.



Figure 7-1. TAF/METAR weather card

Pilot Report (UA)

Aircraft in flight are the only means of directly observing cloud tops, icing, and turbulence; therefore, no observation is more timely than one made from the cockpit. While the FAA encourages pilots to report inflight weather, a report of any unforecast weather is required by regulation. A Pilot Report, or PIREP (identified by the letters "UA") is usually transmitted in a prescribed format. See Figure 7-2.

Turbulence and icing should be reported by using the intensity tables shown in Figure 7-3 and Figure 7-4.


Figure 7-2. Pilot report form


Figure 7-3. Icing intensities


Figure 7-4. Turbulence reporting criteria

Terminal Aerodrome Forecast (TAF)

A Terminal Aerodrome Forecast (TAF) is a concise statement of the expected meteorological conditions at an airport during a specified period (usually 24 hours). TAFs use the same code used in the METAR weather reports (See Figure 7-1).

TAFs are issued in the following format:

TYPE / LOCATION / ISSUANCE TIME / VALID TIME / FORECAST

Note: The "/" above are for separation purposes and do not appear in the actual TAFs.

Aviation Area Forecast (FA)

The aviation Area Forecast is a forecast of general weather conditions over an area the size of several states. It is used to determine forecast enroute weather and to interpolate conditions at airports which do not have TAFs issued.

The Area Forecast is issued 3 times a day, and is comprised of the four sections: a communications and product header section, a precautionary statement section, and two weather sections; a SYNOPSIS section and a VFR CLOUDS/WX section.

Winds and Temperatures Aloft Forecast (FD)

The winds and temperatures aloft forecast is displayed in a 6-digit format (DDffTT). It shows wind direction (DD), wind velocity (ff), and the temperature (TT) that is forecast to exist at specified levels. For example, "23450" decodes as: winds from 230 degrees true north, at 45 knots, temperatures 02°C. When the wind speed (ff) is between 100 and 199 knots, the wind direction (DD) portion of the code will be greater than 50. In cases such as this, you will need to subtract 50 from the coded wind direction, and add 100 to the coded wind speed in order to decipher the code. For example, "734502" decodes as: winds 230 degrees, greater than 200 knots, temperature 02°C. Temperatures with a negative symbol in front of them (DDff-37) are negative. For flight levels above 24,000, temperatures are always negative and will not have a negative symbol. Light and variable winds or wind speeds below 5 knots are indicated by 9900, followed by the forecast temperature. For example, the coded winds aloft forecast for flight level FL270 (flight level 27,000) is "990017" and decodes as: winds are light and variable, temperature negative 17. The observed winds aloft chart shows temperature, wind direction, and speed at selected stations. Arrows with pennants and barbs indicate wind direction and speed. Each pennant is 50 knots, each barb is 10 knots, and each half barb is 5 knots. Wind direction is shown by an arrow drawn to the nearest 10 degrees, with the second digit of the coded direction entered at the outer end of the arrow. Thus, a wind in the northwest quadrant with the digit 3 indicates 330 degrees, and a wind in the southwest quadrant with the digit 3 indicates 230 degrees.

Inflight Weather Advisories (WA, WS, WST)

Inflight Weather Advisories advise pilots en route of the possibility of encountering hazardous flying conditions that may not have been forecast at the time of the preflight weather briefing.

airmets (WA) contain information on weather that may be hazardous to single engine, other light aircraft, and VFR pilots. The items covered are moderate icing or turbulence, sustained winds of 30 knots or more at the surface, widespread areas of IFR conditions, and extensive mountain obscurement.

sigmets (WS) advise of weather potentially hazardous to all aircraft. The items covered are severe icing, severe or extreme turbulence, and widespread sandstorms, dust storms or volcanic ash lowering visibility to less than 3 miles.

sigmets and airmets are broadcast upon receipt and at 30-minute intervals (H + 15 and H + 45) during the first hour. If the advisory is still in effect after the first hour, an alert notice will be broadcast. Pilots may contact the nearest FSS to ascertain whether the advisory is pertinent to their flights.

convective sigmets (WST) cover weather developments such as tornadoes, lines of thunderstorms, and embedded thunderstorms; they also imply severe or greater turbulence, severe icing, and low-level wind shear. When a SIGMET forecasts embedded thunderstorms, it indicates that the thunderstorms are obscured by massive cloud layers and cannot be seen. Convective SIGMET bulletins are issued hourly at H + 55. Unscheduled convective sigmets are broadcast upon receipt and at 15-minute intervals for the first hour (H + 15; H + 30; H + 45).

En Route Flight Advisory Service (EFAS)

En Route Flight Advisory Service (EFAS) is a service specifically designed to provide en route aircraft with timely and meaningful weather advisories pertinent to the type of flight intended, route of flight, and altitude. It provides communications capabilities for aircraft flying at 5,000 feet above ground level to 17,500 feet MSL on a common frequency of 122.0 MHz. Contact Flight Watch by using the name of the ARTCC facility identification serving the area of your location, followed by your aircraft identification, and the name of the nearest VOR to your position.

Radar Weather Report

Radar Weather Reports are of special interest to the pilot because they indicate the location of precipitation along with type, intensity, and trend.

The Radar Summary Chart will yield a three-dimensional view of clouds and precipitation when used in conjunction with other charts and reports. Since radar detects only drops or ice particles of precipitation size, it does not detect clouds or fog. It is the only chart that shows lines and cells of hazardous thunderstorms.

Areas from which precipitation echoes were received are shown on the Radar Summary Chart, with echo intensity indicated by contours as shown in Figure 7-5. Echo heights are displayed in hundreds of feet MSL. Tops are entered above a short line, while any available bases are entered below a short line. Movement of an area of echoes is indicated by a shaft and barb combination with the shaft indicating the direction, and the barb indicating the speed. A whole barb is 10 knots, a half barb is 5 knots, and a pennant is 50 knots. Individual cell movement is indicated by an arrow that shows the direction of movement with the speed in knots entered as a number. Severe weather watch areas are outlined by heavy dashed lines, usually in the form of a large


Figure 7-5. Radar weather report

Surface Analysis Chart

The Surface Analysis Chart depicts frontal positions, pressure patterns, temperature, dew point, wind, weather, and obstructions to vision as of the valid time of the chart. Pressure patterns are shown with isobar lines around the highs and lows. Isobars are solid lines depicting sea level pressure patterns. They are usually spaced at 4-millibar intervals; close spacing of the isobars indicates a strong pressure gradient. When the pressure gradient is weak, dashed isobars are sometimes inserted at 2-millibar intervals to more clearly define the pressure pattern.


Figure 7-6. Total sky cover symbols


Figure 7-7. Examples of plotted data on the weather depiction chart

Weather Depiction Chart

The Weather Depiction Chart, which shows conditions that existed at the valid time of the chart, allows the pilot to readily determine general weather conditions on which to base flight planning.

A legend in the lower right-hand corner of the chart describes the method of identifying areas of VFR, MVFR, and IFR conditions.

Total sky cover is depicted by shading of the station circle as shown in Figure 7-6. Cloud height is entered under the station circle in hundreds of feet above ground level. Symbols for weather and obstructions to vision are normally shown to the left of the station circle.

Visibility (in statute miles) is entered to the left of any weather symbol. If the visibility is greater than 6 miles it will be omitted. Figure 7-7 shows examples of plotted data.

Constant Pressure Chart

A Constant Pressure Analysis Chart is an upper air weather map where all the information depicted is at the specified pressure-level of the chart. Each of the Constant Pressure Analysis Charts (850 MB, 700 MB, 500 MB, 300 MB, 250 MB, and 200 MB) can provide observed temperature/dewpoint spread, wind, height of the pressure surface, and the height changes over the previous 12-hour period.

Tropopause Height/Vertical Wind Shear Prognostic Chart

The Tropopause Height/Vertical Wind Shear Prognostic Chart is a two-panel chart containing a maximum wind prog and a vertical wind shear prog. The chart is prepared for the contiguous 48 states and is available once a day with a valid time of 18Z. It shows the temperature, pressure, and wind at the tropopause.

Significant Weather Prognostics

The Low-Level Significant Weather Prognostic Chart (surface to 24,000 feet) portrays forecast weather which may influence flight planning, including those areas or activities of most significant turbulence and icing. It is a four-panel chart; the two lower panels are 12- and 24-hour surface progs. The chart is issued four times daily. The chart uses standard weather symbols as shown in Figure 7-8.

The High-Level Significant Weather Prognostic Chart (24,000 feet to 63,000 feet) outlines areas of forecast turbulence and cumulonimbus clouds, shows the expected height of the tropopause, and predicts jet stream location and velocity. The chart depicts clouds and turbulence as shown in Figure 7-9.

The height of the tropopause is depicted in hundreds of feet MSL and is enclosed in a rectangular box. Areas of forecast moderate or greater Clear Air Turbulence (CAT) are bounded by heavy dashed lines and are labeled with the appropriate symbol and the vertical extent in hundreds of feet MSL. Cumulonimbus clouds imply moderate or greater turbulence and icing.


Figure 7-8. Significant weather prognostics


Figure 7-9. Prognostic chart depictions

Composite Moisture Stability Chart

Composite Moisture Stability Charts are analysis charts using observed upper air data. These charts are available twice daily with valid times of 12Z and 00Z. They contain four panels: freezing level, precipitable water, average relative humidity, stability.

The stability panel outlines areas of stable and unstable air. Two stability indices are computed for each upper air station:

Lifted index (LI) -- This index indicates the stability at 500 mb (18,000 feet MSL).

K index (K) -- This index examines the temperature and moisture profile of the environment. The number above is the lifted index, and the number below the line is the K index. A positive lifted index means that a parcel of air, if lifted, would be colder than existing air at 500 mb. For example, 23 over 28 indicates that the air is very stable. There will be stratified cloudiness and steady precipitation. A negative lifted index means that the low-level air, if lifted to 500 mb would be warmer than existing air at 500 mb. For example, -1 over 35 indicates that the air is unstable, and there is an air-mass thunderstorm probability of 61% to 80%.