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times reports may be as small as 150 feet between altitudes. Standard “wind barb” depictions are used to represent the wind direction and speed at different altitudes. 

Plymouth State University in Plymouth, New Hampshire operates a Weather Center with a website for locating VAD winds. This user-friendly website offers pilots a convenient selection of reporting stations at http://vortex.plymouth. edu/lnids_conus.html. 

## Aviation Forecasts 

Observed weather condition reports are often used in the creation of forecasts for the same area. A variety of different forecast products are produced and designed to be used in the preflight planning stage. Pilots need to be familiar with the following printed forecasts: wind and temperature aloft forecasts (FD reports), the terminal aerodrome forecast (TAF), aviation area forecast (FA), and inflight weather advisories (SIGMET, AIRMET). 

## Wind and Temperature Aloft Forecast (FD Report) 

Wind and temperature aloft forecasts provide wind and temperature forecasts for specific locations in the contiguous United States, plus network locations in Hawaii and Alaska. The forecasts are made twice a day based on the radiosonde upper air observations taken at 0000Z and 1200Z. Up through 12,000 feet are true altitudes; at and above 18,000 feet are 

pressure altitudes. Wind direction is always in reference to true north, and wind speed is given in knots. The temperature is given in degrees Celsius; no winds are forecast when a given level is within 1,500 feet of the station elevation. Similarly, temperatures are not forecast for any station within 2,500 feet of the station elevation. If the wind speed is forecast to be greater than 100 knots but less than 199 knots, the computer adds 50 to the direction and subtracts 100 from the speed. A sample FD report is shown in Figure 4-37 . 

To decode this type of data group, the reverse process must be accomplished. For example, when the data appears as “731960,” subtract 50 from the 73 and add 100 to the 19; the wind would be 230° at 119 knots with a temperature of –60°C. If the wind speed is forecast to be 200 knots or greater, the wind group is coded as 99 knots. When the data appears as “7799,” subtract 50 from 77 and add 100 to 99; the wind is 270° at 199 knots or greater. When the forecast wind speed is calm or less than 5 knots, the data group is coded “9900,” which means light and variable. 

## Aviation Routine Weather Report (METAR) 

METAR is an observation of current surface weather reported in a standard international format. While the METAR code has been adopted worldwide, each country is allowed to make 

