Aviation Weather Services

Also reported were tornadoes, hail to 2 inches in diameter, and altitude is 6,000 feet for the forecast winds. In this case, wind gusts to 65 knots possible in the North Dakota portion. “2714” means the wind is forecast to be from 270° at a speed of 14 knots. Winds and Temperature Aloft Forecast (FB) Winds and temperatures aloft forecasts (FB) provide wind A six-digit group includes the forecast temperature aloft. and temperature forecasts for specific locations throughout The elevation at Denver (DEN) is 5,431 feet, so the lowest the United States, including network locations in Hawaii reportable altitude is 9,000 feet for the winds and temperature and Alaska. The forecasts are made twice a day based on the forecast. In this case, “2321-04” indicates the wind is forecast to radiosonde upper air observations taken at 0000Z and 1200Z. be from 230° at a speed of 21 knots with a temperature of –4 °C. Altitudes through 12,000 feet are classified as true altitudes, Weather Charts while altitudes 18,000 feet and above are classified as Weather charts are graphic charts that depict current or altitudes and are termed flight levels. Wind direction is forecast weather. They provide an overall picture of the always in reference to true north, and wind speed is given in United States and should be used in the beginning stages of knots. The temperature is given in degrees Celsius. No winds flight planning. Typically, weather charts show the movement are forecast when a given level is within 1,500 feet of the of major weather systems and fronts. Surface analysis, station elevation. Similarly, temperatures are not forecast for weather depiction, and significant weather prognostic charts any station within 2,500 feet of the station elevation. are sources of current weather information. Significant weather prognostic charts provide an overall forecast weather If the wind speed is forecast to be greater than 99 knots but picture. less than 199 knots, the computer adds 50 to the direction and subtracts 100 from the speed. To decode this type of data Surface Analysis Chart group, the reverse must be accomplished. For example, when The surface analysis chart depicts an analysis of the current the data appears as “731960,” subtract 50 from the 73 and surface weather. [Figure 13-10] This chart is transmitted add 100 to the 19, and the wind would be 230° at 119 knots every 3 hours and covers the contiguous 48 states and with a temperature of –60 °C. If the wind speed is forecast to adjacent areas. A surface analysis chart shows the areas of be 200 knots or greater, the wind group is coded as 99 knots. high and low pressure, fronts, temperatures, dew points, wind For example, when the data appears as “7799,” subtract 50 directions and speeds, local weather, and visual obstructions. from 77 and add 100 to 99, and the wind is 270° at 199 knots Surface weather observations for reporting points across or greater. When the forecast wind speed is calm, or less than the United States are also depicted on this chart. Each of 5 knots, the data group is coded “9900,” which means light these reporting points is illustrated by a station model. and variable. [Figure 13-9] [Figure 13-11] A station model includes: • Sky cover—the station model depicts total sky cover Explanation of Figure 13-9: and is shown as clear, scattered, broken, overcast, or The heading indicates that this FB was transmitted on the obscured/partially obscured. 15th of the month at 1640Z and is based on the 1200Z upper air data. The valid time is 1800Z on the same day and should • Sea level pressure—given in three digits to the nearest be used for the period between 1400Z and 2100Z. The tenth of a millibar (mb). For 1,000 mbs or greater, heading also indicates that the temperatures above FL240 prefix a 10 to the three digits. For less than 1,000 mbs, are negative. Therefore, the minus sign will be omitted for prefix a 9 to the three digits. all forecast temperatures above FL240. • Pressure change/tendency—pressure change in tenths of mb over the past 3 hours. This is depicted directly A four-digit data group shows the wind direction in reference below the sea level pressure. to true north and the wind speed in knots. The elevation at • Dew point—given in degrees Fahrenheit. Amarillo, Texas (AMA) is 3,605 feet, so the lowest reportable • Present weather—over 100 different standard weather FB KWBC 151640 symbols are used to describe the current weather. DATA BASED ON 151200Z • Temperature—given in degrees Fahrenheit. VALID 151800Z FOR USE 1400-2100Z TEMPS NEGATIVE ABV 24000 • Wind—true direction of wind is given by the wind FB 3000 6000 9000 12000 18000 24000 30000 pointer line, indicating the direction from which the AMA 2714 2725+00 2625-04 2531-15 2542-27 265842 wind is blowing. A short barb is equal to 5 knots of DEN 2321-04 2532-08 2434-19 2441-31 235347 wind, a long barb is equal to 10 knots of wind, and a Figure 13-9. Winds and temperature aloft forecast. pennant is equal to 50 knots. 13-13