www.nws.noaa.gov, the NOAA site at http://aviationweather. gov/adds/metars/ and Dr. Jack’s regional forecasts at http:// www.drjack.info/BLIP/RUC. Interpreting Weather Charts, Reports,  and Forecasts Graphic Weather Charts Winds and Temperatures Aloft Forecast The winds and temperatures aloft forecast (FB) is a 12- hour product that is issued at 0000Z and 1200Z daily.  [Figure 9-34]  It is used primarily to determine expected  wind direction and velocity, and temperatures for the altitude  of a planned cross-country flight. The forecast contains nine  columns that correspond to forecast levels 3,000; 6,000;  9,000; 12,000; 18,000; 24,000; 30,000; 34,000; and 39,000  feet MSL. Soaring pilots planning to attempt proficiency  for altitude should be aware that the levels below 18,000  feet are based on local altimeter settings. Above 18,000  feet, flight levels are based on standard altimeter setting of  29.92 "Hg. For example 19,000 feet is FL190. Wind direction  is from true north. No winds are forecast within 1,500 feet of  station elevation. Also, no temperatures are forecast for the  3,000 foot level or for any level within 2,500 feet of station  elevation. Temperature is in whole degrees Celsius and  assumed to be negative above 24,000 feet.  Figure 9-34  shows  an example winds aloft message as well as how to decode it.  Reports of observed weather are graphically depicted in a  number of weather products. Among them are the surface  analysis chart, radar summary chart, weather depiction  chart, winds and temperature aloft chart, and the composite  moisture stability chart. For detailed information about  the surface analysis chart, weather depiction charts, and  radar summary chart, refer to the Pilot’s Handbook of  Aeronautical Knowledge and Advisory Circular 00-45G,  “Aviation Weather Services.” Knowing how and where to gather weather information  is important but the ability to interpret and understand the  information requires additional knowledge and practice.  Weather charts and reports are merely records of observed  atmospheric conditions at certain locations at specific times.  Trained observers using electronic instruments, computers,  and personal observations produce the weather products  necessary for pilots to determine if a flight can be conducted  safely. This same information can be used by soaring pilots  to determine where they can find lift and how long the lift is  usable for soaring flight. Figure 9-33.  Soaring forecast (continued). Maximum Rate of Lift - An estimate of the maximum strength of thermals. It is computed from an empirical formula which combines the expected  maximum height of thermals with the difference in the environmental  temperatures between the maximum height of thermals and the temperature 4,000 feet above the ground. After this computation, further empirical adjustments are made based on the value of the K-index and the amount and opacity of middle and high level cloudiness expected between the time of trigger temperature and the time of overdevelopment. Middle/High Clouds - The amount and opacity of middle (altostratus,  altocumulus) or high (cirrus, cirrostatus, cirrocumulus) clouds. Broken means that between 60% and 90% of the sky is covered by the cloud, with overcast conditions occurring when more than 90% of the sky is covered by the cloud. Thin implies that the clouds are predominantly transparent, meaning that some sunlight is reaching the ground, in contrast to opaque which suggests that little sunlight is reaching the ground. Potential Height of Wave - The minimum of the following two heights: 1. Level above the height of stable layer (or 14,000 feet if none exists) where the wind direction changes by 30 degrees or more 2. Level above the height of stable layer (or 14,000 feet if none exists) where the wind speed no longer increases with height PVA/NVA - Positive vorticity advection (PVA)/negative vorticity advection  (NVA) on the 500 millibar isobaric surface (approximately 18,000 feet  above mean sea level). Weak PVA has been shown to assist in mountain  wave soaring. Soaring Window - The time between the time the trigger temperature is reached and the time of overdevelopment. Thermal Index - The difference between the environmental temperature and the temperature at a particular level determined by following the dry adiabat through the forecast maximum temperature up to that level. Negative values are indicative of thermal lift. Thermal Soaring Index - An adjective rating (for sailplanes) based on the computed maximum rate of lift, and the wind speed and middle and high cloud cover expected during the soaring window (the time of the trigger temperature and the time of overdevelopment) according to the following: Maximum rate of lift Adjective Rating ≥ 800 fpm Excellent ≥ 400 and < 800 fpm Good ≥ 200 and < 400 fpm Fair < 200 fpm Poor Time of Overdevelopment - The time one or more of the following phenomena, which essentially shut off thermal lift, is expected to occur: 1. formation of broken to overcast convective cloud cover 2. formation of scattered to numerous downbursts 3. initiation of widespread precipitation Time of Trigger Temperature - The time the surface temperature is expected to reach the trigger temperature. Trigger Temperature - The surface temperature required to make the first 4000 feet of the atmosphere dry adiabatic. Wave Soaring Index - An empirical, adjective rating (for sailplanes) which attempts to combine a variety of phenomena important in mountain wave soaring into a single index number. Objective points are assigned to these phenomena: wind speed and direction at 14,000 ft MSL, the static stability in the 12,000-18,000 ft MSL layer, the wind speed gradient above the stable layer, jet stream location and frontal and upper trough movements. 9-23