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.
Graphic Weather Charts
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.”
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.
NOTE: The winds aloft forecasts were formally known as FD.
Composite Moisture Stability Chart
The composite moisture stability chart is a four-panel chart, which depicts stability, precipitable water, freezing level, and average relative humidity. It is a computer-generated chart derived from upper-air observation data and is available twice daily with a valid time of 0000Z and 1200Z. This chart is useful for determining the characteristics of a particular weather system with regard to atmospheric stability, moisture content, and possible aviation hazards, such as thunderstorms and icing. [Figure 9-35]
For the purpose of soaring flight, the stability panel located in the upper left corner of the chart [Figure 9-35 Panel A] can be useful when obtaining weather as it outlines areas of stable and unstable air. The numbers on this panel resemble fractions; the top number is the lifted index (LI) and the lower number is the K index (KI). [Figure 9-36] The LI is the difference between the temperature of a parcel of air being lifted from the surface to the 500 mb level (approximately 18,000 feet MSL) and the actual temperature at the 500 mb level. If the number is positive, the air is considered stable. For example, a lifted index of +8 is very stable, and the likelihood of severe thunderstorms is weak. Conversely, an index of –6 or less is considered very unstable, and severe thunderstorms are likely to occur; however, the instability may give rise to favorable soaring conditions. A zero index is neutrally stable. [Figure 9-37]
The chart in Figure 9-37 shows relative instability in two ways. First, the station circle is darkened when the lift index is zero or less. Second, solid lines are used to delineate areas that have an index of +4 or less at intervals of 4 (+4, 0, –4, –8). The stability panel is an important preflight planning tool because the relative stability of an airmass is indicative of the type of clouds that can be found in a given area. For example, if the airmass is stable, a pilot can expect smooth air and, given sufficient moisture, steady precipitation. On the other hand, if the airmass is unstable, convective turbulence and showery precipitation can be expected.
The K index indicates whether the conditions are favorable for airmass thunderstorms. The K index is based on temperature, low-level moisture, and saturation. A K index of 15 or less would be forecast as a 0 percent probability for airmass thunderstorms, and an index of 40 or more would be forecast as 100 percent probability. [Figure 9-38]
Caution should be exercised if the K index is high, indicating moisture is sufficient for storm development. Although the lifting index may be very negative and good for soaring, too much moisture can make good soaring conditions very dangerous once the storms develop. A very negative LI with high KI may mean a better day to soar close to the gliderport, whereas very negative LI and low KI values make for clear soaring conditions in general.