information at lower altitudes, and charts with lower pressure  altitudes present information at higher altitudes.  Symbology on the constant pressure analysis chart is  the same as that of the surface analysis chart. This chart  depicts the information at a specific pressure altitude. When  compared with the surface analysis of the same time frame,  a three-dimensional concept of the atmosphere can be  conceptualized, and the pilot can gain a greater understanding  of the atmospheric dynamics involved in weather patterns.  left corner of each panel. The upper two panels show forecast  significant weather, which may include nonconvective  turbulence, freezing levels, and IFR or MVFR weather. Areas  of moderate or greater turbulence are enclosed in dashed  lines. Numbers within these areas give the height of the  turbulence in hundreds of feet MSL. Figures below the line  show the anticipated base, while figures above the line show  the top of the zone of turbulence. Also shown on this panel  are areas of VFR, IFR, and MVFR. IFR areas are enclosed by  solid lines, MVFR areas are enclosed by scalloped lines, and  the remaining, unenclosed area is designated VFR. Zigzag  lines and the letters “SFC” (surface) indicate freezing levels  in that area are at the surface. Freezing level height contours  for the highest freezing level are drawn at 4,000 foot intervals  with dashed lines. Significant weather prognostic charts  [Figure 4-34]  display  the observed or forecast significant weather phenomena at  different flight levels that may affect the operation of aircraft.  They are available for low-level significant weather from the  surface to FL 240 (24,000 feet), also referred to as the 400  mb level, and high-level significant weather from FL 250 to  FL 600 (25,000 to 60,000 feet). This discussion involves the  low-level significant weather prognostic chart.  Most weather information is derived from radiosondes, or  weather balloons, that are released from over 100 stations in  the United States twice daily (00Z and 12Z). The observations  of temperature and humidity at various pressure altitudes are  transmitted back to the releasing station; the radiosondes are  also tracked by radar in order to determine wind speed and  direction. This information is plotted to create a diagram  known as a Skew-T/Log-P plot, commonly referred to as a  Skew-T. These plots can be found at many different online  The low-level chart comes in two forms: the 12- and 24-hour  forecast chart, and the 36- and 48-hour surface only forecast  chart. The first chart is a four-panel chart that includes 12- and  24-hour forecasts for significant weather and surface weather.  Charts are issued four times a day at 0000Z, 0600Z, 1200Z,  and 1800Z. The valid time for the chart is printed on the lower  $_{Figure 4-34.}$  Significant weather prognostic chart. 4-34 Additional Weather Information  Skew-T Plots Significant Weather Prognostic Charts