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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. 

## Significant Weather Prognostic Charts 

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. 

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 

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. 

## Additional Weather Information Skew-T Plots 

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 

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