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Unformatted text preview: G 1 September 11, 2007 SKEW-T, LOG-P DIAGRAM ANALYSIS PROCEDURES I. THE SKEW-T, LOG-P DIAGRAM The primary source for information contained in this appendix was taken from the Air Weather Service Technical Report TR-79/006. 1 The Skew-T, Log-P Diagram is the standard thermodynamic chart in use in most United States weather service offices today. This diagram is a graphic representation of pressure, density, temperature, and moisture, in a manner such that the basic atmospheric energy transformations are visually depicted. A unit of area on the diagram represents a specific quantity of energy. This diagram, when plotted with the various meteorological elements received from an upper air sounding, presents a vertical picture of the atmospheric conditions present at the time of observation and allows for computations of various parameters required by forecasters. The Skew-T, Log-P diagram is preferred because: (a) most of the important isopleths are straight rather than curved, (b) the angle between the adiabats and isotherms is large enough to facilitate estimates of the stability, (c) the ratio of area on the chart to thermodynamic energy is the same over the whole diagram, (d) the vertical in the atmosphere approximates the vertical coordinate of the diagram (i.e. the isobars are plotted to a logarithmic scale and pressure in the atmosphere decreases nearly logarithmically with height), and (e) an entire sounding to levels in the stratosphere can be plotted on one chart. The "parcel" method is the process used in determination of unmeasured parameters and analysis of the characteristics of the atmosphere above a station. In the parcel method, a small quantity of air is moved upward or downward in the atmosphere and the changes to its characteristics are determined and compared with the surrounding, unchanged air. One might imagine this parcel to behave as a very thin balloon, which expands or contracts as the parcel moves upward or downward, but which does not allow mixing with the surrounding air. This is, of course, somewhat unrealistic since as air rises (e.g., as thermals) or sinks, it mixes, to some extent, with the surrounding air. The Skew-T, Log-P diagram is also considered a "pseudo-adiabatic diagram" in that it is derived from the assumption that the latent heat of condensation is used to heat the air parcel, and that condensed moisture falls out immediately. Similarly, the above assumption does not represent the observed changes which occur as air is lifted. However, the results are sufficiently accurate to provide estimates in the right order of magnitude, and when used with other forecasting considerations, will prove useful. G 2 September 11, 2007 FIG. 1. Coordinate system of the Skew-T, Log-P Diagram. ...