The plan position indicator (PPI) scope provides range and bearing information on weather echoes. Figure 10-4-1, views A and B, illustrates precipitation echoes as seen on a PPI scope. A standard scope presentation is shown in view A, and a contoured echo display is shown in view B. The latter is a feature available on many of

Figure 10-4-1.Precipitation echoes as seen on a PPI scope. A. Typical PPI showing convective precipitation echoes and ground clutter. Range markers are every 25 nautical miles. B. Contoured echo display on PPI of same echoes.

todays weather radars. The intensity levels are color coded, thereby permitting a quick deter-mination of intensity gradients (strong to weak, weak to strong, etc.) and maximum intensities. Weather echoes are classified according to their cinfiguration, coverage, continuity of pattern, and meteorological processes. The echo systems are then grouped for ease in reporting, according to the following definitions:

. AREARelated or similar echoes that can be readily associated geographically.

. LINERelated or similar convective echoes having a length-to-width ratio of at least 5 to 1. The line must be at least 30 nautical miles long.

. CELL(S)One or more convective echoes that, for reasons of isolation, intensity, or character, cannot be grouped with other echoes. 

. STlLATIFIED ELEVATED ECHOPre-cipitation aloft. 

. SPIRAL BAND AREAEchoes asso-ciated with tropical storms and hurricanes/ typhoons and systematically arranged in curved lines. These echoes may include a wall cloud.

. EYEA well-defined central region of a tropical storm or hurricane.

. CENTERA poorly defined central region of a tropical storm or hurricane.

. FINE LINEA narrow nonprecipitation echo pattern associated with the cold air outflow in advance of a squall line or the leading edge of a sea breeze front.

The appearance of echoes on a PPI scope can give you some idea of the weather to which they are related. For instance, scattered echoes may be related to air-mass weather; lines of convective echoes may be related to squall lines or fronts; certain characteristic curves in echo lines maybe related to frontal waves; and widespread relatively uniform echo patterns may be related to stable warm frontal precipitation. Now, lets discuss the appearance of some specific weather phenomena on the PPI scope.

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