PRIMUSr 880 Digital Weather Radar System
When the antenna is tilted downward for ground mapping, two phenomena may occur that can confuse the pilot. The first is called ”The Great Plains Quadrant Effect”that is seen most often when flying over the great plains of central United States. In this region, property lines (fences), roads, houses, barns, and power lines tend to be laid out in a stringent north- south/east- west orientation. As a result, radar returns from these cardinal points of the compass tend to be more intense than returns from other directions and the display shows these returns as bright north/south/east/west spokes overlaying the ground map.
The second phenomenon is associated with radar returns from water surfaces (generally called sea clutter), as shown in figure 5- 3. Calm water reflects very low radar returns since it directs the radar pulses onward instead of backward (i.e. the angle of incidence from mirrored light shone on it at an angle). The same is true when viewing choppy water from the upwind side. The downwind side of waves, however, can reflect a strong signal because of the steeper wave slope. A relatively bright patch of sea return, therefore, indicates the direction of surface winds.
REFLECTION 
CALM WATER OR WATER WITH |
| CHOPPY WATER PROVIDES | ||
SWELLS DOES NOT PROVIDE |
| GOOD RETURN FROM | ||
GOOD RETURN. |
| DOWNWIND SIDE OF WAVES | ||
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| WIND DIRECTION AT | |
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| SURFACE OF WATER | |
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PATCH
OF SEA
RETURNS
AD- 12056- R2@
Sea Returns
Figure 5- 3
Radar Facts | A28- 1146- 102- 00 |
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