Simrad SH90 - REFERENCE MANUAL REV A Reference Manual page 187

Figure 45 Water temperatures on warm days
Figure 46 Water temperatures on cold days
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When the temperature decreases (A) closer to the bottom, the
sonar beam is deflected down as indicated by the illustration. The
school of fish (B) you thought you would see is not visible at all.
On cold days, you may experience the opposite: the cold water
will be close to the surface, and the water temperature increases
with increasing depth.
When the temperature decreases (A) closer to the surface, the
sonar beam is deflected up as indicated by the illustration. The
school of fish (B) you thought you would see is once again not
visible at all, and you will also see a ring of surface echoes.
Basic theory
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