Simrad SH90 - REV A, Operator'S Manual

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Basic theory
Deflections
On warm days – especially during summer – the sea surface will
be heated by the sun, and the water temperature will gradually
decrease with increasing depth.
Figure 19 Water temperatures on warm days
(CD015005G)
B
A
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.
Figure 20 Water temperatures on cold days
(CD015005H)
B
A
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.
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