To be fair, unless he was an metereologist, he would hardly know how fogs and winds appeared and vanished. The fisherman must of had a life long experience for him to know the wind patterns of the river, still that doesn't deny the fact that he was the best strategist ever.
Likewise, Liu Bei is depicted in the Romance as the most virtuous and kind of all people, though in reality he was much more 'grey'. For instance, it was Liu Bei himself who ordered the beating of Cao Xing which led to the attack of Lu Bu. The Romance also has Liu Bei look like a weak or mediocre general, while in reality he was far more able in commanding armies than in the Romance.
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