Why did the old man know there would be a breeze all night?

After the old man has caught the great fish, prepared it, and lashed it to his boat, he looks up at the sky. He sees that "there (are) high cumulus clouds and enough cirrus above them so that the old man (knows) the breeze would last all night". As he raises his sail into the gentle wind, he notes that it is "steady...it had backed a little further into the north-east and he (knows) that (means) that it (will) not fall off".

The old man is very attuned to nature. Long years of living and working on the sea have enabled him to read the subtle nuances of ocean swells and the weather. He "(does) not need a compass to tell him where southwest (is). He only (needs) the feel of the trade wind and the drawing of the sail". Over the years, Santiago has learned to read the sky and the elements like we in our more modern times would read a barometer or a compass. He is constantly noting the sun and the moon, the sky and the winds. Their position and condition tell him all he needs to know about the weather and where he is.

Santiago is so attuned to the natural conditions in his environment that he is able to predict what the weather will be like by through observation. On the night before he landed the big fish, he noticed that "the sky was clouding over to the east and...it looked now as though he were moving into a great canyon of clouds and the wind had dropped". He rightly predicted that "there will be bad weather in three or four days...but not tonight and not tomorrow".

Here is a simple explination:

There were high cumulus clouds and enough cirrus above them so that the old man knew the breeze would last all night.