How did Captain John Nicholas of the Adventure Know that there was a storm coming up?

The wind stopped and the waters were completely calm, which gave Gulliver a source of comfort after the weather they'd sailed through. Captain John, however, an experienced seaman, knew it was merely the calm before the storm.

But he, being a man well experienced in the navigation of those seas, bid us all prepare against a storm, which accordingly happened the day following: for the southern wind, called the southern monsoon, began to set in.