Explain the purpose of the descriptions at the beginning of the act

Storms are always of great significance in Shakespeare; here, the storm is a symbol of unrest and the discord to come. The storm marks the end of the peaceful part of the play, a signal that Iago's mischief is about to begin. The storm is also a great example of Shakespeare's deft use of language. The characters that comment on the storm are mariners, alluding to Ursa Minor and stars used for navigation; this is a testament to Shakespeare's ability to craft credible dialogue for a great diversity and range of characters. You can check more out here: