According to the 'Use' section, point 6 states: 'We use it to talk about weather. e.g. It's a clear sunny day.'
Class 6 › English Grammar and Composition › Chapter 7: Introductory 'There' & 'It' › Topic: Understanding the use of 'it' for time, day, month, date, weather, and situations
Aligned to the NCTB national curriculum.