Halliday & Matthiessen (2014: 309-10):
On this borderline between the ‘existential’ and the ‘material’ there is a special category of processes to do with the weather: meteorological processes like it’s raining, the wind’s blowing, there’s going to be a storm. Some are construed existentially, e.g. there was a storm/hurricane/breeze/gale/shower/blizzard. Some are construed as material events, e.g. the wind’s blowing, the sun’s shining, the clouds are coming down. Some are construed as relational attributives: it’s foggy/cloudy/misty/hot/humid/sunny/frosty; here the it can be interpreted as a Carrier, since it is possible to substitute the weather, the sky or the (time of) day. Finally, some are construed as it + a verb in the ‘present in present’ tense: it’s raining/hailing/snowing/freezing/pouring/drizzling/lightning/thundering.