Thanks, Wendy.

Another thought, maybe as a continuation from the previous sentence, you could write this : But, it's increasingly rare today.

Sorry Mark, re-reading the text as a whole, I realise that this sentence is an adjunct to the previous sentence and refers to the fact that the sun seldom appears today, as is confirmed in the following sentence. So it does work if you change the punctuation at the end of the previous sentence to a comma or semi-colon. Or you could write this sentence as : This is increasingly rare today.

Mark, because the previous sentence ends in a full stop, it doesn't really work to start this one with 'Which'.
Also to avoid the repeition of 'rare' - perhaps 'It's seen even less often these days. ?