'Fine' or 'iawn' isn't an adjective that would be used that often for describing the weather in Welsh, more likely to be 'clir' or 'braf' for 'clear' and 'nice'. But for your question, it's difficult to say. 'Hi' is most certainly feminine, and the weather is a feminine word. But without context, especially in welsh, the emphasis usually falls on the masculine for unknown objects. So in this example, if it was 'it' without any context we would make it masculine; 'Dwi'n siwr bod o'n iawn'. Also 'it' without context, or 'it' as a sisutaion would be seen as 'mae' rather than 'bod', because 'bod' implies that the object is alive. So for your sentence it would be; 'Dwi'n siwr mae'n iawn'.