Complex sentence: The monsoon conditions had been bad and climbing had not been possible over and it was not realised that in 1935 the monsoon had been exceptionally late (starting 26 June)--at this time the timing of the monsoon was not really understood at all.
Split into simpler sentences: The monsoon conditions had been bad and climbing had not been possible over. Also, it was not realised that in 1935 the monsoon had been exceptionally late (starting 26 June)--at this time the timing of the monsoon was not really understood at all.