Most professional green houses are completely automated with climat control and kept at a constant 80% humidity with around 15°C to 25°C night and day temperatur through out the year. During the summer periode when it's extremely hot outdoors temps will obviously rise in the greenhouse. Air conditioning to control this doesn't realy work, because air conditioning is air recycling. This isn't realy cost effective in a ventilated area..
In case you grow a variety of different plant spieces in the same greenhouse requiring different humidity levels. Than they use semi translucent plastic tents to devide the surface into sections. Than inside these tents humidity will rise up to over 90% while outside the tent it stays lower.
The average outdoor humidity on dry days is about 60% in temporate regions and on wet days up to 90%. Than obviously you are stuck with that, keeping an area at 80% with a refreshing it with 90% humidity can't be controled. But it doesn't mater, it should be best at least 80% humidity.
So because of these climate controlling difficulties a supplier as for example Dennerle grows their Java ferns via partners in Asia and imports them. Because the climate there is much more constant through out the year. This way they have a much more constant supply of plants in simmular condition all year long. 🙂
How plants excactly handle all the light they recieve is partialy still a big mystery.. We simply do not fully understand.. For example we use terms as compensation points which means how much PAR does a plant need to react to it and grow or not grow. I've grown plants in a greenhouse and experienced some plants in a specific corner growing deformed flowers. After a lot of thinking and searching i found out the cause, it was a power socket with a switch and this switch was iluminated with a small red light. It was a very faint red light comming from this switch and theoreticaly it shouldn't be enough to even get near the plants compensation point. But still that little light burning all night long confused the plant and it didn't know what to do, is it night or is it day? Should i flower or should i grow? And it did both and grew mallformed flowers with rather large internodes in between as only plant in the entire greenhous. I don't know if it was compensation point or wave lenght causing it.. I only know the plant reacted to it and in theory it shouldn't be strong enough, but it still was. I covered the lit switch with black tape and after that all was ok again and the plant did what it was suposed to do, grow normal flowers. 🙂 It beats me, it threw a lot of theories i red out the door..