Humidity Questionsssss

Hi,
For some reason I seem to have a dysfunctional humidity understanding gene and I'm hoping someone here can help me resolve this issue. I understand that the higher the temp, the more water the air will hold and that the lower the temp the less water it takes to raise the humidity. What I don't understand is the best or proper way to control the humidity in my greenhouse. I'm growing mostly paph's, bromeliads and various other tropicals. From extensive reading and scanning forums I understand that the humidity for these types of flowers is recommended to be kept as a rough debatable guide between 60% & 80%. With the installation of a good weather station and the data being recorded and graphed on my pc I have come to the realization that the humidity here in Washington state (and everywhere else I assume) cycles daily from morning to night on average from a low of 40% in the day to as high 100% at night. (I never said I was quick…..) I’ve also installed a temp/humidity sensor in my greenhouse so I can monitor and track conditions in the greenhouse as well. You can see a 3 day chart of the temp and humidity inside my greenhouse at http://www.friedfish.us/Greenhouse.htm
I have to think that the humidity in the places most of these orchids originate has the same kind of cycle only the low being higher in the 60% range, but still peaking in the 100% range as here. My current control strategy is to try and maintain 60% - 80% as I mentioned above. I use a misting system to control the humidity during the day and a dehumidifier to keep it down at night. My reasoning for trying to keep it down to 80% at night is that from my understanding (which could be greatly skewed) is that plants take in moisture during the day and they release it at night and that in order for them to function properly they need to be able to evacuate a large portion of this moisture at night. If the humidity in the greenhouse is 100% and it’s raining in there even with 3 fans running for ventilation I don’t see how they can do this very effectively, hence the dehumidifier. But in the wild, isn’t that what these plants are exposed to more or less anyhow? Isn’t that why everything is covered with dew in the morning? Maybe I’m making more of this than I should, but that’s why places like this exist. Any input would be appreciated.

Thanks,

Marty

Your humidity measurements are relative humidity measurements. The air holds a certain amount of water vapor, the higher the temp, the more it holds. Air with a given partial pressure of water vapor can be cooled until the vapor condenses into dew...the dew point. The relative humidity is determined by how close the air temp is to the dew point. Air much hotter than the dew point (afternoon) has a greater capacity to hold water vapor and the RH is lower. Cooler air (night) is much closer to the dewpoint and so it has a higher RH. So, you have the same water vapor pressure all the time, but the RH, both in and outside of the GH, is 96% in the morning and (for me) 35-40% in the afternoon.

Transpiration, the loss of water from the plant, is greater during the day when the temps are higher and the plant is photosynthesizing. It may be one of the things that helps keep the plant cool. I don't know that moisture is "released" at night, but plants do respire at night, turning sugars back into water, CO2, and energy.

Dew has nothing to do with plants per se...think of dew drops on a spider web.

If I werre you, I'd aim for a min RH of 60% daytime, and then just let it go (without the dehumidifier) at night. I'd turn off the misters or foggers a couple of hours before dusk to see that the foliage gets dry.

SB

Thanks for the informative explanation SB. That's what I'm here for!