Greenhouse and humidity - how much frost protection does high humidity give?

Hello all! I was wondering how much protection a high humidity level in a greenhouse gives when the temperature hits freezing? My understanding is that if the humidity level is high then plants can better deal with freezing temperatures but I'm not sure how much protection this gives. Would a plant that is good down to say 33 F be able to handle a frost (say at 30 to 32 F) if the humidity level was high? I'm partially wondering about this because I currently have a mini-greenhouse and while it is not the best at holding a good temperature it does effectively increase humidity levels and I'm trying to figure out what is the benefit of having the high humidity level. Thanks!

Daron,
I haven't heard this. Can you explain the reasoning or give where you saw this piece of advice?
It's true that when water freezes it gives off a bit of heat via the phase change... a reason why sometimes people spray crops before a freezing night... but I don't see how water vapor in the air would have the same effect. Interested to learn more.

Lindsey Schiller wrote:Daron,
I haven't heard this. Can you explain the reasoning or give where you saw this piece of advice?
It's true that when water freezes it gives off a bit of heat via the phase change... a reason why sometimes people spray crops before a freezing night... but I don't see how water vapor in the air would have the same effect. Interested to learn more.

Here are some links that talk about this a bit - they talk about humidity and frost protection but not specifically greenhouses:

Principles of Frost Protection

Frost Protection Techniques

Here is a quote from the second link:

The dew point is the temperature at which the relative humidity reaches 100% as the air cools. At this point, water vapor in the air condenses into fog or dew, which gives off heat, slowing the temperature drop. The risk of having a frost becomes greater as the dew point becomes lower. If the dew point is below freezing, so that condensation and heat release does not take place until below freezing, temperatures can drop to damaging levels extremely rapidly. In this case, the white crystals typically seen in a frost or freeze may not form, a condition sometimes referred to as a "black frost".

I have read similar things from a number of different sites - some just general info sites and others like the two above from education sites. Seems like the key is to keep the humidity level high so when the temperature drops in the greenhouse it reaches 100% while being above freezing. Then as the temperature continues to drop the water vapor will condense (this may form ice on the greenhouse cover) and give off a little bit of heat. In my small greenhouse this may not provide much if any protect but I could see it being beneficial for a larger greenhouse - something that could help give a couple degrees of protection?

This is all new to me and I would appreciate any thoughts you all have on this!

Thanks Lindsey!

Hi Daron,

I see. Yes, these are referring to the same effect as I mentioned above... that water gives off some heat as it changes from a liquid (water) to solid (ice) through latent heat. They just assume that the vapor is condensing from humidity first, which of course makes sense.

But basically your greenhouse is going to be humid at night regardless. Plants respire water during the day, so you often get high humidity levels during the day (hopefully not too high...) at night the temperature drops. Colder air can hold less water so the relative humidity drops... usually to 100% percent (= condensation). So you will get this effect no matter what if your greenhouse freezes.. you don't have to increase humidity Most greenhouse growers try to keep humidity down, bc it has many negative effects for plant health.

Note that it's not a super dramatic heating effect or anything. Most people try to keep their greenhouses above freezing anyway so I have never heard of someone trying to utilize this effect. I have mostly heard about it for outdoor farmers... their crops are going to freeze anyway on a freezing night. To give a degree or two (my best guess) of protection they spray crops to take advantage of latent heat as the water freezes.

Thanks for the response. In my area this time of the year we are generally above freezing at night but occasionally it might dip just below freezing for a single night. A lot of people here have very simple greenhouses since we don't need much protection. I was trying to figure out if the heat given off through the phase change of water would be enough to get through these rare freezing nights. My greenhouse being so small with so little thermal mass can't hold the temp much above the air temp at night. I have added some thermal mass but I'm very limited in regards to space. But it can keep the humidity at a high level so for this type of simple and small greenhouse I got curious if this would provide enough protection to get through our rare freezing nights.

Thanks again for your response!

Not sure exactly how it would apply, but cranberry growers here all used to "watch frost".  Now it is all automated, but the same applies.  On nights that the temps go below freezing, they run sprinklers on the berry vines all night and it keeps the frost from damaging them.  Without the sprinklers, they could lose the entire crop.  Maybe high enough humidity has some of the same effect?  I don't know exactly how cold it can get without harming the cranberries with the sprinklers running, but I have a couple friends that are growers, I could ask them.