Humidity Versus Air Temperature
For a long time I have been confused about how colder air can hold less moisture. I would have thought that the reverse was true, with cold air being denser. Regardless of HOW it happens, here is why I feel it should be important to the terrarium keeper:
Humidity is a measurement of how much water is in the air, versus how much it is capable of holding. As the temperature goes down, air can hold less water and the humidity rises because the CAPACITY is going down with the temperature. It's like having a glass that is 80% full, and then you shrink the glass. The water spills over, leaving a glass that is 100% full, and a mess on the floor. That's how it is when the air cools. The "spilled water" is the dew all over everything in the morning. As the temperature rises, the glass is getting bigger again, and while a little water is returned (the dew evaporating), much is lost as air currents (and terrarium fans) carry it away, and needs to be replenished.
With a highland terrarium, where the temperature drops every night, this can cause problems the next morning. A good solution is a misting system, which replaces the lost moisture, sometimes faster than it is lost. An other solution for smaller terrariums is frequent misting, but this can get to be quite a drag over time! Yet another is a sort-of swamp cooler/mini wet wall set up, where air is drawn in through a sheet of water-saturated material. Some people have gotten really fancy, and hooked up a humidistat to their fogging system, and it would hold a preset range automatically.
Just my two cents.
If you shake a rain stick, you get rain. I need a hamata stick.
The temp/humidity relationship is similar to the fact that sugar dissolves easier in warm water than it does in cold. The chemistry explanation is something like this...warm air has more energy, so it has the capacity to hold a greater amount of water. As it cools, it energy is lost and so is water in the form of condensation/dew.
I have never grown highland Neps, so forgive me if my method isn't relevant here. I do grow high elevation orchids, so there should be some similarities. My solution to humidity is to run an ultrasonic humidifier for the time that the lights are on. This ensures that the humidity is high at the highest temps (lights on) and is maintained as the temp drops at night. The next morning, when lower temps leads to lower total air moisture, the humidifier kicks back on and the total air moisture rises as the temperature does. This has worked with me for my orchids for years, and I rarely see issues with too much moisture. I just run computer fans 24/7 to keep the air constantly circulating.