If low humidity prevailed even at night, would you consider running
the misting system without a timer (with perhaps a humidistat in
series with the leaf switch)? Or is it unrealistic to encounter low
humidity at night? Being located in Arizona, you are probably
knowledgeable about such things.

Depends upon where your misters are going, as well as the size of
the mist. If your misters would be delivering water straight to the plants
(i.e., where it might accumulate in the crowns or on the leaves or other
locations vulnerable to rot), then you're just asking for trouble. Many
orchids are CAM plants, which means the stomata open at night. When the
stomata are open, the fungi can get in and rot the plants. My mycology
prof did some work with one of the big chip fabs, which made it possible
to produce little slabs of metal with stoma-sized holes in them. The slabs
were also "striped" in the same way that cells run parallel to the length
of the leaf.

When the fungal spores germinated, they followed the parallel
lines, and when they found a hole, BLOOP! Right down into the hole. It was
incredible. Beautiful scanning electron micrograph images of it all. My
only remaining question is why they can't do the same when the stoma are
only partially open, since stoma never really close tightly. You'd think
the mycelia would be able to snake down in there.

ANYWAY. I suspect this is one of the reasons why orchid rule
number 12 (or whatever) is to water and mist during the day. Thanks to the
unusual physiology of orchids, water on the leaves at night is just asking
for trouble. Further, I have no proof, but a sneaking suspicion that the
fertilizers we add to solution encourage fungal growth. Whether these
fungi are pathogenic or not is, of course, an unanswered question. Urea
(as found in Peter's and some other fertilizers) is particularly suspect.

This rule is, I am sure, broken by a number of people who will
report not only complete success, but hundreds of FCC's, CCM's, CBR's,
FBI's, NSA's, and M-O-U-S-E's, despite watering them at night. Until such
time that one is elevated to Diety of Much Orchidaceous Wisdom, keep it
simple and don't water at night, and don't mist such that water gets on
the leaves when the sun isn't up.

Depending upon how your system is run, you may find that humidity
increases after dark (as it gets cooler, water holds less moisture, so
relative humidity goes up without adding any moisture), or decreases (if
your misters are off, and your greenhouse exchanges air with the outside
in significant quantities). Fortunately for us, orchid growers are often
willing to tweak or tinker to achieve desired results.

Cheers,

-AJHicks
Chandler, AZ