I just read this great article on Rusty Burlew’s web site:
If you ever needed an excuse to buy one of those infrared cameras that plugs into a smartphone, this has to be it for the beekeeper! I bought a FLIR One camera for my husband last year, because he was worried about rain leaking in through a wall. The camera can detect the cool wet spot without making the holes in a wall that a humidity detector makes. After we had the leak repaired, he was very relieved to be able to check the wall after rain.
However, now we can use it to check on our bees! Not sure that they will cluster for any length of time in San Diego, but it will keep him busy playing with it.