Man, I was about to Google, FTB Clouds.
F*** all of you. =(
Imagine a cloud block though. Semi-transparent to dark opaque depending on the humidity conditions. It could go along with Bees and act as a visual detector for changing humidities...
But you'd have to account for pressure some how. So then you could include the pressure API from liquiducts to determine the pressure of the liquid in your liquiducts using clouds without having a multi-meter.
You could also use the clouds as a semi-conductor to determine the current flowing through a piece of wire (V = IR, if you know the resistance of the semi-conductor cloud and the voltage, you can determine current.)
SO MANY APPLICATIONS SOMEONE GET ON THIS IT'S GOLD.
/end
(Vauthil was here. Watch the language please. Thank you. =))