So I bought one of those Temperature/Humidity digital thermometers with the little bulb and put the bulb in my dome that covers the clone tray. There is plenty of moisture on the dome, the vents are shut, and the temperature reads 78F but the thermometer only indicates 42% relative humidity. Is this possible?

If it is reading correctly how do I raise the humidity when it is already pretty wet inside. What should it read for fresh cut clones?

Thanks for the help!

I would expect it to read higher than that, but I wouldn't worry... If you want to test your meter then add a bit of heat to the under-side and watch how the RH responds..
You want the warmth at the roots more than above anyways, and that will facilitate evaporation well obviously.. If your upper region is the warmest area, then less evaporation will occur, and on top of that, the warm air up top (I'm assuming where your meter sits) will read lower 'relative humidity' since warm air can hold more moisture overall..
Its quite likely though that conditions are quite ideal at canopy level.. Even at 42%, if you mist them everyday they should do fine.. Adding heat to the undersidecan be risky if you haven't worked out a heating method that you know won't cook them prior to introducing the cuttings to it..
Ideally you want your soil temp at about 80°.. The temperature of the air is less important when rooting..