Water, not air

A gas does not conduct

Air holds water as vapour: separate molecules, far apart, with nothing to carry a current from one loop to the next. At 30 % humidity or 95 %, the gap between two loops is the same insulator. The board reads 0.

Liquid water is different. A drop that lands across a gap touches a VCC loop and a SENSE loop at once, and a small current flows through it. That is the only thing the board detects.

Pick what is on the board. Humid air reads the same as dry. One drop across one gap reads a little. A fogged board, droplets across every gap at once, reads most of the way up. Tap water reads more than condensed steam for the same amount, because what conducts is what is dissolved in the water.

How steam becomes water on the board

Steam is water vapour, too, until it meets something cooler than itself. Then it condenses: on a cold window, on a mirror, and on these loops if they are cooler than the steam. A kettle's spout, a pan, a breath on the board all work this way. The back of the board says what it does, fairly: DETECTS STEAM VIA MOISTURE.

It also means the board stops noticing once it has warmed up in the steam, and that a drop left behind keeps reading after the steam has gone, until it evaporates.

What the number means

The reading is not in any unit. It grows with how much of the pattern is wet, and with how conductive the water is. Condensed steam is nearly pure and conducts poorly; tap water, rain and anything with salt in it conduct far better. So use it to answer yes or no, wet or dry, against numbers you have seen on your own board, not as a measurement. The values in the figure above are a model, not readings from a bench.