The hygrometer measures the level of humidity in the air.

Humidity sensor - hygrometer

It's response can go from 0 (in the improbable case where the air would be completely dry) to the maximum value (127 in standard Midi protocol) indicating a high water saturation.

It can also react to breathing by sensing the condensed moisture when exhaling.

This is a slow sensor, used in installations that monitor an environment.

Especially if used outdoors, it must be protected from hard light and bad weather, by some sort of dark cover. It must still permit air to penetrate the cover so that the sensor may measure the humidity.

Data returned by the sensor over time.