Are there LEGO Hygrometer sensors?
There is not one made by LEGO. However, there are a number of 3rd-party options:


Fatcatlab makes a Humidity Sensor (EV3 only).
Vernier makes a relative humidity sensor that can be used with the Vernier sensor adapter (EV3 and NXT).
There are also several temperature and humidity sensors from Grove that can be used with the mindsensors.com Grove sensor adapter (EV3 and NXT).