we are using HIH7130 sensor for measuring the humidity and temp. Honeywell HumidIcon Digital Humidity/Temperature Sensors are digital output-type relative humidity and temperature sensors combined in the same package. These sensors provide an accuracy level of ±3%RH. With industry-leading long-term stability, true temperature-compensated digital I2C, Industry-leading reliability, Energy efficiency and Ultra-small package size and options. Other accuracies which are available: ±1.7 %RH (HIH9000 Series), ±2.0 %RH (HIH8000 Series), ±3.0 %RH (HIH7000 Series), and ±4.0 %RH (HIH6100 Series).

Why I2C :: I2C protocol is so easy to use and we can chain lot of different different devices with it as long as address don't conflict with each other.

Onion omega is so easy to setup. if you are new to onion omega you can follow this guide. Getting started with onion omega

make sure you install python I2C lib in you onion omega using these commands

opkg update
opkg install python-light pyOnionI2C

after installing these commands run i2cdetect -y 0

you will see I2C address of HIH7130.