Samsung Galaxy S4 is loaded with lot of sensors. Ambient temperature and humidity level are 2 of them. Well, you can get these values with Comfort level feature in S Health application. Or in Temp-Humid test in FactoryKeystring application (How to test Galaxy S4 hardware functions?). But that require few clicks here and there. So XDA Developer, bala_gamer developed a nice Ambient Weather for Galaxy S4. You can get ambient temperature and humidity level straight from it.
Looking forward to newer release which has widget support, refresh interval, toggle between Celsius and Fahrenheit and look enhancement.
Download Ambient Weather for Galaxy S4 here.