I 've found out this great website where you can get historical AWOS weather data. I would like to use it for a commercial application but I don't find any data license. Do you know if the AWOS data are free to use for commercial purposes? Can you point me to a relevant link?
Your website source from Iowa State University may or may not be open for commercial use. As Freeman has said in his comment, it would be difficult for anyone here on the internet not affiliated with the university to know. Your best opportunity to use their data is to contact the university directly. Their contact information is at the bottom of the link you provided.
Otherwise, your best option is to contact NOAA, the NCEP or the National Weather Service (basically, one and the same organization). They collect, consolidate, and archive the data at their source. That is where many commercial applications get their data.