Aamu was originally a boy's name but it is almost never given to boys anymore (don't do it, he will literally be the only boy named Aamu he ever meets who is not in a retirement home). It's ok to give this to a girl now.
Aimo is not that common, but it is for boys. Don't name a girl this. It would be accepted by the register for a girl, but she would be mistaken for male all her life. The girl version is Aino. And that's incredibly popular.
If you want a winter name for a baby girl then call her Lumi. That's snow.
Keep this in mind. Nobody really cares about the meaning of names. People care more if it's a nice-sounding name. The newest trends (especially for girls) is to use names from western countries that were not traditionally Finnish names. English: good. Asian/Arabic/other: bad.
If you want to see the common names given recently it is here.
http://www.vrk.fi/default.aspx?docid=8776
Or the top ten in English.
http://verkkopalvelu.vrk.fi/Nimipalvelu/default.asp?L=3