Maybe you could figure a way to install a vent cover in the window. That way, it could remain open for ventilation, but would not get much water from the rain.
https://www.homedepot.com/b/Heating...rill?NCNI-5&isSearch=true&Ns=P_REP_PRC_MODE|0
https://www.homedepot.com/b/Heating...rill?NCNI-5&isSearch=true&Ns=P_REP_PRC_MODE|0