@westerling Unless the fan (or a shroud) takes up the whole open part of window, putting the fan right on the windowsill results in some air getting sucked in from outside around the edges of the blades and then blown right back out, so moving it back a bit ensures that it's Hot Room Air that gets sent That Way.
But tbh even if you slap the fan right in the frame it's still way more efficient than AC on a cool night :)