Most room air conditioners are designed to fit double-hung windows. The lower sash that pushes up from the bottom creates the gap where the air conditioner sits. Measuring the open window's width and height is necessary to get the right size air conditioner.
Window air conditioners tolerate water, but if the side of the house chosen is exposed to more rain and wind, it could be an issue. These air conditioners have side curtains that pull out to cover gaps caused by the installation. Additional barriers may be needed to keep water from getting into the home.
It's also best to pick a window that is not on the sunniest side of the house. The sun heats up the back of the air conditioner, making it work harder. Pieces of insulation, cut to fit, work well for both the rain and the heat issues.
If possible, the window should be close enough to a three-prong power outlet. If none is nearby, specially labeled extension cords are available. Curtains or drapes can block the air vents, so it's best to choose blinds or shades, as they can be adjusted to pull down to the very top of the unit and still provide privacy.