What is "storm window"

n. A detachable second window attached on the exterior side of a window in climates with harsh winters, to add an insulating layer of still air between the outside and inside.

n. a window outside an ordinary window to protect against severe weather or winter [syn: storm sash]

Storm windows are windows that are mounted outside or inside of the main glass windows of a house. It may also refer to a small openable flap found in the side window on light aircraft.

On houses they serve as a retrofit on existing windows in order to improve their thermal efficiencies. Similarly, storm doors (also called "screen doors") allow similar energy savings with less efficient primary doors - and allow a screen for summer ventilation.

Storm windows can be mounted externally or internally; can be made of glass, rigid plastic panels, or flexible plastic sheets; and may be permanently or temporarily mounted. They function similarly to insulated glazing.

Aside from insulation, storm windows provide an additional measure of protection for homes against damages to costly glass panes during inclement weather.