What are storm windows?
Storm windows are a secondary window that typically rest on the exterior side of the primary sash (occasionally from the interior side) and are were originally constructed of wood and latter of various materials including aluminum.
What do storm windows do?
Storm windows have many functions. It is debatable what the most important function is but I am inclined to think it is a toss up between providing more insulation and protecting the primary sash from the elements (i.e. moisture). Storm windows allow a double pane system to exist which reduces noise pollution. Having a double pane system creates a large dead air space about 3-4 inches deep which slows energy transfer.
How to install storm windows?
Storm windows typically rest inside a rabbet and are fastened in place using half turn bottons or full turn buttons. These snap fasteners are also a great option that allows you to push the window open from the inside in case of fire. Storm windows can also be hung using a zinc storm hanger or stainless steel storm hanger and then propped open using what are called storm stays to operate storm windows so they no longer need to be removed and installed depending on the time of year. Here are two styles of storm window stays. These Stanley storm stays are functionally more pleasant to operate and this one by SRShardware is visually more appealing.