On cold winter days when the interior house door is open, hot air hits the cold storm door, causing the window to frost or steam up. This is condensation, and it's usually a good sign. It means that the interior door its job, stopping the cold air from reaching where the storm door is located. If the storm door frosts all the time, though, even when the interior door is not open, it indicates too much warm air is hitting the storm door on a regular basis. Either way, you can stop the window from frosting.
Hold tissue paper, ribbon or another lightweight object along the edges of your interior door to see if air makes it move to test for air gaps. If you see light between the door and the door frame, there's too much of a gap.
Seal the interior door to stop warm air from leaking to the storm door, if applicable. Weather stripping works well, as does an extender screwed into the bottom inside of the door that reaches the floor to cover any gaps. If the gaps can not be sealed or covered, it may be time to install a new door that fits better.
Raise the expander on the bottom of your storm door to allow for a small gap if your storm door frosts up too quickly when the interior door is open or you just don't prefer the condensation. You'll have to unscrew the current bottom expander and then use a drill and screwdriver to raise it 1/4 to 1/8 inch or so.
Lower the humidity in the home. If you keep the interior door open on a regular basis, even on cold days, using a dehumidifier can help prevent frosting.