When an obstruction to air navigation is presented during construction of a structure at least two lights, each light consisting of a lamp of at least 100 watts enclosed in an aviation red obstruction light globe, should be installed at the uppermost point of the structure. In addition, as the height of the structure exceeds each level at which permanent obstruction lights will be required, two similar lights should be installed at each such level. These temporary warning lights should be displayed nightly from sunset to sunrise until the permanent obstruction lights have been installed and placed in operation, and should be positioned so as to insure unobstructed visibility of at least one of the lights at each level from aircraft at any normal angle of approach. It will be permissible, in the event it is more practicable, to install and operate the permanent obstruction lighting fixtures at each required level, in lieu of the above temporary warning lights, as each such level is exceeded in height during construction.